Modern Concept Science and Technology 8 New

Modern Concept Science and Technology – 8 197 c) When the object is at centre of curvature i) The image is formed at C ii) It is real iii) It is inverted iv) It has same size to the object d) When object is between F and C i) The image is formed beyond C ii) It is real iii) It is inverted iv) It is magnified e) When object is at F i) The image is formed at infinity ii) It is real iii) It is inverted iv) It is highly magnified f) When the object is in between P and F i) The image is formed behind the mirror ii) It is virtual iv) It is erect v) It is magnified Summary of the images formed by a concave mirror SN. Position of an object Position of the image Nature of the image formed 1. At infinity At F Real, inverted, diminished 2. Beyond C Between F and C Real, inverted, diminished 3. At C At C Real, inverted and of the same size to the object 4. Between C and F Beyond C Real, inverted and magnified 5. At F At infinity Real, inverted, enlarged 6. Between F and P Behind the mirror Virtual, erect, enlarged Object Image Object Image Object Image Object Image

198 Light 7.2.10 Image Formation by a Convex Mirror ACTIVITY 6 Take a stainless steel spoon. Bring its outer face near to you and look into it. Do you see your image on it? Is it different from what you see in a plane mirror? Observe the natures of the image thus formed. Observation : The image formed by a convex mirror is virtual, diminished, erect and behind the mirror. Rules to draw the image formed by a convex mirror i) A ray of light parallel to the principal axis appears to diverge from the focus. ii) A ray of light passing through centre of curvature is reflected back along the same path. iii) A ray of light from an object which strikes the pole of the mirror at a certain angle, reflects at the same angle. P F C P F C 450 450 P F C i ii iii Rules to draw the image formed by a convex mirror Images formed by a convex mirror a) When the object is at infinity i) The image is formed at focus 'F' behind the mirror ii) It is highly diminished iii) It is virtual iv) It is erect b) When the object is between infinity and the pole i) The image is formed between P and F behind the mirror ii) It is diminished iii) It is virtual iv) It is erect Applications of the spherical mirrors a) Applications of a concave mirror i) Doctors use a concave mirror to focus the parallel beam of light on the patient's organ like throat, ear, etc. for their close examination. ii) Dentist use a special concave mirror to see back of the teeth by placing it behind the tooth. Object

Modern Concept Science and Technology – 8 199 FACTS WITH REASONS Dentist use a special concave mirror to see back of the teeth. A concave mirror forms an enlarged and erect image of an object when it is in between the pole and focus of the mirror. When a doctor places a concave mirror behind the teeth, it forms a virtual, erect and enlarged image of the teeth. So, dentist use a special concave mirror to see the back of the teeth. iii) It is used as a shaving mirror or make-up mirror. FACTS WITH REASONS Concave mirror is used as a shaving mirror. When our face is in between principal focus and pole of a concave mirror, an erect and magnified image is formed. So, a concave mirror is used as a shaving mirror or make-up mirror. iv) It is used as a reflector in making reflecting telescopes. v) Concave mirrors are used as reflector in the headlights of automobiles, such as cars, trucks, motor bike, etc. vi) It is also used as a reflector in search lights, torch light, etc. FACTS WITH REASONS A concave mirror is used as a reflector in headlight or in torch light. Light rays from the focus of a concave mirror form a parallel beam after reflection. When the bulb in a torch light or in headlights of the vehicles is placed at principal focus of a concave reflector, then a powerful parallel beam of light is formed. So, a concave mirror is used as reflector in headlight or in torch light. b) Applications of Convex Mirror i) Convex mirror is used in the street light as a reflector. FACTS WITH REASONS Convex mirror is used in the street light. A convex mirror is used in the street light to diverge the light rays over a wide range. ii) It is used as a security mirror in the shop, gates, etc. iii) It is used as a rear view mirror in the automobiles. FACTS WITH REASONS A convex mirror is used in vehicles as a rear view. Convex mirror has a wide range of view. It forms a small, erect and diminished image from wide range. Hence, a convex mirror is used in vehicles to look a rear view like the traffic behind him.

200 Light HOT SKILL HIGHER ORDER THINKING SKILL 1. Menuka was playing with spoon. She suddenly noticed that her image was formed inside the curved surface of spoon but it was smaller and inverted. She found it interesting and asked her dad, if why is her image formed in the spoon and why is it inverted? Explain it for her. Menuka was playing with spoon and noticed her inverted and smaller image in the spoon. She asked her dad for reasons. The reason is, first of all a polished curved surface works like a spherical mirrors. The spoon is curved, silvery and smooth. So it is a concave mirror. The inside of the spoon is concave mirror so it collects reflected beam of light at focus where lights actually meet to form real image. The real images are always inverted. So her image is also inverted in the spoon. 2. Images in the plane mirror cannot be printed into a photograph. The beam of light reflected from plane mirrors do not meet. They do not travel at the back side of mirror but only appear to do so. As a result a virtual mirror is formed in the plane mirror which cannot be printed into the photograph. 3. "Objects in the mirror are close than they appear" is written in the side glasses of vehicles. Convex mirrors are used as side mirrors in the vehicles. They cover a wide field of view but objects look smaller and farther than they actually are. So if mistaken vehicles can collide during changing lanes or over taking. Therefore "Objects in the mirror are close than they appear" is written as a safety measure. 4. The concave mirrors are used to make astronomical telescopes. The concave mirrors can collect light at focus. If a huge concave mirrors are used, it can collect huge quantity of light from the source over time. So it can make clear images of celestial objects. Therefore concave mirrors are preferred to make astronomical telescopes. 5. Concave mirror is used as a make-up mirror. When our face is in between principal focus and pole of a concave mirror, an erect and magnified image is formed. So, concave mirror is used as a make-up mirror. 6. Concave mirror can be used in the solar cooker. Concave mirror converges parallel rays of light at the focus. As a result, accumulation of heat takes place to cook food. 7. Write any two differences between concave mirror and convex mirror. Differences between concave mirror and convex mirror are: S.N. Convex mirror S.N. Concave mirror 1 In this mirror, the outer surface is reflective surface and inner surface is polished. 1 In this mirror, the inner surface is reflective surface and the outer surface is polished. 2 It is also called a diverging mirror. 2 It is also called a converging mirror.

Modern Concept Science and Technology – 8 201 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) For an object at infinity, a concave mirror forms an image: i) at F ii) at 2F iii) beyond 2F iv) between F and 2F b) For what position of an object, a concave mirror forms a real image equal in size to the object? i) at F ii) at C iii) beyond C iv) between F and C c) Where is the image formed if an object is at focus in a concave mirror? i) focus iii) center of curvature ii) within the focus iv) infinity d) Which mirror is used in search lights? i) concave ii) convex iii) plane iv) none of these e) Where is the image formed if an object is placed between focus and pole of a concave mirror? i) real, inverted and small ii) real, inverted and same size iii) real, inverted and enlarged iv) virtual, erect and enlarged 2. Write True for the correct and False for the incorrect statements. a) A concave mirror is also called a diverging mirror. b) A virtual image cannot be obtained on the screen. c) Convex mirror is used as a reflector in headlights of a car. d) Convex mirror has wider range of view as compared to that of concave mirror. e) The angle of incidence is equal to the angle of reflection. 3. Fill in the blanks using suitable words. a) The center of a spherical mirror is called …… . b) Real focus is present in …… mirror. c) The image which can be obtained on a screen is called ……. image. d) The focal length of a convex mirror is half of its……. of curvature. e) Shaving mirror is made from ............ mirror. 4. Answer the following questions in one word. a) What is the angle of incidence when the angle between the incident ray and the reflected ray is 60o ? b) Which type of mirror always form an image equal to the size of object? c) Which type of mirror used in vehicle to see the traffic at the back side? d) Which mirror is used to see an enlarged image of our face? e) What is called to a perpendicular line drawn at the point of incidence? STEP2 5. Write any two differences between: a) Real image and virtual image b) Concave mirror and convex mirror c) Plane mirrors and spherical mirrors

202 Light 6. Give reasons. a) A plane mirror is used to look our face. b) A concave mirror is also called a converging mirror. c) Dentist use a special concave mirror to see the back of the teeth. d) Concave mirror is used as a reflector in the headlights of automobiles. e) Concave mirror is used as a reflector in a solar cooker. f) Convex mirror is used in street light. 7. Answer the following questions a) Define the following terms with required examples. i. Light energy ii. Center of curvature iii. Principal axis iv. Principal focus v. Reflection of light vi. Spherical mirrors b) What is an image? Write down its types. c) What is a plane mirror? Write down the characteristics of the image formed by it. STEP3 8. Answer the following questions a) Write the characteristics of image formed by plane mirror. b) Write the applications of a plane mirror. c) Write the applications of a concave mirror and convex mirror. d) Copy and complete the ray diagram to show the image formation. Also, write the nature of the image formed. C F P (a) (c) C F P (b) F C e) Draw the diagram to show image formed by a concave mirror when the object is placed at following positions. Also, write the nature of the image formed. i) beyond 2F ii) at 2F (or C) iii) in between F and 2F iv) at F f) Draw the diagram to show the principal focus of a concave mirror and convex mirror. g) Draw the diagram to show the image formed by a convex mirror. Project Work Project Work 1. Take a rubber ball and cut a portion of it with a knife. Identify the concave surface and convex surface. 2. Draw the ray diagrams to show image formed by a concave mirror and convex mirror in different conditions on a chart paper. Present it in your classroom and discuss the nature and variation in size of the image formed in different conditions. Make a list of the uses of such mirrors.

Modern Concept Science and Technology – 8 203 Key terms and terminologies of the unit 1. Sound : Sound is a form of energy which produces the sensation of hearing. 2. Medium for sound : The matter or substance through which sound is transmitted is called medium for sound. 3. Speed of sound : The distance travelled per unit time by a sound wave propagating through a medium is called speed of sound. 4. Pitch of the sound : The shrillness or sharpness of the sound is called pitch of the sound. 5. Loudness : Loudness is the characteristic of a sound that distinguishes a feeble sound from a loud sound of the same frequency. 6. Tone : The sound of a single frequency is called tone. 7. Timber of sound : The quality of sound is called timber of sound. 8. Note : Sound of a mixture of several frequencies is called a note. 9. Sound spectrum : The orderly classification of sound waves according to their wavelength or frequency is called the sound spectrum. 10. Infrasound : The sound waves with frequencies below 20Hz are called infra sound or subsonic wave. 11. Audible sound : The sound wave that a human ear can hear is called an audible sound. The frequency of an audible sound is 20Hz to 20,000Hz (20kHz). 12. Ultrasound : The sound above the frequency of 20,000Hz is called an ultrasound. 13. Reflection of sound : The bouncing back of sound when it strikes a hard surface is called reflection of sound. 14. Sound pollution : The disturbance produced in the environment by undesirable, loud and harsh sounds from various sources is called sound pollution or noise. Sequence of Curriculum Issued by CDC  Simple calculations on velocity of sound  Audible sound, infrasound and ultrasound  Measurement of intensity of sound  Causes, effects and control measures of sound pollution UNIT Sound 7.3 Estimated teaching periods Theory Practical 6 1 Robert Boyle is a famous Irish scientist for proposing the Boyles law. He was born in England on 25th January and died on 31st December 1691. He was a natural philosopher, chemist, physicist, alchemist and inventor. He is best known for Boyle’s law and corpuscularianism. He is considered one of the founders of modern chemistry and a pioneer of the modern experimental scientific method. He was educated at Eton college. He discovered that sound cannot pass through a vacuum. It needs a material medium such as solid, liquid or gas to pass through. Robert Boyle About the Scientist

204 Sound 15. Complete wave : The combination of a compression and a rarefaction of longitudinal wave is called a complete wave of longitudinal wave. 16. Frequency : The number of complete cycles made in one second is called frequency. 17. Time period : The total duration of time required to form one complete cycle is called time period. 18. Wavelength : The distance travelled by the sound while forming one complete wave is called wavelength. 19. Amplitude : The maximum extent of vibration of particles from the mean position of propagation during the propagation of sound energy is called amplitude. Introduction We hear many sounds every day. These sounds are produced from humans, birds, bells, machines, vehicles, televisions, radios, etc. After production, these sounds propagate through the material medium and reach to our ears. Thus, sound is a form of energy which produces the sensation of hearing. It is an important part of existence for many living beings. Animals use sound to hunt and to warn of the approach of predators. Sounds are of various types. The sound of a siren makes people more attentive, while the sound of music has a tremendous effect of relaxing the mind. In this chapter we are going to learn how sound is produced and how it gets transmitted through a medium in the form of a wave. We will also learn about the characteristics of sound, sound spectrum, reflection of sound, refraction of sound and noise pollution. FACTS WITH REASONS Light wave is an electromagnetic wave, but sound wave is a mechanical wave. Light wave consists of both the electric and magnetic fields. It doesn't need a material medium for its propagation. It can travel through vacuum. But, sound wave needs material medium for its propagation. So, light wave is an electromagnetic wave, but sound wave is a mechanical wave. Production of Sound Sound is produced from different sources due to their vibrations. The vibrating body causes the medium like water, air, etc. around it to vibrate. Such vibrations in a medium are called waves. They carry energy from one place to another. ACTIVITY 1 1. Take a drum and beat it. Do you hear the sound of the drum? 2. Touch the membrane of the drum. What do you feel? 3. When the sound stops, touch the membrane of the drum again. Do you feel vibrations on the membrane of the drum? It can be concluded that the vibrating membrane of the drum produces sound. Propagation of Sound The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener in the form of longitudinal waves.

Modern Concept Science and Technology – 8 205 A vibrating object vibrates the particles of the medium around it. The particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle to displace it and returns to its original position. This process continues in the medium till the sound reaches our ear. FACTS WITH REASONS The sound of explosion is not heard on the moon. Moon has no atmosphere. The sound never propagates through a vacuum. If some explosion takes place on the moon, the sound of explosion will not be propagated. So the sound of explosion is not heard on the moon. Terms Related to Sound Sound travels in the form of longitudinal waves. When a sound wave passes through air, the particles of air vibrate back and forth parallel to the direction of propagation of sound wave. It forms regions of compressions and rarefactions. i. Complete wave The combination of a compression and a rarefaction of longitudinal wave is called a complete wave of longitudinal wave. In case of transverse wave, the combination of a crest and a trough is called a complete wave. ii. Frequency (f) The number of complete cycles made by wave in one second is called frequency. It is denoted by 'f'. The SI unit of frequency is hertz (Hz). The larger units of frequency are kilohertz (kHz) and megahertz (MHz). \ 1 kHz = 1000 Hz = 10³ Hz iii. Time period (T) The total duration of time required to form one complete cycle is called time period. Mathematically it is equal to reciprocal of frequency. T = 1 f iv. Wavelength (λ) The distance travelled by the sound while forming one complete wave is called wavelength. Its SI unit is meter. v. Amplitude (a) The maximum extent of vibration of particles from the mean position of propagation during the propagation of sound energy is called amplitude. Its SI unit is meter. ACTIVITY 2 Make a list of different types of instruments which produce sound. Note the part of the instrument that vibrates during the production of sound.

206 Sound Speed of Sound The distance travelled per unit time by a sound wave propagating through a medium is called speed of sound. Sound propagates with different speeds in different media. Its speed is 343 m/s when travelling through the air at 20o C. Wave Equation: Relationship among frequency, wavelength and velocity The speed of sound wave is equal to the product of wavelength (λ) and frequency (f). Mathematically, v = f × λ FACTS WITH REASONS Sound waves of all frequencies propagate with same speed through a medium. When frequency of a wave increases, its wavelength decreases and vice versa. The product v = f × λ remains the same. So, sound waves of all frequencies propagate with same speed through a medium. Solved Numerical 6.1 When the C4 key on a piano keyboard is pressed, a string inside the piano is struck by a hammer and begins vibrating back and forth at approximately 260 cycles per second. a) What is the frequency in Hertz of the sound wave? b) Assuming the sound wave moves with a velocity of 343m/s, what is the wavelength of the wave? Solution: Given, Velocity of sound waves (v) = 343 m/s Number of cycles per second in vibrating string = 260 Frequency of vibrating string (f) = 260 Hz According to the wave equation, Wavelength of the sound wave, λ = v f = 343 260 = 1.32 Therefore, the wavelength of the sound wave is 1.32 m. Speed of Sound in Different Media The speed of sound varies greatly in different media. It is the property of a medium that determines the speed of sound in it. The more compact the medium, the faster the speed of sound. Temperature also affects the speed of sound. i) In solids : Solids are packed together tighter than liquids and gases, i.e., solids are denser than liquids and gases. The distances between MEMORY TIPS Of the three mediums (gas, liquid, and solid) sound waves travel the slowest through gases, faster through liquids, and fastest through solids. vgas < vliquid < vsolid

Modern Concept Science and Technology – 8 207 molecules in solids are very small. They can collide very quickly. It takes less time for a molecule of the solid to colloid into its neighbour. Hence, sound travels the fastest in solids. ii) In liquids : Liquids are denser than gases, but less dense than solids. The distances between molecules in liquids are shorter than in gases, but longer than in solids. So it takes a bit more time to share vibrations in liquids than in solids. Hence, sound travels faster in liquids than in gases but slower than that in solids. iii) In gases : Gases are the least dense. The molecules in gases are very far apart compared with solids and liquids. It takes more time for molecules in sharing vibrations in gases than in solids and liquids. Hence sound travels the slowest in gases. FACTS WITH REASONS When a firework explodes, the light is perceived before the sound. Sound travels more slowly than light. So, when a firework explodes, the light energy is perceived before the sound energy. Factors Affecting Speed of Sound in Air i) Density : At a constant pressure, speed of sound in air is inversely proportional to the square root of the density of air. Speed (v) ∝ 1 √ density The speed of sound becomes greater in air when its density decreases. ii) Temperature : The speed of sound is directly proportional to square root of temperature, Speed (v) ∝ √ temperature (T) It means, the speed of sound increases with increase in temperature. FACTS WITH REASONS We hear more clearly on a hot day than on a cold day. The speed of sound increases with the increase in temperature. For example, the speed of sound waves in air at 0°C is 332 m/s and the speed of sound at 20°C is 343 m/s. So, we hear more clearly on a hot day than on a cold day. iii) Humidity : When moisture is present in the air, the density of air decreases. It is because the density of water vapors is less than that of dry air. As the density of air decreases due to moisture, the speed of sound increases. Therefore, an increase in humidity in air increases the speed of sound and decrease in humidity in air decreases the speed of sound. Density of moist air < density of dry air Therefore, Speed of sound in moist air > speed of sound in dry air. MEMORY TIPS v = f × λ or, 1 f = λ v or, T = λ v Hence, time period is equal to the ratio of wavelength to wave speed.

208 Sound FACTS WITH REASONS On a rainy day sound travels faster than on a dry day. An increase in humidity in air increases the speed of sound and decrease in humidity in air decreases the speed of sound. So, on a rainy day sound travels faster than on a dry day. iv) Direction of Wind : The speed of sound increases in the direction of wind. i.e. Speed of sound = speed of sound in still air + speed of wind If the wind blows in the opposite direction in which the sound travels, the speed of sound decreases. Characteristics of Sound A sound can be characterized by its frequency (pitch), amplitude (loudness) and quality. Pitch Do you distinguish the shrillness of the voice in boys and girls? The voice of boys is less shrill than the voice of girls. A more shrill sound is said to be of a high pitch and the less shrill sound is said to be of a low pitch. The shrillness or sharpness of the sound is called pitch of the sound. It is the effect produced in the ear due to the sound of some particular frequency. Pitch is not a measurable quantity but it can be given a name on the musical scale. For example, the middle C note has a frequency of 262 Hz. Relationship between Pitch and Frequency of a Sound Pitch depends on the frequency of a vibrating body. Pitch and frequency of a sound are directly proportional. The higher the frequency of a sound, the high is the pitch and vice versa. A body vibrating with high frequency produces a sound of high pitch and the body vibrating with low frequency produces a sound of low pitch. For example, a mosquito’s wings vibrate rapidly producing a sound of high pitch. FACTS WITH REASONS There is a variation in the pitch of sound in boys and girls. The tighter vocal cords in girls vibrate more rapidly and the sound produced is of a high pitch. This causes the girls to produce sound in higher pitches than that of the boys. Factors Affecting Pitch of Sound i) Thickness of Vibrating String The pitch of sound increases with the decrease in thickness of the vibrating wire. It is for the same reason that stringed musical instruments like guitar are provided with strings of different thickness, so as to produce notes of different pitches. FACTS WITH REASONS Small thickness of vibrating string produces a high pitched sound. Small thickness of vibrating string vibrates rapidly. That is why; it has high frequency and high pitch of sound.

Modern Concept Science and Technology – 8 209 ii) Length of the Vibrating String Small length of vibrating string produces a high pitched sound and vice versa. This is the reason why musicians plunk the same string at different points so as to have a sound of different frequencies or pitches. FACTS WITH REASONS Small length of vibrating string produces a high pitched sound. Small length of vibrating string vibrates rapidly. That is why; it has high frequency and high pitch of sound. Loudness or Intensity of Sound Loudness is the characteristic of a sound that distinguishes a feeble sound from a loud sound of the same frequency. Its SI unit is W/m2 . It is the human perception of how much energy a sound wave carries. The loudness of sound is commonly measured in the unit decibels (dB). A small change in the dB value of a sound represents a large change in its loudness. 1 decible increase in the magnitude of intensity, will increase the loudness by 10. For example, the sound of 20 dB is 100 times louder than the sound of 10 dB. Human ears can percept sounds from 10 dB to 180 dB. A sound of loudness in between 50 dB to 60 dB is considered as normal for hearing. If a person cannot hear sound whose intensity ranges from 40 dB to 65 dB, it is called moderate hearing loss. If a person cannot hear the sound of 80 dB, it is called deaf. Sounds Sound level in dB 1. Threshold of hearing 0 2. Swishing of leaves 0-20 3. Whisper 0-20 4. Normal conversation 40-60 5. Moving vehicles 60-80 6. Factory noise 80-90 7. Loud music in disco 130 8. Jet aircraft 140 9. Noise of railway station 85-110 10. Sound of printing press 70-80 11. Sound of motor car 110-120 12. Heavy street traffic 60-70 13. Listening limit of hurt 120-140 14. Sound produced due to mechanical failure 140-160 Factors Affecting Loudness (intensity) of Sound i. Amplitude (A) Amplitude depends on the energy the source puts into the wave. So the loudness of a sound is determined by the amplitude of the sound wave. A larger amplitude means a

210 Sound louder sound, and a smaller amplitude means a softer sound. Thus, the loudness of a sound is directly proportional to the square of the amplitude of the sound. Loudness of sound ∝ (amplitude)2 FACTS WITH REASONS In the given figure, which sound has higher pitch and which one is louder. In the given figure the frequency of sound A is higher than the frequency of sound B and C. So sound A has higher pitch than the sound B and C. The amplitude of sound C is longer than that of sounds A and B. So, sound C has more intensity and more louder. ii. Distance from the Source Loudness of sound decreases with the increase in distance of the observer from the source. This is the reason why in a classroom, if a student at the back benches cannot hear the teacher, then the student is asked to sit on a front bench. FACTS WITH REASONS When we are close to the loud speaker, we hear a loud sound. Loudness of sound decreases with the increase in distance of the observer from the source. When we are close to the loud speaker, we hear a loud sound as compared to far distance. iii. Area of Sounding Surface A larger surface of a sounding object sets more molecules in vibration than a smaller sounding surface. Thus, a larger sounding surface produces a louder sound than that from a smaller sounding surface. For example, a large drum will produce a louder sound than a smaller drum when they are struck equally. The intensity at a point is defined as the amount of vibrational energy associated per unit area per unit time. Its SI unit is watt per square metre (W/m2 ). It can be calculated by the following formula. Intensity of sound (I) = Sound energy Time × area 90 dB 1 m 2 m 4 m 84 dB 78 dB Fig. Source of sound

Modern Concept Science and Technology – 8 211 Solved Numerical 6.2 If a sound of horn has a power of 2x10³ watts and can be heard with in 200 m² area. Calculate intensity of sound. Solution: Power (P) = 2 × 10³ watts Area (A) = 200 m² Intensity of sound (I) = ? Using formula, I = P A = 2 × 103 200 = 10 dB FACTS WITH REASONS The sound produced from the big bell can be heard for a long distance than the small bell. Loudness of sound is directly proportional to the surface area of the sounding body. Thus, a larger bell produces a louder sound than that from a small bell. iv. Density of the medium The loudness of sound is directly proportional to the density of the medium, i.e. the greater the density of the medium, the louder is the sound. v. Frequency of sound: The loudness of sound is directly proportional to the frequency of sound. Differences between Loudness and Pitch Loudness Pitch 1. Loudness is the characteristic of a sound that distinguishes a feeble sound from a loud sound of the same frequency. 1. The shrillness or sharpness of the sound is called pitch of the sound. 2. Loudness of sound is determined by the amount of sound energy carried by the sound. 2. Pitch of sound is determined by the frequency of the sound wave. 3. Loudness does not change with change in frequency. 3. Pitch of a sound is directly proportional with its frequency. Quality The quality of sound is also called timber of sound. It is one of the characteristics of sound that enables us to differentiate between two different types of sound. Even as the vocal cords are of the same length and thickness in the human beings, their sounds do not resemble. It is due to the modification of sound produced depending upon the shape of mouth cavity, shape of teeth, shape of lips, etc. Thus, the quality of sound is peculiar to an individual.

212 Sound FACTS WITH REASONS The sound of different persons can be recognized. Sounds produced from different people have different timber. Because of the difference in timber of sound, the sound of different persons can be recognized. Tone and Note of Sound The sound of a single frequency is called tone and the sound of a mixture of several frequencies is called a note. A note is pleasant to listen. Music and Noise Music consists of sound waves that are produced by regular and periodic vibrations. It is pleasant to hear because of the good quality called timber. Noise is produced when vibrations are irregular and non-periodic. A noise is unpleasant because of bad quality. Amplitude Time Graphical representation of a musical sound Amplitude Time Graphical representation of a noise Musical Sound Noise 1. It has a pleasing effect on the ears. 1. It does not have a pleasing effect on the ears. 2. It is produced by regular periodic vibrations in a material. 2. It is produced by irregular vibrations in a material. 3. The amplitude of vibration and its frequency do not change suddenly. 3. The amplitude of vibration and its frequency change suddenly. Spectrum of Sound Waves The orderly classification of sound waves according to their wavelength or frequency is called the sound spectrum. Infrasound, audible sound and ultra sound are the three types of sound on the basis of frequency. Infrasound (Infrasonic) or Subsonic The sound waves with frequencies below 20Hz are called infrasound or subsonic wave. Infrasound is produced because of very slow vibrations. We cannot hear infrasound but many animals; such as whales, rhinoceros, elephants, etc. can produce and hear infrasound. Animals use infrasound for their communication. Elephants communicate using sound waves at 10 Hz. Rhinoceroses communicate with one another by using infrasonic sound of frequency as low as 5 Hz. Earthquake waves are subsonic waves. We do not hear these waves but some

Modern Concept Science and Technology – 8 213 animals do. As they hear infrasonic sounds produced by the earthquake, they show abnormal behavior before we feel the earthquake. Audible Sound The sound wave that a human ear can hear is called an audible sound. The frequency of an audible sound is 20 Hz to 20,000 Hz (20 kHz). Sounds produced from musical instruments, our vocal cord, bell, etc. are all audible sounds. Human ear is not equally sensitive to all frequencies. Most people cannot hear sounds with frequencies below 20 Hz or above 16,000 Hz. Older people are less sensitive to frequencies above 10,000 Hz. By age 70, most people cannot hear sounds with frequencies above 8000 Hz. However, children under 5 years of age can hear the sound up to 25,000 Hz. 10–20,000 Hz 20–20,000 Hz 15–50,000 Hz 60–65,000 Hz 40–12,000 Hz Ultrasound Sound above the frequency of 20,000 Hz is called an ultrasound. Humans cannot hear the ultrasound. However, many animals such as dog, cat, bat, monkey, deer, etc. can detect sound beyond the audible range of humans. Bats and dolphin can produce and hear ultrasound. Dogs and cats cannot produce ultrasound, but they can hear it. Uses of Ultrasound Due to high frequency of ultrasound, it is associated with more energy and can penetrate upto a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some of its uses are given below. i) Doctors can identify the diseases of inner body and locate the position of the tumour with the help of an ultrasound. ii) An ultrasound scan of the womb helps to obtain the image of a human foetus in the womb. iii) A concentrated beam of ultrasound can be used to break up kidney stone without a surgery. iv) Bats produce ultrasounds and detect the reflected sound waves coming from obstacles on the way. This helps bats to identify the position of preys or obstacles on the way. v) Dolphins use ultrasound to catch their prey. vi) Ultrasonic waves are used in fathometer to calculate the depth of water in sea. Fig: Ultrasonography

214 Sound vii) To clean machinery parts which are beyond reach without disassembling of their parts. viii) It is used to detect the deformities in metal blocks. ix) It is used in detection of any blockade in a pipe line. FACTS WITH REASONS How can a bat identify location of preys and obstacles? Bats produce ultrasounds and detect the reflected sound waves coming from obstacles on the way. This helps bats to identify the position of preys or obstacles on the way. Differences between Infrasound and Ultrasound Infrasound Ultrasound 1. The sound waves of frequency below 20 Hz is called infrasound. 1. The sound waves of frequency above 20,000 Hz (20kHz) is called ultrasound. 2. Seismic waves are infrasound. 2. Bats and dolphins produce ultrasound. Sound Pollution or Noise The disturbance produced in the environment by undesirable, loud and harsh sounds from various sources is called sound pollution or noise. It is an adverse or unpleasant effect. Causes of Sound Pollution (Noise) The increasing dependence of the man on various kinds of machines at home or workplace or factories is the cause of noise pollution. Sources of Noise i) Noise at homes : Television, radio, power music system, washing machines, mixture/ grinder, vacuum cleaner, etc. are the sources of noise at our homes. ii) Noise in surroundings : Loudspeaker, exploding crackers on various functions and festivals, vehicle horns, announcements, sound from construction works, etc. are the sources of noise in our surroundings. iii) Noise in factories : Various machines used in small and big factories produce noise. Effects of Sound Pollution i) Noise reduces concentration, increases stress, causes headache etc. ii) Noise results loss of work efficiency. iii) It causes anger, irritation and high blood pressure. iv) Due to sound pollution, victims may even suffer heart attacks. v) A constant hearing of noise in the level of 130 dB to 140 dB may result in the loss of hearing or deafness. Ways to Reduce Sound Pollution i) Sound pollution can be reduced by using sound absorbing materials like thermocol, thick curtains, carpets, etc.

Modern Concept Science and Technology – 8 215 ii) Vehicles exceeding allowed noise limits should be penalized. iii) It can be reduced by restricting the use of loudspeakers in public places. iv) It can be reduced by using noise control techniques in industries. v) Airports and noise producing factories and industries must be shifted far away from the residential areas of the city. HOT SKILL HIGHER ORDER THINKING SKILL 1. How is sound produced? Name the organ that produces sound in a human body. Sound is produced by the vibration of the object like loud speakers, bell, tuning fork, etc. Vocal cord vibrates to produce sound in human beings. 2. Have you felt and earthquake. It feels like vibration that flows from ground into your boyd? What is it? Yes! I have felt an earthquake that flows from earth into the body. We feel it like a vibration. So it must be sound. Since we cannot hear but feel it. It must be infrasound (Sound having frequency less than 20 Hz). 3. We do not hear sounds of bats. Sound produced by bats is an ultrasound. Its frequency is beyond 20 kHz. So, it is not audible to us. 4. Differentiate between music and noise. The differences between music and noise are: SN Music SN Noise 1 Music has regular and periodic vibration. 1 Noise has irregular and nonperiodic vibration. 2 Music is pleasant to hear. 2 Noise is unpleasant to hear. 5. Speed of sound in air is 332 m/s. Find the wavelength of a sound wave having a frequency of 10 Hz. Solution: Speed of sound waves (v) = 332 m/s Frequency of vibrating string (f) = 10 Hz According to the wave equation, Wavelength of the sound wave, λ = v f = 332 10 = 33.2 m Thus, wavelength is 33.2 m. 6. A person has a hearing range from 20 Hz to 20 kHz. Find the minimum and the maximum wavelengths of the sound waves that normal people can hear. Take the speed of sound in air as 332 m/s.

216 Sound Solution: When frequency is minimum the corresponding wavelength is the maximum. The minimum frequency of the audible sound fmin = 20 Hz Now, according to the wave equation, Wavelength of the sound wave, λ = v f = 332 f = 332 20 = 16.6 m The longest wavelength of the audible sound is 16.6 m When frequency is maximum the corresponding wavelength is the minimum. The maximum frequency of the audible sound fmax = 20 kHz = 20,000 Hz Now, according to the wave equation, Wavelength of the sound wave, λ = v f = 332 f = 332 20000 = 0.017 m The minimum wavelength of the audible sound is 0.017 m. 7. Study the given table and answer the following questions. i) Explain the relation between speed of sound and density of gas. The velocity of sound in gas is inversely proportional to the square root of density of the gas. If density of medium is high then the velocity of sound will be less and vice versa. ii) Which gas has minimum density among A, B and C? Why? Medium (B) has minimum density because this medium has the highest velocity of sound. iii) Which gas has the highest density among A, B and C? Medium (C) has the highest density because it has lowest velocity of sound. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) What is the source of hearing? i. sense of hearing ii. sense of vision iii. vibration iv. ability to hear b) Through which medium sound cannot travel? i. fluid ii. liquid iii. solid iv. vacuum Medium Speed of sound A 332 m/s B 1270 m/s C 258 m/s

Modern Concept Science and Technology – 8 217 c) Which equation is true for wave equation? i. v = f × λ ii. f = v × λ iii. λ = v × f iv. Q= msdt d) What is the shrillness of the sound called? i. timber ii. loudness iii. music iv. pitch e) The loudness of sound of large bell is more than small bell. Why? i. larger bell has more frequency of sound ii. larger bell has higher pitch of sound iii. larger bell has higher amplitude of the sound iv. larger bell has more velocity of sound 2. Write True for the correct and False for the incorrect statements. a) Pitch depends on the frequency of a vibrating body. b) A small change in the dB value of a sound represents a large change in its timber. c) The loudness of sound is directly proportional to the density of the medium. d) The sound of a single frequency is called tone and the sound of a mixture of several frequencies is called a note. e) Doctors can identify the diseases of inner body and locate the position of the tumour with the help of an infrasound. 3. Fill in the blanks using a suitable word. a) The substances through which a sound is transmitted is called a ............ b) The distance travelled per unit time by sound wave propagating through a medium is called ............ c) Sound travels slowest in gases because gas molecules are ............ d) A body vibrating with high frequency produces a sound of ............ e) The disturbance produced in the environment by undesirable, loud and harsh sounds from various sources is called ............ 4. Answer the following questions in very short. a) Which organ of our body vibrates to produce sound? b) Write down the range of audible sound to humans. c) Write the relationship between the pitch and wavelength of a sound. d) Write the SI Unit of the amplitude and intensity of the sound. e) Write two examples of animals which use ultrasound. STEP2 5. Give Reasons a) Sound waves are longitudinal waves. b) Sound waves of all frequencies propagate with the same speed through a medium. c) Sound travels the fastest in solids. d) Sound travels the slowest in gases. e) Sound produced by a mosquito is shrill. f) The thinner strings of a guitar give out shriller sounds than thicker strings.

218 Sound g) The skin of a TABALA is tightened to produce shrill sounds. h) Sound produced from different people can be recognized. 6. Differentiate between the following a) sound wave and light wave b) intensity of sound and pitch of sound c) musical sound and noise d) infrasound and ultrasound. 7. Answer the following in short a) Define the followings terms with required examples. i) Sound ii) Musical sound iii) Mechanical wave iv) Noise v) Transverse wave vi) Sound spectrum vii) Trough viii) Amplitude of sound ix) Wavelength x) Intensity of sound xi) Longitudinal wave xii) Timber of the sound xiii) Sound pollution xiv) Medium for sound xv) Speed of sound xvi) Pitch of the sound xvii) Loudness xviii) Tone xix) Note xx) Infrasound xxi) Audible sound xxii) Ultrasound b) How is sound produced? c) Explain the factors affecting the speed of sound in air. d) What factors determine the pitch of a sound? Explain is short. e) What causes the variation in the pitch of sound in boys and girls? f) How is loudness related to amplitude? g) How does the distance of a listener from a source of sound affect the loudness of the sound? h) If the wavelength of a sound wave is reduced by half, what happens to the wave frequency and speed of the wave? i) What will happen to the loudness of the sound if i) the amplitude of a wave is tripled. ii) the surface area of the vibrating body is decreased. iii) the wind blows in the direction of propagation of sound j) Explain which of the following is (i) louder (ii) shriller i) sounds produced by a drum and a guitar ii) vibrating mosquito wings and roar of a lion iii) voice of a girl and voice of a boy k) All vibrating bodies produce sounds. Are we able to hear all the sounds? Explain.

Modern Concept Science and Technology – 8 219 STEP3 8. Answer the following questions. a) Write three applications of an ultrasound. b) What is sound pollution? How is it caused? c) What are the harmful effects of sound pollution? d) What measures can be taken to minimize sound pollution? e) Explain the effect of density, temperature, humidity and direction of wind in speed of sound when it propagates through air. f) Explain the effects and control measures of sound pollution. g) Explain the factors affecting intensity of sound. 9. Numerical Problems a) Speed of sound in air is 332 m/s. Find the wavelength of a sound wave having a frequency of 10 Hz. [Ans: 33.2m] b) The wavelength of a wave is 22m and its frequency is 15Hz. Find its speed. [Ans: 330m/s] c) A wave propagates with the speed of 340m/s. Its wavelength is 1.7 × 10-2m. Find the frequency of the wave. [Ans: 20,000 Hz] d) A wave of frequency 1kHz has wavelength of 100cm. How long will it take to move through a distance of 2km? [Ans: 2s] e) A person has a hearing range from 20 Hz to 20 kHz. Find the minimum and the maximum wavelengths of the sound waves that normal people can hear. Take the speed of sound in air as 332m/s. [Ans: 16.6m and 0.017m] f) A dog has a hearing range from 67 Hz to 47 kHz. Find the minimum and the maximum wavelengths of the sound waves that the dog can hear. Take the speed of sound in air as 332 m/s. [Ans: 4.96m and 0.0071m] g) A boy whistles with a power of 0.9 ×10−4 W. What will be the sound intensity at the area of 7 square meters? Project Work Project Work 1. Go out of your building and shout in a loud voice. Press the button of a stop watch when you shout and stop your stop watch when you listen to the echo. Note the time taken to hear the echo. Make a calculation for the rough estimate of the distance between you and the reflecting surface. 2. Visit a temple in your locality. Go near the bells. Hit the different sized bells with the suspended gung and listen to their sound. Which one produces a loud sound and which one produces a very sharp sound? Note down your answers and discuss about your findings in the classroom.

220 Electricity and Magnetism Electricity and Magnetism Unit 8 Introduction Magnetism is the property of the magnet. A magnet can pull iron and nickel. They are magnetic substances. Loadstone is a natural magnet. Bar magnets, u-shaped magnets and horseshoe magnets are artificial magnets. Natural magnets have weak magnetic fields so it is not used in electrical equipment. Artificial magnets have very powerful magnetic fields so it is used in electrical equipment. All of the magnets and magnetic substances are made up of tiny magnets. Each molecule of magnet has its north pole and south pole. The magnets can lose their property if hammered, heated or broken. The haematite rock found on the earth has magnetic properties. It gained magnetic properties from the earth. The earth itself is a huge magnet. The magnetic properties of the earth is called terrestrial magnetism. The magnetic energy can be changed into electricity by a generator and dynamo. The electricity passes through electric circuits. In our home, we can see lots of wires, bulbs, power sockets, switches, meter boxes and fuses. The arrangement of electrical parts and components in homes to supply electricity is called domestic wiring. There are three distinct wires in domestic wiring. They are live wire, phase wire and a neutral wire. The live wire brings electricity into the circuit. The neutral wires take the returning current away from the domestic circuit. The earthing wires pass leakage current from machines into the earth. We should learn to fix a plug but it is a very dangerous task. Electric shock can kill.

Modern Concept Science and Technology – 8 221 Key terms and terminologies of the unit 1. Magnet : The substance which attracts magnetic substances like iron, nickel, cobalt, etc. is called a magnet. 2. Natural magnet : The naturally occurring substance (like the lodestone), which possesses the property of attraction to the magnetic substances, is called a natural magnet. 3. Artificial magnet : Magnets which are made in laboratories or factories are called artificial magnets. 4. Temporary magnets : Those magnets in which magnetic property remains for a short time are called temporary magnets. 5. Permanent Magnets : Those magnets in which the magnetic property remains for a long time are called permanent magnets. 6. Magnetic substances : Those substances which get attracted towards a magnet are called magnetic substances. 7. Non-magnetic substances : Those substances which are not attracted by a magnet are called non-magnetic substances. 8. Magnetism : The various properties of a magnet are called magnetism. 9. Molecular theory of magnetism: According to the molecular theory of magnetism, "Each and every molecule of a magnet and magnetic substances is a complete magnet in itself, having north pole and south pole of equal strength. Sequence of Curriculum Issued by CDC  Introduction to natural magnets and artificial magnets, their uses and differences between them.  Molecular theory of magnetism  Introduction and causes of demagentization  Introduction to terrestrial magnetism, comparision of magnetism of the earth to the magnetism of a bar magnet UNIT Magnetism 8.1 Estimated teaching periods Theory Practical 4 1 William Gilbert is well known for his study of magnetism. He was born in England on 24th May 1544 and died on 30th November 1603. He was a physician, physicist and natural philosopher. He was the first person to realize that the earth itself is magnetic and that was the reason compasses point north. He also explained that the poles of a magnet cannot be separated by cutting. He got his education from St John’s College, Cambridge. William Gilbert About the Scientist

222 Magnetism 10. Molecular magnet : All magnets and magnetic substances are composed of tiny molecules called molecular magnets. 11. Magnetic field : The space around a magnet within which magnetic effect can be experienced is called the magnetic field. 12. Magnetic line of force : The curve path around a magnet along which a unit north pole moves is called magnetic line of force. 13. Magnetization : The process of making magnet from the soft magnetic materials is called magnetization. 14. Magnetic induction : Magnetic induction is a process by which a magnetic material develops magnetic properties in presence of a magnet. 15. Demagnetization : The process by which a magnet loses its properties is called demagnetization. 16. Terrestrial magnetism : The magnetic properties of the entire earth is called terrestrial magnetism. 17. Earth's magnetic field : The area where the earth'smagnetic influence is felt is called the earth'smagnetic field. 18. Dip needle : A dip needle is a magnetic needle which is free to rotate in the vertical plane. 19. Angle of dip : The angle between the dip needle and the horizontal line at a certain place is called the angle of dip. 20. Dip circle : The instrument which is used to measure the angle of dip is called a dip circle. 21. Geographical meridian : The vertical plane passing through the true geographic north and geographic south is called the geographical meridian. 22. Magnetic meridian : The vertical plane passing through magnetic south and magnetic north is called the magnetic meridian. 23 Angle of declination : The angle between the geographical meridian and magnetic meridian at a place is called the angle of declination. Introduction The substance which attracts magnetic substances like iron, nickel, cobalt, etc. is called a magnet. Magnet always rests in the north-south direction when freely suspended. It attracts only magnetic materials like iron, nickel, cobalt, etc. Thus, those substances which get attracted towards a magnet are called magnetic substances. Magnet also attracts some alloys which contain magnetic substances. Two such alloys are steel and alnico. Steel is an alloy of iron and carbon and alnico is an alloy of aluminium, nickel and cobalt. Substances which are not attracted by a magnet are called non-magnetic substances. For example, wood, plastic, brass, aluminium, copper, etc. Magnetic materials can be used to make magnet but non-magnetic materials cannot. Discovery of a Magnet The discovery of magnets is credited to the shepherd boy, Magnes. There does not have exact date or year when he discovered the magnet. Magnes used to live in a town of Magnesia in Asia Minor (now Turkey). One day while roaming on Mt. Ida, he found that his iron-strapped sandals Magnetite Lodestone

Modern Concept Science and Technology – 8 223 got stuck to the mountain and created difficult for walking. This was due to a black stone containing iron in it. This stone has a power to attract Iron. He also observed that the long thin stone taken from the mountain always pointing in a certain direction when it is freely suspended. Due to this direction indicating property of the stone, it was named lodestone which means 'leading stone'. Magnetism The various properties of a magnet are called magnetism. For example, a magnet attracts magnetic substances, it has two opposite poles, i.e. North Pole and South Pole, like poles repel and opposite poles attract, and magnet has more magnetic power at the ends, etc. Properties of a magnet i) Attracting property: A magnet attracts magnetic substances. ii) Property of indicating direction: A magnet when suspended freely with a thread comes to rest in the north-south direction. iii) The magnetic force is more at the ends of a bar magnet. iv) The two unlike poles of the magnets attract and like poles repel. v) The poles of a magnet cannot be separated. 8.1.1 Natural Magnet The naturally occurring substance (like the lodestone), which possesses the property of attraction to the magnetic substances, is called a natural magnet. Lodestone is a black ore of iron called magnetite. Its chemical formula is Fe3 O4 . The name magnetite is derived from the place where it was first found, i.e. Magnesia in Asia Minor. It is the only known natural magnet to man. Properties of Lodestone i) It attracts small pieces of iron. ii) A lodestone suspended freely with the help of a thread rests in the north-south direction. Use of Lodestone in Ancient Times It was used by the Chinese sailors to guide their ships. For this purpose, magnets were made in a desired shape and size from the lodestone. However, the natural magnets have no practical value in the modern days because of the following reasons: i) They have a weak magnetic force, which is not enough to be used in modern devices. ii) They are found in irregular structures, in quite odd shapes. FACTS WITH REASONS Lodestone is a natural magnet. Lodestone is natural magnet because it is formed naturally and has magnetic properties. Lodestone MEMORY TIPS The region around a magnet up to which the magnet can affect is called the magnetic field.

224 Magnetism 8.1.1 Artificial Magnets Magnets which are made in laboratories or factories are called artificial magnets. They are of various shapes and sizes. Some artificial magnets are bar magnet, horseshoe magnet, U-shaped magnet, cylindrical magnet, magnetic needle, etc. An artificial magnet which is formed when current flows in an insulated coil around a soft iron core is called electromagnet. Bar magnet Horseshoe magnet Compass U-shaped magnet Electromagnet Need of Artificial Magnet Nowadays, powerful magnets are used in a number of devices such as a loud speaker, radio, television, hydro-power station, etc. Following are the reasons of using artificial magnets. i) They can be made very powerful, which is not possible in the case of a natural magnet. ii) They can be made in any desired shape and size. Types of Artificial Magnets On the basis of temporary and permanent magnetic properties, there are two types of artificial magnets. They are temporary magnets and permanent magnets. a. Temporary Magnets Those magnets in which magnetic property remains for a short time are called temporary magnets. They are formed from the substances like soft iron. For example, when an iron nail is attracted by a magnet, magnetism gets induced in the iron nail. The magnetic properties of the nail disappear immediately. b. Permanent Magnets Those magnets in which the magnetic property remains for a long time are called permanent magnets. They are made of substances like steel or alnico. These substances possess a high power of magnetism after the magnetizing source is removed. Now-a-days, permanent magnets of different shapes and sizes are being made. These magnets have wide applications because of being light, strong and permanent. FACTS WITH REASONS Bar magnet is called a permanent magnet. A bar magnet is called a permanent magnet because its magnetic property remains in it for a long time. MEMORY TIPS 1. A bicycle dynamo uses permanent magnet. 2. The earth itself acts a big magnet.The magnetic property of the earth is called the terrestrial magnestim.

Modern Concept Science and Technology – 8 225 Differences between Temporary and Permanent Magnets S.N. Temporary Magnets S.N. Permanent Magnets 1. Those magnets in which magnetic property remains for a short time are called temporary magnets. 1. Those magnets in which the magnetic property remains for a long time are called permanent magnets. 2. Temporary magnets like an electromagnet can be magnetized to a very high strength. 2. Permanent magnets cannot be magnetized to a very high strength. 3. They are used to make generators, electric motors, fans etc. 3. They are used to make speaker, radio, etc. 4. They are made from soft iron. 4. They are made from steel or alnico. Differences between Natural Magnets and Artificial Magnets: S.N. Natural Magnets S.N. Artificial Magnets 1. Magnets formed naturally are called natural magnets. 1. Man-made magnets are called artificial magnets. 2. Natural magnets are not found everywhere on the earth. 2. Artificial magnets can be made everywhere. 3. Natural magnets are not found in a definite shape. For example lodestone. 3. Artificial magnets are made in different shapes and sizes such as, bar, horseshoe, U-shape, etc. Uses of magnets (i) Magnets are used in loudspeakers, radios, generators, dynamos, electric motors, computers and telephones. (ii) Magnets are used in magnetic compasses, refrigerator doors, pencil boxes, etc. (iii) Magnetic tapes are used in tape recorders and video recorders. (iv) Magnets are also used in certain toys to give a magic effect. (v) It is also used in MRI (Magnetic Resonance Imagers) machines in hospitals. 8.1.2 Molecular Theory of Magnetism Molecular theory of magnetism was introduced by Weber and prompted by James Alfred Ewing. Each molecule of a magnet behaves like a magnet. It shows all the properties of a

226 Magnetism magnet. According to the molecular theory of magnetism, all magnets and magnetic substances are composed of tiny molecular magnets. In a magnetic substance, the molecular magnets are randomly scattered in the form of a closed chain and in a magnet they are lined up in one direction in the form of a open chain. In the magnetic substances like iron, nickel, cobalt, the molecular magnets are scattered in the form of a closed chain. This pattern of molecular magnets in the form of closed chain neutralizes the adjacent molecular magnets and magnetic effect is removed. Fig: Arrangement of molecular magnets in a magnet Fig: Arrangement of molecular magnets in a magnetic substance FACTS WITH REASONS An iron bar has molecular magnets but it does not attract another iron bar. According to the molecular theory of magnetism, in a magnetic substance like iron bar, the molecular magnets are scattered in the form of a closed chain. This pattern of molecular magnets in the form of a closed chain neutralizes the adjacent molecular magnets. So, the magnetic effect is removed. On the other hand, a magnet has its molecular magnets in the straight line direction. In such arrangement, the north pole of each molecular magnet points in one direction and the South Pole faces to the opposite direction. This arrangement of the molecular magnets in the form of open chain produces a strong effective north pole at one end and a strong effective south pole at the other end. FACTS WITH REASONS A bar magnet has an effective north pole at one end and an effective south pole on the other end. In a bar magnet, the north pole of each molecular magnet points in one direction and the south pole points in opposite direction. The opposite poles of the molecular magnets in the middle of a bar magnet neutralize each other. So, a bar magnet has an effective north pole at one end and an effective south pole at the other end. Evidences of the molecular theory of the magnetism Following properties of a magnet support the molecular theory of magnetism. i) The poles of a magnet cannot be separated. ii) Only the magnetic substances can be magnetized. iii) A magnet gets demagnetized when it is hammered or heated. iv) A bar magnet has more force at its poles than that at its middle. MEMORY TIPS Magnetic saturation means all molecular magnets are arranged in close chain in a magnetic substance.

Modern Concept Science and Technology – 8 227 FACTS WITH REASONS The strength of a bar magnet is more at its poles. A bar magnet has its molecular magnets lined up in a particular direction. In such arrangement, the north pole of each molecular magnet points in one direction and the south pole point in opposite direction. However, the opposite poles of the molecular magnets in the middle of the bar magnet neutralize each other. So, the strength of a bar magnet is more at its poles. Main points of the molecular theory of the magnetism 1. All magnets and magnetic substances are composed of tiny and independent molecular magnets. 2. In a magnet, molecular magnets are arranged in a straight chain. 3. In a magnetic substance, molecular magnets are arranged in a closed ring. 4. The poles of a magnet cannot be separated. Memory Tips Magnetic Field The space around a magnet within which magnetic effect can be experienced is called a magnetic field. A strong magnet has a large magnetic field and small magnet has a small magnetic field. Magnetic field can be represented with the help of imaginary magnetic lines of force drawn around a magnet. Magnetic lines of force If a unit north pole is placed in a magnetic field of a bar magnet, the north pole of the bar magnet repels the unit north pole and the south pole of the bar magnet attracts the unit north pole. So, a unit north pole moves from the north pole of the bar magnet to the south pole along a curved path called magnetic lines of force. The curved path around a magnet along which a unit north pole moves is called magnetic line of force. Magnetic lines of force can be traced by using a small magnetic compass. The pattern of magnetic lines of force is different in different magnets. ACTIVITY 1 1. Take a drawing board and fix a card board paper with the help of pins. 2. Sprinkle fine iron powder (iron filings) evenly on the paper. 3. Place a bar magnet in the middle of the card board paper and beat the paper gently. Does the iron filings rearrange in the form of curves? These curves represent the magnetic lines of force. Magnetic Induction : A Method of Magnetization The process of making a magnet from a soft magnetic materials is called magnetization. The magnetic materials like iron, nickel, cobalt, etc. can be made magnet using different methods. This process is also called magnetic induction. Thus, magnetic induction is a process by which a magnetic material develops magnetic properties in presence of a magnet. In magnetic Magnetic lines of force

228 Magnetism induction, an ordinary piece of a magnetic substance acquires temporary magnetic properties. For example, take an iron or steel bar and attach to the clamp connection as shown in the figure (a). Put some iron nails near to it. In this condition, the iron or steel bar does not attract iron nails. Again, take a bar magnet near to the iron bar as shown in the figure (b). Now, the iron or steel bar attracts iron nails. It shows that in presence of a magnet, the steel bar also develops magnetic properties. Iron nails Unmagnetised steel bar Stand Stand Bar magnet Steel bar behaves as a magnet Iron nails (a) (b) Magnetic induction ACTIVITY 2 1. Take a bar magnet and keep on a table slightly facing outside. 2. Touch the pins with a bar magnet one by one as shown in the given figure. 3. You will see a long chain of pins. 4. You will find that the pins are attracted one another and with the magnet. This is due to the fact that the iron pins also become magnet if we touch them with a magnet. 8.1.3 Demagnetization The magnetic properties can get lost in mishandling of magnet. According to the molecular theory of magnetism, if a bar magnet is broken into many pieces, each small piece becomes a complete magnet. When the arrangement of the molecular magnets in a magnet is disturbed, the magnet gets demagnetized. Thus, the process by which a magnet loses its properties is called demagnetization. A magnet can be demagnetized in various ways. Some common ways are listed below: i) By heating: When a magnet is heated, its molecular magnets gain thermal energy and their kinetic energy increases. The alignment of molecular magnets in an open chain gets disoriented in a closed chain and the magnet is demagnetized. ii) By dropping or hammering: When a magnet is dropped frequently or heated regularly, it changes the alignment of molecular magnets into a closed chain. iii) By rubbing: A magnet loses its magnetic properties by rubbing the like poles of the magnets. MEMORY TIPS The disturbance in the chain of molecular magnets in a magnet is the main cause of demagnetization.

Modern Concept Science and Technology – 8 229 iv) Haphazard storing: A magnet loses its magnetic properties by storing magnets haphazardly without using keepers. When a magnet is kept nearer to another magnet of similar strength with like poles facing each other, then both of them get demagnetized due to induction in few days. v) Electrical method: A magnet loses its magnetic properties when we pass a high voltage alternating current through a solenoid wrapped around it. vi) Self demagnetization: Magnet becomes weaker if their poles are left free for long time. This is called self-demagnetization. It is a natural demagnetization. FACTS WITH REASONS Heating and hammering results demagnetization. When a magnet is heated or hammered, the molecular magnets start vibrating vigorously. It breaks the straight chain arrangement of the molecular magnets. So, heating and hammering a magnet results demagnetization. 9.8 Conservation of the Magnetism Magnet becomes weaker if their poles are left free for long time. This is called selfdemagnetization. Magnets should be stored in pairs, with opposite poles placed side by side after their use. A soft iron piece is placed across the poles of the magnets. These soft iron pieces are known as magnet keepers. They help the magnets to retain their magnetic properties. Some common methods for conserving magnetism are given below: i. We should not heat or hammer a magnet. ii. We should not rub the like poles of two magnets. iii. We should not drop a magnet. iv. We should store magnets by using keepers with opposite poles placed side by side. 8.1.4 Terrestrial Magnetism Our earth behaves like a huge magnet. This fact was first suggested by Dr. William Gilbert 400 years ago. The south pole of the earth's magnet is towards the geographical north and the North Pole is towards the geographical South Pole. The magnetic poles of earth are at Ellesmere island of Canada and Vostok Station of Antartica. Terrestrial magnetism is the study of the earth's magnetism and its various elements. Evidences of Terrestrial Magnetism i) A freely suspended magnetic needle points towards the geographic north and south direction. ii) A piece of iron placed inside the earth becomes a magnet. iii) There exist neutral points. iv) There occurs angle of dip and angle of declination. Bar magnet U-shaped magnet Keeper of soft iron piece Magnetic axis Geographical pole Magnetic field lines Rotation axis North Terrestrial magnetism

230 Magnetism FACTS WITH REASONS A freely suspended magnetic needle points towards the geographic north and south direction. A freely suspended magnetic needle points towards the geographic north and south direction because the free north pole of the magnetic needle is attracted towards South Pole of the earth's magnet. Similarly, the free south pole of magnetic needle is attracted towards North Pole of the earth's magnet. Why do neutral points exist? If we draw the magnetic lines of force due to a bar magnet, then we find a point at which the magnetic field due to the bar magnet is canceled by the earth's magnetic field. So, neutral points exist. Earth's Magnetic field The area where the earth's magnetic influence is felt is called the earth's magnetic field. Like the magnetic field of a bar magnet, earth's magnetic field is the strongest near the magnetic north and south poles and the weakest near the equator. Effect of Geomagnet i. It forms magnetosphere around earth which blocks harmful solar radiation. ii. Magnetic compass responds to terrestrial magnetism, which is very helpful in navigation. ACTIVITY 3 • Bring a bar magnet, an iron nail and a magnetic compass. • Place the iron nail on a table and keep the north pole of the magnet near the nail. Now, place the magnetic compass on another side of the nail. Which pole is formed on the other side of the nail? • Now, remove the magnet from the nail. Bring the south pole of the magnet near the nail. Which pole is formed on the other side of the nail? • Write down the conclusion of this activity. Elements of Terrestrial Magnetism i. Angle of Dip (angle of inclination) A dip needle is a magnetic needle which is free to rotate in the vertical plane. The angle between the dip needle and the horizontal line at a certain place is called angle of dip. It is also called magnetic dip or magnetic inclination or angle of inclination. The instrument which is used to measure the angle of dip is called dip circle. The dip needle remains parallel to the magnetic lines of force of the earth. The value Uses of angle of dip

Modern Concept Science and Technology – 8 231 of angle of dip varies from place to place. In the Kathmandu valley, it is about 42°. It means the dip needle makes 42° with horizontal line. The value of angle of dip at magnetic poles is 90° because the needle remains vertical at the poles. Similarly, the dip needle remains parallel to the ground at magnetic equator. So, its value is zero because the effect of earth's magnetic north pole and south pole are equal on the dip needle. Due to this, the dip needle remains parallel to the horizon. As we move towards the northern hemisphere, the earth's magnetic south pole becomes nearer. So, the value of angle of dip increases. A similar case happens when we go towards the southern hemisphere. The effect of the magnetic north pole of the earth is more on the south pole of the compass. So, the south pole of the dip needle points towards the ground. The magnetic south pole is situated at northern Canada and north pole is situated at the corner of Antarctica. It is used for measuring and correcting errors made by compass. It also helps in navigation of airplanes. It is used for geological mapping. ii. Angle of Declination The geographical poles and the magnetic poles of the earth do not coincide. So, there are two planes. The vertical plane passing through the true geographic north and geographic south is called geographical meridian and the vertical plane passing through magnetic south and magnetic north is called magnetic meridian. The angle between the geographical meridian and magnetic meridian at a place is called angle of declination. The value of angle of declination differs from place to place. To find the true geographical direction, the angle of declination of that place should be known. Angle of declination is measured by magnetic compass, declinometer, declination calculator, aeronautical map and software. A magnetic compass is a simple device which has a magnetic needle which is free to rotate on a pivot at the centre of a round box. In a compass, the needle which is free to rotate always rests in the north- south direction. It helps to find direction, test polarity, detect magnetic fields and find north and south poles. Uses of angle of declination It helps to find the true north and south poles of the earth. It helps in navigation. FACTS WITH REASONS At any given place to know the actual direction, one should know the angle of declination. The angle made by the intersection of the line joining the magnetic north-south and the line joining the geographical north-south at a place is called angle of declination. There is single value of angle of declination at a fixed place. Therefore, the value of angle of declination gives the position of that place. So, at any given place to know the actual direction, one should know the angle of declination. Fig: magnetic compass

232 Magnetism Differences between bar magnet and earth magnet S.N. Earth magnet S.N. Bar magnet 1. Earth magnet is a permanent magnet made from rare earth alloys. 1. Bar magnets are composed mainly of iron. 2. A earth magnet is about 2-7 times stronger than a bar magnet 2. Bar magnet is weaker than earth magnet HOT SKILL HIGHER ORDER THINKING SKILL 1. Can breaking a magnet into two, separate the poles of magnets? Why? Why not? No! breaking a magnet into two cannot separate the poles of magnet. According to the molecular theory of magnetism, all magnets and magnetic substances are composed of tiny molecular magnets. Each molecular magnet has both poles. Therefore, if a magnet is broken into multiple pieces, each piece can generate both poles. 2. What is the main difference between magnet and magnetic substance based on molecular theory? In a magnetic substance, the molecular magnets are randomly scattered in the form of a closed chain and in a magnet molecular magnets are lined up in one direction in the form of an open chain. Magnet has lots of free ends at north and south poles, but magnetic substance does not have any free ends. 3. An iron nail in close contact with a magnet can pull other nails. How is it possible? Explain. An iron nail is made of molecular magnets arranged in a closed chains. When it is in contact with a powerful magnets, the close chains will break and molecules will arange in open chain. It develops poles in them and they act like a magnet. So it can attract other nails. It is called magnetic induction. 4. How can we save magnetism of a magnet? Write down any four methods. We can conserve a magnetism of a magnet by the following ways: i. We should not hammer a magnet. ii. We should not heat a magnet. iii. We should not rub the like poles of two magnets. iv. We should not drop a magnet. v. We should not pass current in the magnet. 5. Write down any two differences between magnet and magnetic substances. Differences between magnet and magnetic substances are: S.N. Magnet S.N. Magnetic substance 1. The substance which attracts magnetic substances like iron, nickel, cobalt, etc. is called a magnet. 1. Those substances which get attracted towards a magnet are called magnetic substances.

Modern Concept Science and Technology – 8 233 2. In a magnet, molecular magnets are lined up in one direction in the form of an open chain. 2. In magnetic substances, the molecular magnets are scattered in the form of a closed chain. 6. In the given figures identify magnet and magnetic substances. Also write down the reason. a. b. The figure (a) is of a magnet. This is because molecular magnets are in a straight chain. The figure (b) is of a magnetic substance. This is because the molecular magnets are in a closed ring. 7. Is magnetism of earth important? Does it help living things to survive? Discuss. The magnetism of earth is called terrestrial magnetism and it is very important. It helps living things to survive by following ways. i. Migratory animals can sense earth's magnetism and use it to navigate. ii. Our magnetic compass works due to magnetism of earth. iii. It blocks solar winds from reaching earth. iv. It blocks solar radiations. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) Where is the magnetic field around a bar magnet strongest? i) at north pole ii) at south pole iii) at poles iv) at middle b) What happens to the magnetic strength, when a bar magnet is heated strongly i) remains the same ii) increases iii) decreases iv) doubles c) What will be the attraction of an iron filing by the poles of a magnet? i) maximum ii) zero iii) medium iv) minimum d) How are molecular magnets organized in an unmagnetized substance? i) an open chain ii) a straight chain iii) closed chain iv) none of these e) Which one is the evidence of terrestrial magnetism? i) a magnet has poles

234 Magnetism ii) earth has gravity iii) a freely suspended magnet rest in north-south direction iv) earth can pull moon 2. Fill in the blanks with appropriate words. a) The discovery of magnet is credited to the shepherd the equator. b) Magnet is the term derived from ……. . c) The property of a magnet is called ……. d) Hammering a magnet violently destroys its …….. e) The magnetic property of the earth is called ……… 3. Write True for the correct and False for the incorrect statements. a) The maximum strength of a bar magnet lies at the ends. b) Cobalt is a magnetic metal. c) Lodestone is an example of an artificial magnet. d) Permanent magnet loses its magnetism after falling on the ground. e) Magnetic field of the earth is strongest at the poles and weakest near the equator. 4. Answer the following questions in one word. a) What is called to the alloy made from aluminium, nickel and cobalt? b) What is called to the process of developing magnetic property? c) What is called to the process in which a magnet loses its property? d) What is called to the process in which the magnets become weaker after some time if their poles are left free? e) What is the magnetic property of the earth called? STEP2 5. Give reasons. a) A piece of soft iron is attracted by a magnet. b) A plastic pen is not attracted by a magnet. c) Most of the iron filing cling near the ends of a bar magnet. d) When a magnet is heated strongly, it gets demagnetized. e) Poles of a magnet cannot be separated. 6. Write any two differences between: a) Magnet and magnetic substances b) Magnetization and demagnetization c) Natural magnet and artificial magnet 7. Answer the following in short. a) What is a magnet? Write any three properties of a magnet. b) What is a molecular magnet?

Modern Concept Science and Technology – 8 235 c) State the molecular theory of magnetism. d) Write the evidences of molecular theory of magnetism. e) Define magnetic field and magnetic lines of force. f) What do you understand by magnetic induction? g) Explain the role of magnet keepers. h) What is terrestrial magnetism? STEP3 8. Answer the following questions. a) Explain an experiment to show the method of magnetization. b) Write down any four activities which cause demagnetization. c) Write any four methods of safe handling of magnets. d) Explain the arrangement of molecular magnets in a magnet and in a magnetic substance with the help of a diagram. e) Earth itself is called a magnet. How do we know that it is a magnet? Discuss. f) Answer the questions with the help of the given figure. figure a figure b i) Which figure represents the magnet and magnetic substance, why? ii) Which theory is explained by the given diagrams? g) Draw the diagram i) to show the magnetic field around a bar magnet. ii) to show the molecular magnets in a magnetic substance and in a magnet. Project Work Project Work 1. Take a strong bar magnet and iron nails. Pull iron nails one after another with the help of a bar magnet in the form of a long chain. What does it suggest? 2. Magnetize a stainless steel blade. Break it into four pieces. Check the magnetic properties in each piece of the blade. Do you find a new piece of magnet? Write your conclusion. Please do this project work in the supervision of the teacher or gurdian.

236 Electricity Key terms and terminologies of the unit 1. Electricity : Electricity is a form of energy which is produced due to the continuous flow of electrons through a conductor. 2. Sources of electricity : The devices which produce electricity are called sources of electricity. 3. Domestic electric circuit : An electric circuit made in industries, factories, houses, etc. is called a domestic electric circuit. 4. Fuse : Fuse is a thin wire made of tin (63%) and lead (37%) having a low melting point and high resistance. 5.. MCB : Miniature Circuit Breaker (MCB) is the advanced form of a fuse. It is used to protect the household wiring from overloading or short circuit. 6. Corporate fuse : The fuse installed in live wire before live wire is connected to meter box is called corporate fuse. 7. Distribution board : The component of domestic wiring that supplies electricity to different sections of the house is called distribution board. 8. Bulb : The bulb is an electric load that converts electricity into light. 9. Switch : The electrical parts that can open or close the electric circuit is called a switch. 10. Sockets : The part of electrical circuits where we can connect electrical appliances to the power supply is called sockets. Sequence of Curriculum Issued by CDC  Household wiring system  Name (main switch, electric meter, distribution board, fuse, miniature circuit breaker, bulb, switch and sockets), function and connection of electrical parts and components of domestic wiring  Introduction of live or phase wire, a neutral wire and earthing wire  Methods of wiring in a plug  Wiring of electric light circuit UNIT Electricity 8.2 Estimated teaching periods Theory Practical 4 1 Michael Faraday shares the title of father of electricity with Nikola Tesla and Thomas Edison for his contribution to electrolysis, diamagnetism and the discovery of electromagnetic induction. He was born in England on 22nd September 1791 and died on 25th August 1867. He was an excellent experimental physicist. He was honoured with the Royal medal, Copley medal, Rumford medal and Albert medal. Michael Faraday About the Scientist

Modern Concept Science and Technology – 8 237 11. Kilowatt hour meter : The meter which measures the electric energy consumed by loads is called kilowatt hour meter. 12. Main switch box : The switch box that can break electric circuit in entire house is called main switch box. 13. Consumers switch box : The switch box that controls the electric circuit flowing in the house flat wise is called consumers switch box. 15. Live wire : The wire that brings electricity from the main line is called live wire. 16. Neutral wire : The wire that provides path for current to return is called neutral wire. 17. Earth wire : The wire that conducts leakage current to the ground is called earth wire. 18. Colour code of wire : The colour present in the live wire, neutral wire and earth wire is called colour code of wire. 19. Electric plug : The device that fits into sockets and draws current is called electric plug. 20. Electric circuit : A continuous path made by connecting an electric source, electric load, switch, etc. with the help of a conducting wire is called an electric circuit. 21. Connecting wires : The metal wires that connect various electrical components and pass current through it are called connecting wires. 22. Electrical appliances : The device which runs from electrical energy is called an electric device or load. 23. Open circuit : The electric circuit through which current cannot flow due to an interrupted path is called an open circuit. 24. Closed circuit : The electric circuit through which current can flow in an uninterrupted path is called a closed circuit. Introduction Electricity is an important source of energy in the present world. It is the most useful and convenient form of energy for mankind. It is used in our home for various purposes like lighting, heating, cooking, etc. Electricity can be converted into different forms of energy easily. It can be stored and transmitted over a long distance. Study of electricity involves the study of charges. Charges can be produced on the surface of a body and can flow through the wires. The charge developed on a body produces static electricity and the flow of charges through a wire produces current electricity. Thus, electricity is a form of energy which is produced due to the continuous flow of electrons through a conductor. Current electricity is very useful to us. In this unit, we will discuss about the sources of electric current, domestic electrification, some electric appliance, fuse, MCB, etc. Source of Electricity We use a large number of electrical and electronic devices in our daily life. For example, computer, television, radio, camera, washing machine, refrigerator, calculator, watch, etc. We need electrical energy to operate these appliances. This electrical energy is derived from the various sources. Thus, the devices which produce electricity are called sources of electricity. For example, dry cell, photo cell, dynamo, generator, etc. Dry cell Photo cell Generator

238 Electricity 8.2.1 House Wiring System An electric circuit made in industries, factories, houses, etc. is called a domestic electric circuit. It is a circuit for alternating current (A.C.) of 230 volts. In domestic circuit, all kinds of loads are connected in parallel. The electricity generated in a hydro-power station is brought to our houses through overhead wires on the poles. The two wires from the pole and a third wire connected to the earth are distributed on the walls of our house to connect electric appliances. These three wires are live wire (L), neutral wire (N) and the earth wire (E). Bulb Fan Distribution board Main fuse Main switch box(S) Meter box (kWh meter- M) Corporation Fuse(F1 ) Consumer's fuse (F2 ) Socket E N L L N E House wiring system Before distributing the electricity in our house, a kilowatt hour kWh meter (M) and a fuse wire are connected in series with the live wire. This fuse is called corporation fuse or electricity authority's fuse (F1 ). The kilowatt hour meter (M) measures the electric energy consumed by the various electric appliance in our home. From the kilowatt hour meter, both live and neutral wires are connected to the main switch box (S). After the main switch, there is a consumer fuse (F2 ) to control the electric current flowing in our house. The house wiring system is divided into power circuit and lighting circuit. The power circuit is made using live wire, neutral wire and earthing wire. It is used to power heavy electrical equipments such as fridge, television, iron, computers and electric motors. The lighting circuit is made using a live wire and a neutral wire only. It does not contain earthing wire. It is usually used to run electrical bulbs. A bulb is placed in a holder. A wire is connected to two terminals of the holder. A switch is connected in the live wire. Then the bulb is connected into lighting circuit. When the circuit is closed the bulb will glow. FACTS WITH REASONS In domestic electric circuit, if one appliance stops functioning, the other keeps on working. In domestic electric circuit, all electric loads are connected in parallel. There is an individual circuit and switch for every load. All loads are supplied with equal potential difference. So, in a domestic electric circuit, if one appliance stops functioning, the other keeps on working.

Modern Concept Science and Technology – 8 239 Devices Used in a Domestic Circuit Main switch box (S) or Master switch From the kWh meter, both live and neutral wires are connected to a main switch box (S). It is used to switch off the supply to the entire house whenever it is necessary to repair any fault in the wiring. Kilowatt hour meter (M) It is a meter which measures the electric energy consumed by various electric appliances in kilowatt hour or unit. It is the commercial unit of electric energy consumed. The rate at which electrical work is done or the rate at which electrical energy is consumed, is called electric power consumption. It is measured in kilowatt hour (kWh). It is calculated by the given formula: The electrical energy consumed = P × N × t Where, i. 'P' is power rating of the appliance ii. 'N' is number of devices used iii. 't' is time for which the device is used FACTS WITH REASONS Kilowatt hour meter is installed in domestic wiring. Kilowatt hour meter is installed in domestic wiring to measure the quantity of electricity that we consume per month. ACTIVITY 1 Make a 3D model of a kilowatt hour meter. Worked out Numerical 1 Calculate the electrical energy consumed by 5 tube lights of 40 W each when they are used for 50 hours. Solution: Power (P) = 40 W = 40/1000 = 0.04 kW Number of tube light = 6 Time (t) = 50 hour Electricity consumed (EC) = ? MEMORY TIPS Electricity consumed in house is measured in kilowatt hour. One kilowatt hour is the amount of electrical energy consumed by an electrical appliance having a power rating of 1 kW in 1 hour. 1 kWh = 1 unit = 3.6 megajoules.

240 Electricity We have, Electrical energy consumed = P × N × t = 0.04 × 5 × 50 = 10 kWh Therefore, Electrical energy consumed = 10 kWh = 10 unit Worked out Numerical 2 In a building, 12 tube lights of 40 W each are used for 4 hours daily, 3 TVs of 40 W each are used for 4 hours daily, 2 irons of 750 W each are used for 2 hours a week. Calculate the total cost of electricity in one month. The cost of 1 unit is Rs. 7. Solution: i. For the lights Power of the bulb (P) = 40 W = 0.04 kW Number of bulb (n) = 12 Time of the bulb (t) = 4 x 30 = 120 hours Electricity consumed by lights in a month = P x N x t = 0.04 kW x 12 x 120 = 57.6 units ii. For TV Power of TV (P) = 40 W = 0.04 kW Number of TV (N) = 3 Time of TV (t) = 4 x 30 = 120 units Electricity consumed by TV in a month = P x N x t = 0.04 x 3 x 120 = 14.4 units iii. For iron Power of iron (P) = 750 Watt = 0.75 kW Number of iron (n) = 2 Time of iron (t) = 2 hours a week = 8 hours a month Electricity consumed by iron in a month = P x N x t = 0.75 x 2 x 8 = 12 units Total electricity consumed in that house per month = Electricity consumed by lights + Electricity consumed by TV + electricity consumed by iron Total EC = EC by lights + EC by TV + EC by iron = 57.6 + 14.4 + 12 = 84 units Total bill = units x rate = 84 × 7 = Rs. 588

Modern Concept Science and Technology – 8 241 FACTS WITH REASONS The kilowatt hour (kWh) meter is connected before the main switch. The kilowatt hour (kWh) meter is connected before the main switch so that the total current drawn by all the appliances can be calculated easily. Distribution board The component of domestic wiring that supplies electricity to different sections of the house is called distribution board. It contains main switch, trip switch and circuit breakers. Miniature circuit breaker (MCB) MCB is a circuit breaker. It is used in place of a fuse to open a circuit automatically for protection against over-current. Thus, miniature circuit breaker' (MCB) is the advanced form of a fuse. It is used to protect the household wiring from overloading or short circuit. A MCB allows current flow as long as the current in the circuit does not exceed the current rating of the fuse. If excess current flows, then the MCB switches off making the circuit open. Advantage of a MCB over fuse A fuse melts during overloading. It must be replaced to use the circuit again. But a MCB can be reset after the fault has been corrected. FACTS WITH REASONS MCB is better than fuse. A fuse melts during overloading or short circuit. It must be replaced to use the circuit again. But MCB trips when there is a fault in the circuit and current exceeds the safe limit. It can be reset after the fault has been corrected. MCBs are safer, easier, and offer more protection. So, MCB is better than fuse. Fuse Fuse is a safety device. It avoids the flow of unwanted current in the circuit. Thus, fuse is a thin wire made of tin (63%) and lead (37%) having a low melting point and high resistance. Fuse limits the value of current that can pass through a circuit. For example, a fuse of 5A is manufactured so that this fuse melts and breaks the circuit, if a current greater than 5 ampere (A) flows through it. If any fault arises in a circuit, its fuse wire melts without affecting the other circuit. It protects the circuit from fire and avoids fire hazards. Generally, 5A to 30A fuses are used in the domestic use. MCB Fuse Fuse wire melts and breaks the circuit

242 Electricity FACTS WITH REASONS A fuse wire is of high resistance and low melting point. Fuse is a safety device of the circuit. During excessive heating of an electric circuit, the fuse wire melts and opens the circuit. As a result, it protects the other circuit components from damage and fire. The high resistance of the fuse wire helps in heating and low melting point helps to burn fast. So, the fuse wire is of high resistance and low melting point. Consumer's fuse (F2) After the main switch, there is a consumer fuse to control the electric current flowing in the house. It protect the electric circuit connected in our hose. Corporate fuse or electricity authority's fuse (F1) Before the live and the neutral wires enter into the kilowatt hour kWh meter (M), a fuse wire is connected in series with the live wire. It is called corporate fuse (or electricity authority's fuse). Switch The electrical parts that can open or close the electric circuit is called switch or key. Each electrical appliance has at least one switch. Socket The part of electrical circuit where we can connect electrical equipments to the power supply is called sockets. Bulb Bulb is an electric load because it consumes electricity. Bulb is connected in parallel in domestic circuit such as house and school. Bulbs are connected in series in festival lights, disco lights etc. Bulb could be filament bulb, fluorescent lamp, Compact Fluorescent Lamp (CFL) or LED (light emitting diode). Filament bulb has thin wire of high resistance to produce light. Fluorescent lamp has harmful mercury vapour that can produce UV-rays which touches fluorescent powder and become white. We should be very careful while using bulb. Some of them contain poisonous gas and some of them can cut us. Differences between corporation fuse and consumer's fuse S.N. Corporation fuse or electricity authority's fuse S.N. Consumer's fuse 1. The electricity authority fits the main fuse in a house. 1. An electrician fits the consumer's fuse in the house. 2. It is connected before a kilowatt hour (kWh) meter. 2. It is connected after a main switch. 3. It is used for protection of the meter as well as for protection of the domestic circuit. 3. It is used for protection of the domestic circuit. 4. It is sealed by the electricity authority. 4. It is not sealed. Corporate fuse

Modern Concept Science and Technology – 8 243 8.2.2 Wires Connected in Domestic Electric Circuit Two wires from a pole and a third wire connected to the earth are distributed on the walls of our house to connect electrical appliances. These three wires are live wire (L), neutral wire (N) and the earth wire (E). a) The live wire (phase wire) : Live wire or phase wire is connected to the high potential of 220 V. It brings current. If accidentally, we come in direct contact with this wire, we may get an electric shock. b) The neutral wire : Neutral wire is kept at zero potential by connecting it to the earth. It provides return path for the current. A live wire is at the potential of 220 V and the neutral wire is at the ground potential of zero volt. The potential difference between the live wire and the neutral wire is 220- 0 = 220 V. c) Earth wire (E) : The third wire which carries the leakage current from a faulty or live appliance to the ground is called earth wire. One end of the earth wire is connected to the copper plate buried deep under the earth. It conducts leakage current to the ground. The earth wire protects a person from the possible electric shocks. Colour code in domestic wiring In domestic wiring, three wires are distributed on the walls. The electrical appliances with metal case like refrigerator, electric iron, etc. have three core of flexible cable. These wires have different colours. The colour present in the live wire, neutral wire and the earth wire is called colour code of the wire. Wire Old Convention New International Convention Live Red Brown Neutral Black Light blue Earth Green Green and yellow Advantages of using colour code for domestic wiring i) Colour code makes easy to identify the live, neutral and earth wire. ii) Repairing in underground domestic wiring becomes easier by using colour code. FACTS WITH REASONS Colour code is used for wires in domestic wiring. There are three different types of wires in a domestic circuit. They are live wire, neutral wire and earth wire. In order to identify them, the wires are coated with specific coloured insulator. It helps to repair the circuit. Earthing wire Live wire Neutral wire Wire

244 Electricity 8.2.3 Wiring in a Plug and Socket Unscrew the little screws on each of the plug's pins. Insert the twisted copper wires into the holes in the pins. The green or green with yellow stripes wire must always be inserted into the top pin. It is the earthing wire. The blue wire is inserted into the left pin. The pin is marked with a blue spot or the letter N. It is a neutral wire. Similarly the red or brown coloured wire in inserted into the right pin. It indicates the live or phase wire. Fig: Connection of wires in a plug 8.2.4Electric Circuit ACTIVITY 2 1. Take out a torch bulb, holder, dry cell, a switch and a conducting wire. 2. Connect the two terminals of the cell with the bulb. 3. Connect a switch on a wire. Does the bulb glow when you switch it on? The circuit becomes closed when the switch is on. The current flows in a closed circuit and the bulb glows. An electrical circuit is a complete path through which electrons flow. Electric source, electric load and a conducting wire are the three main components of an electric circuit. Electric load is a device which consumes electricity. For example, electric fan, bulb, TV, heater, etc. Thus, a continuous path made by connecting an electric source, electric load, switch, etc. with the help of a conducting wire is called an electric circuit. Components of an Electric Circuit A complete electric circuit normally consists of the following components. a) Electric Source The device which produces electricity is called a source of electricity. For example, cell, photo cell, dynamo, generator etc. Copper wire Switch Dry cell Bulb Electric circuit

Modern Concept Science and Technology – 8 245 b) Conductors or the Connecting Wires The substance through which electrons can flow is called a conductor. Metals are conductors of electricity. For example, silver, gold, copper, etc. But graphite, a nonmetal, also conducts electricity. c) Electric Device/Load The device which runs from electrical energy is called an electric device or load. For example, electric bulb, fan, television etc. It uses electrical energy and converts it into other forms of energy like heat, light, etc. Many electric devices have transformer. Transformer can change voltage of AC so that it will be suitable to operate devices. d) Switch The current flows in a circuit when a load is connected to the source by some connecting wire. To break the flow of the current, there is a device in a circuit. A device which is used to open or close an electric circuit is called a switch. It is connected in a series with the electric source, electric load, etc. So, it can interrupt a circuit. With its switch in an ‘on’ position, the circuit is closed and the circuit becomes open when the switch is in the ‘off’ position. Types of Electric Circuit a) Open Circuit An open circuit does not have a complete path for current to flow. The electric circuit through which current cannot flow due to an interrupted path is called an open circuit. A circuit becomes open when the switch is turned off. The electric load does not work in an open circuit. b) Closed Circuit A closed circuit has a complete path for current to flow. The electric circuit through which current can flow in an uninterrupted path is called a closed circuit. A circuit becomes closed when the switch is turned on. The electric load works in a closed circuit. Direction of Current in a Closed Circuit In an electric circuit, there is a flow of electrons. However, electrons were not known at the time when the phenomenon of electricity was first observed. So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current. Conventionally, in an electric circuit, the direction of electric Copper wire Switch off Dry cell Bulb Open circuit Copper wire Switch on Dry cell Bulb glowing Close circuit MEMORY TIPS 1. Emf is more than potential difference in a open circuit. 2. In closed circuit, electric appliance like bulb, heater, television etc. work.

246 Electricity current is taken as from the positive terminal to the negative terminal, which is opposite to the direction of the flow of electrons. Electric circuit of bulbs and tube-lights In domestic electrification, the bulbs and tubelights are connected in parallel. As a result we can control all the bulbs and tube-lights by separate switch. In parallel connection, if one of the bulbs or tube-light gets fused, other bulbs and tube-lights can work independently. HOT SKILL HIGHER ORDER THINKING SKILL 1. What is the main difference between a fuse and MCB? A fuse melts during overloading. It must be replaced to use the circuit again. But a MCB can be reset after the fault has been corrected. 2. Why is fuse called a safety device? : A fuse prevents overloading and overflow of current by breaking the electric circuit. As a result, it protects various electrical and electronic devices in our home from electrical hazards. So, fuse is called a safety device. 3. Ramesh noticed that electrical wires are available in multiple colours. He was wondering if it is just for show or it has certain meaning. Help him understand the colour code of the wire in domestic wiring. Ramesh saw electrical wires in multiple colours. He was wondering if the colours are just for decoration or it has meaning. To clarify his confusion we can explain colour code of domestic wiring for him. Domestic wiring uses various colours to mark the wires. Live wire : Red or brown Neutral wire : blue or black Earthing wire : Yellow or green or striped 4. The plug of an iron melted and Lavadev wants to replace the plug of iron with the new one. How should she proceed? If Lavadev wants to replace the damaged plug with new one, she should learn the method of wiring a plug. It is a simple task. i. Unscrew the plug. ii. Unscrew each pin of the plug. iii. Twist the copper of red wire and put in one pin. iv. Twist the copper of black wire and put in another pin. v. Twist the copper of green wire and put in the longest pin at top. vi. Screw all the pins then screw the plug. Now the plug is ready. No naked wires should touch each other inside the plug. Fig: Circuit of bulbs and tube-lights