Oasis School Science and Technology 8 New

Oasis School Science and Technology - 8 197 Rule 3 : A ray of light coming from an object which strikes the pole of the mirror at a certain angle, reflects at the same angle. Fig. 7.2.26 450 450 P F C Images Formed by a Convex Mirror When an object is kept in front of a convex mirror at any point, its image is formed behind the mirror. The image is virtual, erect and diminished. A F C 1 B1 Object Image A B Fig. 7.2.27 Image formed by a convex mirror Uses of Mirrors Uses of a plane mirror i. For a looking glass at home, shops or hair cutting saloons ii. For making periscope, kaleidoscope, etc. iii. For the practical purposes in laboratory Reasonable Facts Concave mirror is used in torch light and search light. The search lights is used for searching something. Since, it would be better as much as area it can illuminate. Concave mirror also spreads light and illuminate larger area as compared to the plane mirror. So, concave mirror is used in torch light and search light. Uses of a concave mirror i. For shaving and making cosmetic mirror ii. For making astronomical telescope

198 Oasis School Science and Technology - 8 iii. For making the reflector of searchlights, torches, etc. iv. For making solar cooker to converge solar radiation Reasonable Facts Concave mirror is used in headlight of vehicles. Concave mirror allows the light rays to be focused as a single beam and gives more power to the light that makes more efficient for seeing and to be seen by others. So, concave mirror is used in headlight of vehicles. Uses of a convex mirror i. To make street light reflector so that light can be spread in wider regions ii. To make the back view mirror in automobiles as it has wide field of view. Reasonable Facts Convex mirror is used as side view mirror of vehicles. Convex mirror gives a diminished, virtual and an erect image of the side or rear with wider field of view. A convex mirror enables the driver to view much larger area than would be possible with a plane mirror. So, convex mirror is used as side-view mirror of vehicles. Activity 4 Take a concave mirror. Hold it facing the sun. Focus the reflected light on the paper. Adjust the distance between the paper and the mirror until you get a sharp bright spot. What will happen if you hold it for few minutes? Explain. Reasonable Thinking Skill R T S 1. Moon is called non-luminous but the sun is called luminous. Moon is called non-luminous because it does not emit light. But, the sun is called luminous because it can emit light. 2. The mirrors can make an image but the walls cannot. The mirrors can make an image but the walls cannot because the mirrors are smooth that can make a regular reflection whereas the walls are rough that cannot make a regular reflection. Only the regular reflection can form an image. 3. Under what condition a concave mirror can make an image at infinity? A concave mirror can make an image at infinity when the object is in the focus of the concave lens. Fact File Focal length is half the length of radius of curvature.

Oasis School Science and Technology - 8 199 4. Why is a concave mirror not used as a side mirror of vehicles? The concave mirror is not used as a side mirror of vehicles because it forms the real and inverted image. The inverted image will be very confusing while driving. 5. Differentiate between the concave mirror and the convex mirror. The differences between the concave mirror and convex mirror are: SN Concave mirror SN Convex mirror 1 The spherical mirrors in which reflecting surfaces are inside are called concave mirrors. 1 The spherical mirrors in which reflecting surfaces are outside are called convex mirrors. 2 The concave mirror can converge parallel beams of light. 2 The convex mirror can diverge parallel beams of light. 6. Our images are inverted on the concave side of the steel spoon. Explain. Our images are inverted in the concave side of the steel spoon because the inner sides of the spoons are concave mirrors. They make real and inverted images. 7. Write the nature of an image formed when an object is 3 cm away from the concave mirror of the focal length of 2 cm. When an object is 3 cm away from the concave mirror of the focal length 2 cm, then the nature of the image will be real, inverted and enlarged. 8. Our image in the plane mirror is called a virtual image. Explain. Our image in the plane mirror is called a virtual image because it is formed without an actual meeting of reflected rays. This image cannot be brought on screen. Exercises 1. Choose the best answer from the given alternatives. a. What is the speed of light in a vacuum? i. 3 × 108 km/s ii. 3 × 108 m/s iii. 2 × 108 m/s iv. 8 × 103 m/s b. What is the perpendicular ray of light to the reflecting surface called? i. angle of incidence ii. normal iii. refracted ray iv. incident ray c. Where should an object be so that an image will be formed at the centre of curvature in a concave mirror? i. at focus ii. at the centre of curvature iii. between f and C iv. beyond C d. Which lens is used in searchlight? i. concave mirror ii. convex mirror iii. plane mirror iv. spherical mirror

200 Oasis School Science and Technology - 8 e. What is the nature of an image formed by a concave mirror when objects are in focus? i. real, inverted and diminished ii. virtual, erect and enlarged iii. real, inverted and enlarged iv. virtual, inverted and diminished 2. Define the following terms with required examples. a. Light b. Ray c. Focus d. Centre of curvature e. Pole of mirror f. Spherical mirror g. Reflection of light m. Regular reflection n. Irregular reflection o. Mirror p. Real image q. Virtual image 3. Very short questions a. What is the point where the reflected parallel rays of light converge? b. What is the midpoint of the aperture of a mirror called? c. What kind of image cannot be obtained on the screen? d. Where should an object be so that a concave mirror can form a virtual image? e. What is the type of beam produced by torchlight? 4. Give reasons. a. We use a plane mirror to see our faces. b. Concave mirror is called a converging mirror. c. Convex mirror is used as side mirrors to see the back side of the road in a vehicle. d. Virtual image cannot be brought in screen. e. Concave mirror is called a converging mirror. f. Convex mirror is called a diverging mirror. g. Dentists use a special concave mirror to see back of the teeth. h. Concave mirror is used as a shaving mirror. i. A concave mirror is used as a reflector in the headlight or torch light. j. Image is formed at infinity when object is kept at focus of a concave mirror. 5. Differentiate between: a. Concave and Convex mirror b. Real image and Virtual image c. Regular reflection and Irregular reflection d. Image and Object

Oasis School Science and Technology - 8 201 6. Short question answers a. Draw a diagram showing the reflection of light on a plane mirror. b. Write any two uses each of concave mirror and convex mirror. c. Draw the diagram of the reflection of light in a plane mirror. d. Write any four uses of the mirror. e. Write the nature of an image formed when an object is 2 cm away from the concave mirror of a focal length of 2 cm. 7. Long question answers a. What is a beam? Enlist and describe its types. b. Complete the given ray diagrams and write down the properties of the image formed. C F P (a) (c) C F P (b) F C c. Draw the ray diagram for a concave mirror when the object is placed i. at F. ii. at C. iii. between F and C. iv. between F and P. Also, write down the properties of the image formed.

202 Oasis School Science and Technology - 8 Key terms and terminologies 1. Sound : A sound is a form of energy which produces the sensation of hearing. 2. Source of sound : The object that produces sound is called the source of the sound. 3. Spectrum of a sound wave : The group of various types of sound waves is called the spectrum of the sound wave. 4. Infrasound : The sound having a frequency less than 20 Hz is called infrasound. 5. Audible sound : The sound having a frequency between 20 Hz and 20 kHz is called audible sound. 6. Ultrasound : The sound having a frequency of more than 20 kHz is called ultrasound. 7. Music : A continuous and uniform sound produced by regular and periodic vibrations that produces a pleasing effect on our ears and mind is called music. 8. Noise : A discontinuous and non-uniform sound produces by irregular and non-periodic disturbances that produces an unpleasant effect on our ears and mind is called noise. UNIT 7.3 SOUND Estimated teaching periods Theory 6 Practical 1  Simple calculations on velocity of sound  Audible sound, infrasound and ultrasound  Measurement of intensity of sound  Causes, effects and control measures of sound pollution The Sequence of Curriculum Issued by CDC 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. About the Scientist Robert Boyle

Oasis School Science and Technology - 8 203 9. Pitch : The shrillness of a sound is called pitch. 10. Loudness : Loudness is the property by which a loud sound can be distinguished from a faint one, both having the same pitch. 11. Timbre : The quality (timbre) is the property by which two sounds of the same pitch and possibly of the same intensity given by two different instruments can be distinguished from each other. 12. Noise pollution : Unwanted harsh and loud sound from any source that causes the discomfort of any kind is called noise pollution. 13. Complete wave : The combination of a compression and a rarefaction of longitudinal wave is called a complete wave of longitudinal wave. 14. Frequency : The number of complete cycles made in one second is called frequency. 15. Time period : The total duration of time required to form one complete cycle is called time period. 16. Wavelength : The distance travelled by the sound while forming one complete wave is called wavelength. 17. Amplitude : The maximum extent of vibration of particles from the mean position of propagation during the propagation of sound energy is called amplitude. Introduction In our everyday life, we hear a variety of sounds around us. At home we hear the sound of family members. We also hear the sound of telephone ring, mobile phone, television, etc. At school, we hear the sound of students, teachers, school bell, etc. All these sounds produce sensation of hearing. It is a form of energy produced by vibrating objects. Thus, sound is a form of energy which produces the sensation of hearing. We can hear the sound coming from a distant place through the material medium. Sound travels through the medium in the form of waves is called sound waves. Actually, there are two types of waves: longitudinal wave and transverse wave. Sound wave is a longitudinal wave. Production of Sound Sound is produced from different sources due to their vibrations. So, we can say sound is produced due to the vibration of a body. The vibration of the source of sound creates the vibration in the surrounding particles which propagates in the form of a longitudinal wave. It is the wave in which particles of the medium (the substance through which sound is travelling) vibrate along the direction of the propagation of sound. Sources of Sound Sound is produced due to the vibration of a body. It requires material medium to travel. So, it cannot travel in vacuum. Sound travels in the form of longitudinal waves. The object

204 Oasis School Science and Technology - 8 that produces sound is called the source of sound, e.g. loudspeaker, temple bell, radio, etc. Terms related to the sound wave 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. Compression is a region in the wave where molecules of the medium are closer. Rarefaction is a region in the wave where molecules of the medium are loose. 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 1 • Make a list of different types of instruments which produce sound. Note the part of the instrument that vibrates during the production of sound.

Oasis School Science and Technology - 8 205 Spectrum of Sound Wave The group of various types of sound waves is called spectrum of sound wave. The frequency of sound wave ranges from 1 Hz to 108 Hz. The frequency of sound wave differs according to the source of sound. The sound waves having the same speed may have different frequencies. The sound of children and girls is shrill due to high frequency. But the sound of boys becomes dull (hoarse) after getting maturity. The frequency of sound of adult men is about 6.5 kHz and that of adult women is about 8.5 kHz. Human ear cannot hear all the sounds having different frequencies. Human ear can hear the sound of frequencies 20 Hz to 20,000 Hz. On the basis of frequency, the sound is classified into three types. They are: a. Infrasound b. Audible sound c. Ultrasound a. Infrasound The sound having a frequency less than 20 Hz is called infrasound. This sound is also called sub-sound or infrasonic sound. It is produced during the earthquake and volcanic eruptions. Similarly, simple pendulum and animals like whale, elephant also produce infrasonic sound. Rhinoceros also produce infrasound of less than 6Hz. This sound cannot be heard by human beings but can be felt by touching. b. Audible sound The sound having a frequency between 20 Hz and 20 kHz is called audible sound. This sound is audible to human ear and hence is called audible sound. The range of frequency from 20 Hz to 20 kHz is known as the frequency range of hearing in human beings. The sound produced while talking, tuning radio, playing guitar, singing song, etc. are some examples of audible sound. The frequency range of hearing in different animals is shown below: Fig. audible sound for different animals 20–20,000 Hz 20–20,000 Hz 15–50,000 Hz 60–65,000 Hz 40–12,000 Hz c. Ultrasound The sound having frequency more than 20 kHz is called ultrasound. It is also called ultrasonic sound. This sound is not heard by human ear. But some animals like whale, bat, mouse, insects and dolphin can produce, detect and use ultrasound. Similarly, cat and dog can also hear ultrasound. Fig. ultrasonography

206 Oasis School Science and Technology - 8 The ultrasound produced by bat is reflected back by other objects in their path, and the echo thus formed is detected by bats. As a result, bat can fly at night without colliding with other objects. Due to very high frequency, i.e. greater than 20000 Hz, ultrasound has a greater penetrating power than ordinary sound. So, it can be used to detect objects under the sea and organs inside human body. These days ultrasound is widely used for various purposes. Some of them as mentioned below: Uses of Ultrasound i. It is used in SONAR to find the depth of oceans, seas, lakes, etc. and to locate shoals of fish, shipwrecks, submarines, icebergs in sea, etc. ii. It is used to find tumours and foreign objects inside the human body. iii. It is used by some animals like dolphin, bat, etc. for locating their body. iv. It is used to kill bacteria and to detect the fault in metals or rocks. v. It is used in the treatment of muscular pain and arthritis. vi. It is used by doctors to determine the sex of embryo in mother's womb and its growth. Reasonable Facts Ultrasound is used in SONAR. Ultrasound is used in SONAR because it has enough energy to reach bottom of ocean and reflect back to the detector. Activity 2 • Take a drum and beat it. Observe the surface of the drum. What do you find? You will notice the vibrations of the surface of the drum. So, sound is produced by the vibration of the body. Speed of Sound in Different Media Sound travels with different velocity in different media. Solids are more elastic in nature than gases and liquids, i.e. Egas < Eliquid < Esolid. The velocity of sound is maximum in solids and minimum in gases. This is because the molecules are packed closer in solids and liquids than in gas. Since molecules carry the vibrations, they do so more effectively when they are close together. One can press his ears against the railway track to find whether a train is approaching or not. This is because we know that the speed of sound in solids is much more than that in gases. The sound produced by the moving wheel of the train travels much faster along Fact File Dolphins uses ultrasound to locate nearby objects.

Oasis School Science and Technology - 8 207 the steel track than through the air. So, the sound is heard through the steel track before it is heard through the air. The speed of sound (v) can be calculated by the given formula : speed of sound (v) = frequency (f) × wavelength (λ) ∴ v = f × λ Worked out Numerical 1 The speed of sound in a medium is 330 m/s and its wave length is 3.3 m. Calculate the frequency of the sound. Solution: Speed of sound (V) = 330 m/s Wave length (λ) = 3.3 m Frequency (f) = ? We know, f = v λ = 330 3.3 = 100 Hz ∴ The frequency of the sound is 100 Hz. Factors Affecting the Speed of Sound in Gases The speed of sound in any gaseous medium is affected by a number of factors. These factors are discussed below: 1. Density (d) The velocity of a sound in a gas is inversely proportional to the square root of density of the gas. For example, the density of oxygen is 16 times greater than that of hydrogen. So, the velocity of sound in hydrogen is 4 times greater than the velocity of sound in oxygen, i.e. 1 density Velocity of sound ∝ 2. Temperature (T) The velocity of sound is directly proportional to the square root of its absolute temperature, i.e. Velocity of sound ∝ T

208 Oasis School Science and Technology - 8 This is due to the reason that with an increase in temperature, the density will decrease and the speed of sound increases. Reasonable Facts We hear more clearly on a hot day than on a cold day. We hear more clearly on a warm day than on a cold day because speed of sound increases with increase in temperature. 3. Humidity The amount of water vapour present in the air is called humidity. The density of the gaseous medium changes due to the water vapour. The water vapour reduces the density of air, i.e. Density of dry air > Density of moist air Therefore, velocity of sound in moist air is higher than that of the velocity of the sound in dry air. That is why sound travels faster on a humid day than on a dry day. 4. Direction of motion of air The velocity of sound is higher in the direction of motion of air than in the opposite direction. Characteristics of Sound All sounds which produce the sensation of hearing may be divided into two categories, i.e. (i) musical sound and (ii) noise. However, the distinction between a musical sound and a noise is subjective, i.e. a sound which is musical for someone may be a noise to others. i) Musical sound A continuous and uniform sound produced by regular and periodic vibrations that produces pleasing effect on our ears and mind is called musical sound, e.g. the sound produced by piano, violin, flute, harmonium, etc. The graphical representation of a musical sound is given below: Amplitude Time Fig. Graphical representation of a musical sound ii) Noise A discontinuous and non-uniform sound produced by irregular and non-periodic disturbances that produce unpleasant effect on our ears and mind is called noise,

Oasis School Science and Technology - 8 209 e.g. the sound produced by horns of vehicles, engines in factories, a falling brick, the sound of a crowd, etc. The graphical representation of a noise is given below: Amplitude Fig. Graphical representation of noise Time Differences between Musical sound and Noise S.N. Musical sound S.N. Noise 1. It produces pleasing effect on ears and mind. 1. It produces unpleasant effect on ears and mind. 2. It produces regular wave form. 2. It produces irregular wave form. 3. The frequency of musical sound is high. 3. The frequency of noise is low. 4. There is no sudden change in amplitude. 4. There is a sudden change in amplitude. Characteristics of sound A musical sound has three basic characteristics, viz. pitch, loudness and quality. i) Pitch: The shrillness of a sound is called pitch. It enables us to differentiate between two sounds with equal loudness coming from different sources and having different frequencies. If the pitch is high, the sound is said to be shrill and if the pitch is low, the sound is flat (hoarse). Pitch is a sensation which depends upon the wavelength or frequency. The pitch has subjective existence while the frequency has an objective existence. The pitch is purely a physical sensation while the frequency is a measurable quantity. The voice of a woman has a higher pitch than that of a man because the frequency of a woman's voice is greater than that of a man's voice. So, the woman's voice is shriller than the man's voice. Pitch of a sound depends on the frequency or wavelength. Thus, the higher the frequency of a sound, the higher is its pitch and vice-versa. The thinner strings of a guitar give out sound of higher pitch than the thicker strings because the thin strings vibrate with higher frequency than the thick strings. Reasonable Facts Boys have hoarse sound than girls. Boys have hoarse sound because their sound has lower frequency than the girls. ii) Loudness (Intensity): Loudness is the property by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch. Loudness depends on the intensity or the amplitude of the wave. The intensity is a measurable quantity while loudness is a sensation. The intensity at a point is defined as the amount of

210 Oasis School Science and Technology - 8 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 Measurement of loudness of sound (Intensity) The loudness of sound is measured in decibels (dB). The zero point on the decibel scale is the lowest sound that can be heard by the human ear. It is also called the threshold of hearing. The loudness of a whisper is 20 dB. The sound of conversation, i.e. 60 dB is about 10,000 times louder than the whisper. The sound wave of loudness more than 120 dB damages human ears. If the sound wave of loudness 40 dB to 65 dB is not heard by human ear it is called hearing impairment and if the sound wave of 80 dB is not heard by human ear, it is called deafness. The sound levels of different sounds is tabulated below: Sounds Sound level in dB 1. Threshold of hearing 0 2. Rustling 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. Environment in library 20-40 14. Listening limit of hurt 120-140 15. Sound produced due to mechanical failure 140-160

Oasis School Science and Technology - 8 211 Worked out Numerical 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 × 10³watts Area (A) = 200 m² Intensity of sound (I) = ? Using formula, I = Power (P) Area (A) = 2 × 103 200 = 10 dB Factors Affecting the Loudness of Sound (Intensity) i. Amplitude of the vibration of the source: The loudness or intensity of sound is directly proportional to the square of the amplitude of the sound waves at a given point, i.e. ∴ Intensity ∝ (amplitude)2 ii. Distance from the vibrating body: Loudness decreases with an increase in the distance between the source and the listener. The loudness of sound is inversely proportional to the square of the distance, i.e. loudness ∝ 1 (distance)2 . iii. Surface area of the vibrating body: The greater the surface area of the vibrating body, the greater is the loudness of the sound. It means that loudness increases with an increase in the area of the vibrating body, e.g. a larger drum produces louder sound than a smaller one. 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. Motion of the medium: If wind is blowing in the direction of propagation of sound, loudness is increased and vice-versa. vi. Presence of resonant bodies: The loudness of sound increases due to the presence of sound reflectors and decreases due to the presence of sound absorbers. vii. Frequency of sound: The loudness of sound is directly proportional to the frequency of sound. Quality The quality is the property by which two sounds of the same pitch and possibly of the same intensity given by two different instruments can be distinguished from each other. It is also called timbre. Fact File The sound wave whose intensity is above 120 decibel is called noise.

212 Oasis School Science and Technology - 8 The quality of musical sound depends on the waveform of the sound. The waves produced by different instruments differ in their waveforms. Reasonable Facts Different people have different sound. Different people have different sound because of the timbre of the sound. Activity 3 Find the large bell and small bell. Ring them both and observe the differences in the sound. Also find out the differences. Noise Pollution Unwanted, harsh and loud sound from any source that causes discomfort of any kind is called noise pollution. It is undesirable and can cause irritability, loss of concentration, stress, sleep disturbance and can even damage hearing. Causes of Sound Pollution (Noise) The increasing use of various kinds of machines at home or workplace or factories is the cause of noise pollution. The main causes of sound pollution are given below: 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 noise pollution i. Noise pollution may result in the loss of hearing to deafness. ii. It reduces concentration, increases stress and causes headache. iii. It increases blood pressure and nervous tension. Ways to reduce noise pollution To minimize noise pollution, we need a certain degree of discipline on the part of all of us. The following are the measures one should adopt to keep noise pollution under control: Fact File Schools and hospitals are no horn area.

Oasis School Science and Technology - 8 213 i. The use of loud speakers at functions, programmes, etc. should be avoided or stopped. ii. People should not talk too loudly or tune the music too loudly. iii. People working in factories, etc. where they are subjected to constant loud noise of machinery should take special precaution to protect their ears. iv. Traffic noise could be reduced to a great extent by instilling traffic discipline among bus and automobile drivers. Pressure horns should not be used in automobiles. v. Explosion of fire crackers should be avoided as far as possible in different festivals. Reasonable Facts We should remove factories far from cities and villages. We should remove factories far from cities and villages so that people won't be affected by noise and other pollutants. Activity 4 Whenever you visit a movie theatre, try to notice various things done to reduce noise. Reasonable Thinking Skill R T S 1. Why is Kathmandu valley declared as a ‘no horn’ area? The Kathmandu valley is declared as a ‘no horn’ area to reduce the noise pollution in the city which is very harmful to citizens. 2. How do we use sound in our daily life? We use sound in our daily life in multiple ways: i. We use it to talk, sing and speak. ii. Certain sounds are used as alarms, sirens, music etc. 3. Sound travels faster through the air after rainfall. Sound travels faster through the air after rainfall because humidity lowers the density of air. 4. Ultrasound is used to detect kidney stones. How does it work? Ultrasound is used to detect kidney stones. When the ultrasound is sent toward the kidney, it strikes stones and reflects back. The reflected ultrasound is collected to form an image in the computer. 5. We should use earplugs if we are to stay in noisy places for a long time. What happens if we do not use earplugs? We should use earplugs if we are to stay in a noisy place for a long time otherwise, we can suffer from temporary or permanent hearing loss, headache, irritation, insomnia and high blood pressure.

214 Oasis School Science and Technology - 8 6. Study the given table and answer the following questions. Medium Speed of sound X 330m/s Y 1450 m/s Z 212 m/s i. Explain the relation between the speed of sound and the density of the gas. The velocity of sound in a gas is inversely proportional to the square root of the density of the gas. ii. Which gas has minimum density among X, Y and Z? Why? Medium (Y) has minimum density because the sound has the highest velocity in it. iii. Which gas has the highest density among X, Y and Z? Ans: Medium (Z) has the highest density because it has the lowest velocity of sound. 7. Sound waves have different frequencies but travel at the same speed in a medium. How? Sound waves have different frequencies but travel at the same speed in a medium because their wavelength will change accordingly. The velocity of sound is given by the equation, Velocity = frequency x wavelength 8. Why is infrasound not used for SONAR? Infrasound is not used for SONAR because it does not have sufficient energy to reflect back. Exercises 1. Choose the best answer from the given alternatives. a. What is the source of the sound? i. ears ii. vibration iii. molecules iv. torch b. Why astronauts cannot hear each other in space without using electronic devices to communicate? i. because they wear a helmet ii. because their suit is soundproof iii. because there is no air medium between them iv. because they do not want to listen c. What is the frequency of sound used for ultrasonography? i. below 20 Hz ii. between 20 Hz to 20 kHz iii. above 20 kHz iv. 20 Hz d. Which equation is true for the velocity of the sound? i. v = f x λ ii. f = v x λ iii. λ = v x f iv. v = u + at e. What is the mixing of unwanted sounds called? i. music ii. timbre iii. noise iv. amplitude

Oasis School Science and Technology - 8 215 2. Define the followings. a. Sound b. Infrasound c. Ultrasound d. Audible sound e. Source of sound f. Sound pollution 3. Very short questions a. What is the range of sound waves? b. What is the range of audible sound waves? c. What is the minimum frequency of the sound wave used for SONAR? d. What is the power carried by the sound wave per unit area perpendicular to that area called? e. Give two examples of infrasound. f. What is the relation between the speed of sound, frequency and wavelength? g. Name any three animals which can hear ultrasound. h. What is the pitch of sound? i. What is the intensity of sound? In which unit is it measured? 4. Give reasons. a. The speed of sound in solids is more than that in liquids. b. The sound of a girl is sharp. c. We cannot hear the sound of the rhinoceros. d. Bats can navigate at night. 5. Differentiate between : a. Infrasound and audible sound b. Infrasound and ultrasound 6. Short question answers a. Write two uses of audible sound. b. Write four uses of ultrasound. c. What are the causes of noise pollution? d. What are the ways to control noise pollution? e. How is the sound produced? 7. Long question answers a. Describe in brief the effect of sound on human life. b. The speed of sound in a medium is 1200m/s. If the wavelength of the sound is 15.5m, calculate the frequency of the sound. Is the sound audible to human ears? [Ans: 77.41 Hz] c. If the wavelength of a sound wave is doubled, what happens to the wave frequency and speed of the wave? d. Write four harmful effects of sound pollution on human health. e. 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] f. The wavelength of a wave is 22m and its frequency is 15Hz. Find its speed. [Ans: 330m/s] g. If a sound of horn has a power of 3 × 10³ watts and can be heard within 300 m² area. Calculate intensity of sound. h. 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?

216 Oasis School Science and Technology - 8 8 ELECTRICITY AND MAGNETISM UNIT

Oasis School Science and Technology - 8 217 UNIT 8.1 MAGNETISM Estimated teaching periods Theory 4 Practical 1  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 The Sequence of Curriculum Issued by CDC 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. About the Scientist William Gilbert Key terms and terminologies 1. Magnet : A magnet is a substance which has the property of attracting iron, cobalt, etc. 2. Natural magnets : Naturally occurring substances like lodestone which attract magnetic substances are called natural magnets. 3. Artificial magnets : Magnets made by human beings using artificial methods are called artificial magnets. 4. Temporary magnets : The magnets that can retain their magnetism only for a short time are called temporary magnets. 5. Permanent magnets : The magnets that can retain their magnetism for a long time are called permanent magnets. 6. Molecular theory of magnetism : Molecular theory of magnetism states that "Each molecule of a magnet or a magnetic substance is an independent magnet which is called a molecular magnet." 7. Magnetic induction : The process by which a magnetic substance develops

218 Oasis School Science and Technology - 8 magnetic properties when it is placed near a magnet is called magnetic induction. 8. Demagnetization : The process by which a magnet loses its properties is called demagnetization. 9. Terrestrial magnetism : The magnetic properties of the earth is called terrestrial magnetism. 10. Dip needle : A dip needle is a magnetic needle which is free to rotate in the vertical plane 11. Angle of dip : The angle between the dip needle and the horizontal line at a certain place is called the angle of dip. 12. Dip circle : The instrument which is used to measure the angle of dip is called a dip circle. 13. Geographical meridian : The vertical plane passing through the true geographic north and geographic south is called the geographical meridian 14. Magnetic meridian : The vertical plane passing through magnetic south and magnetic north is called the magnetic meridian. 15. Angle of declination : The angle between the geographical meridian and magnetic meridian at a place is called the angle of declination. 16. Magnetic compass : 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. Introduction A magnet is a substance which has the property of attracting iron, cobalt, etc. The substances like iron, cobalt, nickel, etc. which are drawn towards the magnet are called magnetic substances and the force that the magnet exerts on magnetic substances is called the magnetic force. The discovery of a magnet is attributed to a shepherd boy, Magnes, who while roaming on Mt. Ida, found that his iron-strapped sandals got stuck to the mountain. He could not walk easily and tried to find the cause of the problem. A wise man found that the mountain was made of black stone with iron in it which had a special property to attract iron. That black stone was an ore of iron called magnetite. The stone was called lodestone. It was used by navigators for finding their way in the sea. In our daily life, magnets are widely used in electric appliances such as loud speakers, electric motors, dynamos, toys, 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. S N Fig.: Bar magnet

Oasis School Science and Technology - 8 219 v. The poles of a magnet cannot be separated. Magnets are of two types: natural magnets and artificial magnets. Natural Magnets Naturally occurring substances like lodestone which attract magnetic substances are called natural magnets. Natural magnets are found in irregular shapes and their magnetism (i.e. attracting force) is not strong enough to be used in modern magnets. Reasonable Facts Natural magnets are not used in devices. Natural magnets are not used in devices because they do not have regular shape and they are weak. Artificial Magnets Magnets made by human beings using artificial methods are called artificial magnets. Artificial magnets are of various shapes and sizes. Some of them are as follows: i. Bar magnet ii. Horse shoe-shaped magnet iii. U-shaped magnet iv. Magnetic compass v. Magnetic needle vi. Cylindrical magnet Some of the artificial magnets are shown below: Bar magnet Horse shoe-shaped magnet Compass U-shaped magnet Fig : Magnets of various shapes Artificial magnets are of two types. They are temporary magnets and permanent magnets. Temporary Magnets The magnets that can retain their magnetism only for a short time are called temporary magnets. These magnets are usually made of soft iron or steel by passing electric current through an insulated wire. They Fact File Natural magnets are usually formed from iron ore due to effect of lightning strikes. Fig: Temporary magnet Battery Switch Coil Steel bar Insulated copper wire Iron nails

220 Oasis School Science and Technology - 8 lose their magnetism when the flow of electric current is stopped. Electromagnet is an example of a temporary magnet. Temporary magnets are used in electric bell, electric motor, electric fan, generator, etc. Permanent Magnets The magnets that can retain their magnetism for a long time are called permanent magnets. These magnets are made of steel, cobalt, iron, nickel, etc. These magnets are used in speaker, transistor, radio, tape recorder, etc. Activity 1 • Bring a bar magnet near a heap of iron dust. What do you observe? You will notice that the magnet draws the iron dust towards it. • Bring some iron filings and spread them on a sheet of paper. Roll a bar magnet on the filings and pick up the magnet. What do you observe? What can you conclude from this activity? Fig. Activity 2 • Bring a bar magnet and tie it with a nylon thread. Suspend it in a stand so that it rotates freely. Observe it for a while till it comes at rest. Does the magnet come at rest by pointing north-south direction? Now, turn the direction of the magnet by your hand and observe the direction in which the magnet resettles. What can you conclude from this activity? 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. Differences between natural magnet and artificial magnet Natural magnet Artificial magnet 1. The magnet which is found in nature is called natural magnet. 1. The magnet made by artificial method is called artificial magnet. 2. It is less powerful than artificial magnet. 2. It is more powerful than natural magnet.

Oasis School Science and Technology - 8 221 Molecular Theory of Magnetism If a magnet is broken into smaller pieces, these smaller pieces also contain the magnetic properties. Depending on this fact, a theory was introduced by a British physicist Sir Alfred Ewing. Molecular theory of magnetism states that, "Each molecule of a magnet or a magnetic substance is an independent magnet which is called molecular magnet." In a magnet, the molecular magnets are arranged in a parallel way of alignment in the same direction. Hence, one end of the magnet has many free ends of north pole of molecular magnets and another end has free south poles. Due to these free ends, a magnet can attract iron, cobalt, etc. In a magnet, molecular magnets are arranged in the form of chains. In a chain, north pole of molecular magnet is connected to the south pole of another molecular magnet and so on. Thus, there due to mutual attraction there is no any free end of the magnetic poles in a magnetic substance. Arrangement of molecular Arrangement of molecular magnets in a magnet magnets in a magnetic substance (a) (b) Fig. Showing molecular theory of magnetism From the molecular theory of magnetism, we can conclude the following points: i. Amagnetic substance cannot attract another magnetic substance since the molecular magnets are arranged in closed chains. ii. Poles of a magnet are strong since they have the free ends of the molecular magnets. In the middle of the magnet, the molecular magnets are arranged one after another. As a result, there is the cancellation of the magnetic property among the molecular magnets and hence the attractive power is weak in the middle of the magnet. This is only due to the effect of end points of the magnet. iii. When a long magnet is broken, the broken pieces also behave like a complete magnet. As the magnet is broken, the broken part has the free ends of the molecular magnets and hence the new poles are developed. Due to the existence of the molecular magnets, we cannot make a monopole or single pole of the magnet. iv. When a magnet is hammered, the magnetic properties are lost. In so doing the molecular magnets are disturbed and they form a closed chain structure. As a result, the magnet loses its property. Fact File Magnetic saturation means every molecular magnets are arranged in close chain in magnetic substance.

222 Oasis School Science and Technology - 8 Reasonable Facts Magnetic substance does not show magnetic properties. In a magnetic substance , the north pole of one molecular magnet is attached to the south pole of another molecular magnet in the form of a closed chain. So, the magnetic substance does not show magnetic properties. Reasonable Facts North and South poles of the magnet has more energy. In a magnet, molecular magnets are arranged in parallel in a certain direction. All the north poles are arranged in one side and all the south poles are arranged in another side which form north pole and south pole respectively. So, north and south poles of the magnet has more energy. Reasonable Facts The poles of a magnet cannot be separated. Each magnetic molecule has south (S) and north (N) poles. A magnet consists of numerous molecular magnets. So, when long magnet is broken, the poles of magnet cannot be separated. Activity 3 • Take a blade and break it into two parts as shown in the figure. • Now, take a bar magnet. Rub the magnet against the piece of the blade so that it becomes a magnet. • Break the piece of blade carefully into two parts so that it won't cut hands. the magnetic property of these pieces. • What can you conclude from this activity? From this activity, it can be concluded that magnetic poles exist in pairs and they cannot be separated. A magnet does not have only one pole. Fig. Magnetic Induction When an demagnetized steel bar is held above the iron nails, it does not attract them. But when a bar magnet is kept near the demagnetized steel bar, the bar gets magnetized. As a result, the steel bar attracts iron nails. It shows that the steel bar develops magnetic properties when it is kept near a magnet. This process is called magnetic induction. The process by which a magnetic substance develops magnetic properties when it is placed near a magnet is called magnetic induction.

Oasis School Science and Technology - 8 223 Fig. magnetic induction Iron nails Unmagnetised steel bar Stand Stand Bar magnet Steel bar behaves as a magnet Iron nails (a) (b) When the magnet kept near the steel bar is taken away, the steel bar loses magnetic properties. As a result, the iron nails sticking to the steel bar fall down. It shows that the steel bar behaves as a magnet so long as it remains inside the magnetic field of a strong bar magnet. Activity 4 • Bring a bar magnet and keep it on a table. Now, attach an iron nail to one end of the magnet. Bring another nail in contact with the previous nail. The nail is attached to the previous nail. In this way, it forms a chain of nails as shown in the figure. What is the reason behind it? Now, take the bar magnet away from the nails. What do you observe? Why do nails separate from each other while taking the magnet away from the nails? What can you conclude from this activity? This activity shows that the magnetism acquired by induction is not permanent. Fig. magnetic induction Demagnetization When a magnet is heated, dropped or hammered, it loses magnetic property. The process by which a magnet loses its properties is called demagnetization. The disturbance in the chain of molecular magnets in a magnet is the main cause of demagnetization. When the molecular magnets in a magnet arrange into ring form, the magnet does not behave as a magnet. A magnet gets demagnetized by either of the given activities: i. By heating a magnet The heat energy increases the kinetic energy of molecules. The open chain will distort and magnetism will decrease. Fact File Permanent magnet lose magnetism over time due to collision, breaking, heat etc.

224 Oasis School Science and Technology - 8 ii. By hammering and dropping a magnet Hammering or dropping forces molecules to break from open chain as a result magnetic strength will be lost. iii. By rubbing the like poles of magnets When the like poles of magnets are rubbed, they repel each other. The open chains in both magnet will be distorted. iv. By storing magnets haphazardly without using keepers v. By passing electricity When alternating current is passed through the magnet, molecular magnets of the magnet will disorient due to changing polarity of AC. As a result magnetism will be lost. vi. Magnetism can be lost naturally over time. Activity 5 Wind a insulated wire around the bar magnet and pass AC source of 12 V. Then check its magnetic field. Is it's magnetic field wider or narrowerthan before passing electricity. Reasonable Facts Magnetic properties are lost when a magnet is heated. When a magnet is heated, the array of order of molecular magnets will be disordered. So, magnetic properties are lost when a magnet is heated. Reasonable Facts When a magnet is hammered, it loses magnetic properties. When a magnet is hammered the array of order of molecular magnets will be disordered. So, magnetic properties are lost when a magnet is hammered. Methods of Conserving Magnetism (Geo magnet) Magnets are highly useful. We should conserve magnetism or magnetic energy. Some methods for conserving magnetism are given below: i. We should not heat the magnet. ii. We should not drop the magnet. iii. We should not hammer the magnet. iv. We should not rub the like poles of magnets. v. We should not store magnets in moist places. Fig. magnets stored in keepers Keeper Bar magnet

Oasis School Science and Technology - 8 225 vi. We should store magnets by using keepers of iron. vii. We should store magnets by joining their unlike poles. Terrestrial Magnetism or Geo-magnet Dr. William Gillbert suggested that the earth itself is a huge magnet. It is called geomagnet. The earth shows the magnetic properties. The earth can be considered as a huge magnet called a terrestrial magnet. The geomagnet of earth has its magnetic north pole at geographical south pole of earth. It's magnetic south pole is at geographical north pole of the earth so it can attract north pole of a magnetic compass. The magnetic poles of earth are at Ellesmere island of Canada and Vostok Station of Antartica. The following are the facts of existence that indicate existence of magnetism in the earth. i. A freely suspended magnet always rests in north and south direction. ii. The iron ore has magnetic property. iii. There is the existence of angle of dip and angle of declination. iv. Neutral points are formed in the magnetic field. Differences between bar magnet and earth magnet Earth as a magnet Bar magnet 1. Earth's magnetism is due to flowing electrons in the molten core of earth. 1. Magnetism of bar magnet is due to open chain of molecular magnets. 2. Earth has a very big magnetic field because earth itself is big. 2. Bar magnet has a small magnetic field because bar magnet are usually small. Effects of terrestrial magnetism or 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 6 • 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.

226 Oasis School Science and Technology - 8 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 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. Fig: schematic diagram of a dip angle Fig: dip circle Uses of angle of dip It is used for measuring and correcting errors made by compass. It also helps in navigation of airplanes. It is used for geological mapping. 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.

Oasis School Science and Technology - 8 227 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 northsouth 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. Reasonable Facts 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. Reasonable Thinking Skill R T S 1. Discuss the activities that destroy the close chain of molecular magnets of an iron bar. The activity that destroys the close chain of molecular magnets of a bar magnet is called magnetic induction. It changes the closed chain into the open chain. 2. Ram found that certain faces of the two magnets repel each other. He is forcefully putting these faces together. What will it do to the magnetic strength of the magnet? Ram found that certain faces of two magnets repel each other. Like poles of a magnet repel. He is forcefully putting like poles together. It will distort the open chain in the magnet. As a result, the magnetic strength of the magnet will decrease gradually. 3. Magnetic field of the earth is weak in the equatorial region compared to the poles. What could be the reasons? The magnetic field of the earth is weak in the equatorial region compared to the poles because the magnetic poles of the earth lie at the geographical north pole and the south Fig: magnetic compass

228 Oasis School Science and Technology - 8 poles. So, the polar regions have more number of magnetic lines of force than the equator. 4. Differentiate between the temporary magnet and the permanent magnet. The differences between the temporary magnet and permanent magnet are: SN Temporary magnet SN Permanent magnet 1 The magnets that can retain their magnetism only for a short time are called temporary magnets. 1 The magnets that can retain their magnetism for a long time are called permanent magnets. 2 It is made from soft magnetic materials. 2 It is made from hard magnetic materials. 5. Ram had put a few iron nails and a magnet in his pocket for a day. He discovered that the nails were also working as a magnet. What is this process called? What happened to the nail at the molecular level? Explain. Ram put a few iron nails and a magnet in his pocket for a day. He discovered that the iron nails were also working as a magnet. This process is called magnetic induction. The iron nails are magnetic substances. They have close chains in them. When they are placed in a strong magnetic field, their closed chain will be broken and rearranged as an open chain. When open chains are formed in them, they act as a magnet. 6. Why should we not heat the magnet? We should not heat the magnet because doing so will distort the open chain and the magnet loses its strength. 7. Sabina broke the magnet to separate the north pole from the south. Will she be successful? Why? Why not? Sabina broke the magnet to separate the north pole from the south but she won’t succeed because the poles of the magnet cannot be separated. The magnets are made of molecular magnets. Each molecular magnet has its own north and south pole. 8. What properties make a lodestone a magnet? The lodestone is a magnet because it has a north pole and the south pole. It can produce magnetic lines of force.

Oasis School Science and Technology - 8 229 Exercises 1. Choose the best answer from the given alternatives. a. Which one of the following is a magnetic substance? i. plastic ii. stone iii. wood iv. iron b. In which direction does a freely suspended magnet rest? i. east to west ii. north to south iii. west to south iv. south to east c. Which of the following is a natural magnet? i. lodestone ii. bar magnet iii. magnetic compass iv. U-shaped magnet d. Which of the following is a temporary magnet? i. magnetic needle ii. electromagnet iii. bar magnet iv. compass e. What is terrestrial magnetism? i. magnetic property of earth ii. magnet inside earth iii. flowing of river iv. load stone 2. Define the following terms with required examples. a. Magnet b. Natural magnet c. Artificial magnet d. Temporary magnet e. Permanent magnet f. Molecular magnet g. Magnetic induction i. Terrestrial magnetism j. Demagnetization 3. Very short questions a. What are the poles present in a magnet? b. What is the process by which a bar magnet loses its properties? c. Give two examples of machines that use the magnet. d. What type of magnet is an electromagnet? e. What is the process by which a close chain of magnetic substances changes into an open chain when it is placed near a powerful magnet? f. What is dip circle used for? 4. Give reasons. a. A freely suspended bar magnet always shows north–south direction. b. A magnet has a stronger force of attraction at the poles than in the middle. c. A magnet loses its properties if it is hammered. d. We should not heat a magnet. e. Lode stone cannot be used in electronic devices. f. Magnetic compass is used to find the direction. g. Magnetic poles cannot be separated.

230 Oasis School Science and Technology - 8 5. Differentiate between : a. Magnetic substances and magnet b. Artificial magnet and natural magnet c. Temporary magnet and permanent magnet d. Magnetic induction and demagnetization e. Magnetic compass and dip circle f. Angle of declination and angle of inclination 6. Short question answers a. Write four uses of the magnet. b. Write two properties of the temporary magnet. c. What are the conditions for demagnetization? d. Write a short note on a natural magnet. e. Write the advantage of the artificial magnet. f. Write the uses of magnetic compass. g. Why is angle of inclination and angle of declination important? 7. Long question answers a. Describe the molecular theory of magnetism. Draw a magnet and a magnetic substance based on the molecular theory of magnetism. b. Describe an activity to show magnetic induction. c. What should we do to preserve the magnetism of the bar magnet? d. What is the evidence of terrestrial magnetism? e. Study the given diagrams and answer the following questions. Fig. (a) Fig. (b) i. Which one is a magnet? Why? ii. What type of material is shown in fig. (b)? iii. Which theory is explained by the above diagrams? f. Describe an activity to prove that magnetic poles cannot be separated.

Oasis School Science and Technology - 8 231 UNIT 8.2 ELECTRICITY Estimated teaching periods Theory 4 Practical 1  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/phase wire, a neutral wire and earthing wire  Methods of wiring a plug  Wiring of electric light circuit The Sequence of Curriculum Issued by CDC 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. About the Scientist Michael Faraday Key terms and terminologies 1. Electricity : Electricity is a form of energy which is produced due to the continuous flow of electrons through a conductor. 2. Source of electricity : The devices from which we obtain electricity are called sources of electricity. 3. Domestic circuit : The AC circuit made in industries, factories, houses, etc. is called a domestic electric circuit. 4. Corporation fuse : The fuse which is connected before the kilowatt hour (kWh) meter is known as corporation fuse. 5. Distribution board : The component of domestic wiring that supplies electricity to different sections of the house is called distribution board. 6. Switch : The electrical part that can open or close the electric circuit is called a switch or key. 7. Sockets : The part of the electrical circuit where we can connect

232 Oasis School Science and Technology - 8 electrical equipment to the power supply is called sockets. 8. Kilowatt hour meter : It is a meter which measures the electric energy consumed by various electric appliances in a kilowatt hour or a unit. 9. Fuse : A fuse is a thin wire made of tin (63%) and lead (37%) having a low melting point and high resistance. 10. MCB : Miniature circuit breaker is a specialised reusable fuse that can safely break electric circuits to prevent electrical hazards. 11. Live wire : The wire through which electric current comes into the load is called live wire. 12. Neutral wire : The wire through which electric current returns from the load is called neutral wire. 13. Earth wire : The third wire which carries the leakage current from a faulty or live appliance to the ground is called earth wire. 14. Electric circuit : An electric circuit is a conducting path made by connecting a source, a conducting wire and a load. 15. Bulb : A bulb is a device which converts electrical energy into light energy. 16. Open circuit : The electric circuit in which electricity cannot flow due to a broken path is called an open circuit. 17. Close circuit : The electric circuit in which electricity can flow is called a closed circuit. Introduction Electricity is a form of energy which is produced due to the continuous flow of electrons through a conductor. Every atom is made up of two types of charged particles. They are protons (positively charged particles) and electrons (negatively charged particles). Metallic conductors such as copper, aluminium, etc. contain a large number of free electrons. When these electrons flow continuously in a certain direction, electric current is produced. Electricity is one of the key sources of energy. It can easily be converted into light energy, heat energy, sound energy, magnetic energy, and so on. The devices from which we obtain electricity are called sources of electricity. The major sources of electricity are cell, photocell, dynamo, generator, etc. Household Electrification The AC circuit made in the industries, factories, houses, etc. is called domestic electric circuit. The electric power generated by the turbine is connected by cables through high transmission line at very high voltage. The voltage is decreased to 220 V by using step down transformer and supplied to the household purposes. To operate any electrical appliance, we need two connecting wires. They are live wire (L), and neutral wire (N). A third wire is also connected in the power circuit which is called earthing wire (E). Earthing wire saves us from electric shock and prevents the flow of excessive current, i.e. overloading through the appliances.

Oasis School Science and Technology - 8 233 The fuse which is connected before the kilowatt hour(kWh) meteris known as corporation fuse. This fuse protects the meter from getting damaged. Other fuses which are connected after the kWh meter are called consumer's fuses. These fuses are generally connected after main switch box. Main switch helps to cut off the current throughout the household circuit. The main switch box is made up of iron and a connection is made from the iron box (main switch box) to the earth by a conductor which is known as earthing. Finally, the wires which are emerging out of the main fuse box are connected to the distribution board as required. Bulb Fan Distribution board Main fuse Main switch box Meter box (kWh meter) Corporation Fuse Consumers' fuse Socket E N L L N E Fig. Domestic electric wiring All the electric appliances are connected with separate circuit in parallel way. So, separate switches are used to control the flow of current. All the current flows through the fuse of the meter box. So, it should be of the value 15 A. It is very important to note here that usually there are separate circuits in a house, the lighting circuit with a 6 A fuse and the power circuit with a 16 A fuse. The lighting circuit is one which allows less amount of current to flow but the power circuit draws more current through it. The lighting circuit is used for running electric bulbs, tube lights, fans, radios, etc. On the other hand, power circuit is used for running electric iron, room heater, refrigerator, etc. Each distributive circuit is provided with a separate fuse so that if a fault like short circuiting occurs in one circuit, its corresponding fuse blows off but the remaining circuits remain unaffected. Reasonable Facts Parallel connection of resistors is used in domestic wiring. Parallel connection of resistors is used in domestic wiring so that loads can be used separately. It also provides constant voltage. Fact File Loads are connected in parallel in domestic wiring.

234 Oasis School Science and Technology - 8 Activity 1 Make a 3D model of schematic diagram of domestic wiring. Some Devices in a Domestic Circuit Corporate fuse or electricity authority's fuse (F1) The corporate fuse is the first fuse used in domestic wiring. The live wire and neutral wire passes through it before connected to the kilowatt hour meter. 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. Bulb Bulb is an electric load because it consumes electricity. Bulb can be filament or fluorescent. The filament bulb wastes lots of electrical energy so we should switch to fluorescent lamp. For example tube light, Compact Fluorescent Lamp (CFL), LED bulb, etc. 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. Sockets The part of electrical circuit where we can connect electrical equipments to the power supply is called sockets. Kilowatt hour meter (M) or Electric meter 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 Fig. kilowatt hour meter Fact File 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.

Oasis School Science and Technology - 8 235 Reasonable Facts 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 2 Make a 3D model of a kilowatt hour meter. Reasonable Facts 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. 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) = ? We have, Electrical energy consumed = P x N x t = 0.04 × 5 × 50 = 10 kWh ∴ 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

236 Oasis School Science and Technology - 8 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 Main switch box (S) The main switch box is used to break or supply electricity to entire house when required. It is also called master switch. 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 house. Fuse and MCB A fuse is a thin wire made of tin (63%) and lead (37%) having low melting point and high resistance. The capacity of a fuse is measured in ampere (A). A fuse breaks the circuit by melting itself when the current exceeds the safe value. Hence, it prevents the electric appliances in the circuit from getting damaged and other electric hazards. Generally, 5A to 30A fuses are used in the domestic wiring. Fact File Fuse must be replaced once it breaks. MCB can be reused.

Oasis School Science and Technology - 8 237 Porcelain fuse Fig. Fuse Fuse wire Cartridge fuse Symbol of fuse 'Miniature Circuit Breaker' (MCB) is the advanced form of a fuse. It is used to protect the household wiring from overloading or short circuit. MCB switches off the circuit within a fraction of a second in case of short circuit or overloading. The MCB can be reset when the fault has been corrected. Reasonable Facts MCB is used in meter box rather than the fuse. MCB is used in meter box rather than fuse because MCB can be reused. If we use fuse and it breaks, it should be replaced. But we are not allowed to open meter box. It is sealed. Reasonable Facts Fuse is used in household wiring system. A fuse breaks the circuit by melting itself when the current exceeds the safe value. Hence, it prevents the electric appliances in the circuit from getting damaged due to overflow of electric current. So, fuse is used in household wiring system. Differences between Fuse and MCB Fuse MCB 1. A fuse is a thin wire made of lead (37%) and tin (63%) which is used in electric circuit. 1. MCB is advanced form of fuse which switches off the circuit within the fraction of second in case of overloading. 2. It needs wire to be exchanged. 2. It does not need wire to be exchanged. Reasonable Facts Electricity is a very useful form of energy. Electricity is one of the key sources of energy. Electrical energy is required to operate large number of electrical appliances like computer, television, radio, calculator, refrigerator, etc. So, electricity is a very useful form of energy. Fig. MCB

238 Oasis School Science and Technology - 8 Introduction to live wire, neutral wire and domestic wire a. The live wire (phase wire) Live wire or phase wire is connected to the high potential of 220 V. It brings current. The wire through which electric current comes into the load is called live wire. If accidentally, we come in direct contact with this wire, we may get an electric shock. Its colour code is red or brown. b. The neutral wire Neutral wire is kept at zero potential by connecting it to the earth. It provides return path for the current. The wire through which electric current return from the load is called neutral wire. 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. Its colour code is black or light blue. c. Earth wire (E) The 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. Its colour code is green and yellow. Activity 3 Collect wires and observe their colours. Identify phase wire among them. Reasonable Facts Wires in domestic wiring have a colour code. Wires in domestic wiring have a colour code because it helps to identify wire and it will be easy for wiring, repairing etc. Electric Circuit An electric circuit is a path made by connecting a cell, a conducting wire and a bulb. It is a continuous path through which current flows. The things required for making an electric circuit are described below: a. Cell or Battery: It acts as the source of electricity. b. Conducting wire: It is the medium through which electric current flows. c. Bulb: A bulb is a device which converts electrical energy Fact File Earth wire is green or yellow or green with yellow stripes. Fig. electric circuit A V

Oasis School Science and Technology - 8 239 into heat energy and light energy. A bulb is also called a load. It shows the effect of electric current. d. Switch: It is a device which is used to open or close an electric circuit. Open circuit and Closed circuit When the switch is turned ‘OFF’, the bulb does not glow. Such electric circuit is called open circuit. The electric circuit in which electricity cannot flow due to broken path is called open circuit. In such condition, the current cannot flow continuously through the wire. So a load cannot function when the circuit is opened. When the switch is turned ‘ON’, the bulb glows. Such electric circuit is called closed circuit. The electric circuit in which electricity can flow is called closed circuit. In such condition, electricity flows continuously through the circuit. As a result, the bulb glows. In our home, school or office, electric circuits are closed to light the bulbs or operate other appliances. Electric circuits are closed or opened with the help of switches fixed on the switch boards. 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. Bulbs are connected in series in festival lights, disco lights etc. Bulb could be filament bulb, fluorescent lamp 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 touch 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. Symbols Used in Circuit Diagrams When we draw circuit diagrams, it is very difficult to draw various components used in the electric circuit. For the sake of convenience, various components used in an electric circuit are represented by their symbols. Symbols of various components which are often Copper wire Switch off Dry cell Bulb Fig: Open circuit Fig. Closed circuit Copper wire Switch Dry cell Bulb Fig: Circuit of bulbs and tube-lights

240 Oasis School Science and Technology - 8 used in an electric circuit are given below: S. No. Components Symbol Function 1. Cell + Source of electric current 2. Battery + – Source of electric current 3. Switch To put the current on or off in the circuit 4. Bulb To produce light 5. Wire To make a conducting path 6. Ammeter A + – To measure the current 7. Voltmeter V + – To measure the voltage 8. Galvanometer G To detect the current 9. Load or Resistance To convert electrical energy into other forms of energy like heat and light Connection of wires in the plug 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 strips 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 Physical Quantity SI unit Current (I) ampere (A) Power (P) watt (W) Voltage (V) volt (V) Remember

Oasis School Science and Technology - 8 241 Activity 4 • Take a dry cell, electric bulb, switch, bulb holder and copper wire. Connect them in the supervision of the teacher. • Study the characteristics of open circuit and closed circuit. • Does the bulb glow when the switch is turned ‘off’, why? • Write down the conclusion of this activity. Reasonable Thinking Skill R T S 1. Separate fuses are used for each flat in the house in domestic wiring. What advantages does it have? Separate fuses are used for each flat in a house in domestic wiring. Its advantage s in case of a short circuit, only one flat will lose electricity and other flats can use electricity. 2. Switches are kept in phase wire in the house wiring system. Mention its benefit. Switches are kept in a phase wire in the house wiring system so that they can break the electric circuit successfully. It helps to prevent electricity from flowing into electrical devices when the switch is off. 3. As we can see there are wires of different colours used in domestic wiring. What is its purpose? Wires of various colours are used in domestic wiring. Its purpose is to help in the identification of wires so that it will be easy for wiring and repair. 4. Why is MCB better than fuse? MCBs are better than fuse because they are more sensitive and reusable whereas fuse is not as sensitive as MCB and fuse should be replaced when it breaks. 5. Mention two differences between open circuit and close circuit. The differences between open circuit and close circuit are: SN Open circuit SN Close circuit 1 The electric circuit in which electricity cannot flow due to a broken path is called an open circuit. 1 The electric circuit in which electricity can flow is called a closed circuit. 2 The key is off in open circuit. 2 The key is on in close circuit. 6. Why is the fuse made of lead and tin? The fuse is made of lead and tin because the alloy of lead and tin has very high resistance and low melting point. So it heats, melts and breaks during the overflow of current. 7. Kilowatt hour meter box is sealed by the electricity corporation. We will be fined if we break it. Why do they seal it? Kilowatt hour meter box is sealed by the electricity corporation so that we can't steal electricity.

242 Oasis School Science and Technology - 8 8. Most plugs of heavy appliances have 3 pins. What is the objective of the longest pin? Most plugs of heavy appliances have 3 pins. The objective of the longest pin is to collect leakage current and send them to the earth. It prevents electric shock to us. It also prevents damage to the device. Exercises 1. Choose the best answer from the given alternatives. a. Which of the following brings electricity into the house? i. live wire ii. fuse wire iii. neutral wire iv. earthing wire b. Which of the following is used to break an electric circuit during an emergency? i. kilowatt hour meter ii. distribution board iii. sockets iv. Miniature circuit breaker c. What is a fuse wire made of? i. lead and glass ii. lead and carbon iii. lead and tin iv. tin and copper d. Which wire helps to dispose of leakage current? i. live wire ii. fuse wire iii. neutral wire iv. earthing wire e. What is the device that is used to break or connect the electric circuit as per need called? i. bulb ii. key iii. source v. fuse 2. Define the following terms with required examples. a. Electricity b. Source of electricity c. Electric circuit d. Corporation fuse e. Switch f. Sockets g. Kilowatt hour meter h. Fuse i. MCB j. Live wire k. Open circuit l. Close circuit m. Earthing 3. Very short questions a. What is the energy produced from the continuous flow of electrons called? b. What type of current flows in the domestic circuit? c. Which device measures electricity consumed in our house? d. What is the colour of phase wire? e. What is the conducting path of electricity called?

Oasis School Science and Technology - 8 243 4. Give reasons. a. Kilowatt hour meter is used in domestic wiring. b. An earth wire is connected from the meter to the ground. c. Switch is kept in the live wire. d. MCB is better than the fuse. e. Bulb does not glow in an open circuit. f. Load works in a close circuit. g. Fuse is called a safety device. 5. Differentiate between : a. Load and source b. Live wire and neutral wire c. Fuse and MCB d. Open circuit and close circuit e. Plug and socket 6. Short question answers a. Describe the colour coding of wire in domestic wiring. b. Draw the diagram of domestic wiring. c. Write the uses of the main switch box? d. Write a short note on a live wire and neutral wire. e. Draw the diagram of an open electric circuit which contains a source, a bulb and a switch. 7. Long question answers a. Describe domestic wiring. b. How is wire connected in an electric plug? Describe with diagram. c. How does a fuse work? Describe. d. Describe the types of wire used in domestic wiring in brief. 8. Numerical a. The bulb with the power of 176 watts is used 8 hours daily, calculate the total bill for 30 days, if per unit costs Rs. 7. [Ans: Rs. 295.68] b. 10 tube lights of 20 W each are used in a house for 5 hours daily, calculate the bill for one month for the electricity at the rate of Rs. 7.30 per unit. [Ans: Rs. 4158] c. In a hostel, 20 LED bulbs of 15W are used 5 hours daily, 10 electric heaters each of 2 kW are used continuously for 180 minutes a day, 20 water heaters of 1500 W are used for 5 minutes every day, and 20 irons of 800 W are used for 30 minutes every week. What will be the consumption of electricity in a month? What change in the total consumption will be when all heaters of 2 kW are replaced by 1 kW heaters? If the cost of electricity is Rs. 12 per unit, how much money will be saved in a month? [Ans: 1952 units, 10,800 Rs.]

244 Oasis School Science and Technology - 8 UNIT 9 MATTER Estimated teaching periods Theory 12 Practical 3  Location of proton, neutron and electron in an atom  Atomic structure and electronic configuration based on shells  Introduction to valance orbit, valance electrons and valency  Valency of first 20 elements  Introduction of the periodic table  Modern periodic table  Periodic variation of elements (number of orbits, valency, atomic size and metallic properties)  Introduction to molecular formula, methods to write the molecular formula, molecular formula of a few compounds.  Introduction and calculation of atomic weight and molecular weight  Introduction to chemical equation, word equation and balanced formula equation The Sequence of Curriculum Issued by CDC Henry Moseley is popular for the chemical concept of the atomic number. He was born in England on 23rd November 1887 and died on 10th August 1915. He was a physicist and chemist. Mosley’s law advanced atomic physics, nuclear physics and quantum physics by providing experimental evidence in favour of Bohr’s theory. He was killed in world war I otherwise he was to receive Nobel Prize in Physics in 1916. About the Scientist Henry Gwyn Jeffreys Moseley Key terms and terminologies 1. Matter : Anything which has mass and volume is called matter. 2. Compound : A compound is a substance made up of two or more elements (atoms) in a fixed proportion by weight. 3. Elements : Elements are pure substances made up of only one type of atoms. 4. Atom : An atom is the smallest particle of an element which can take part in a chemical reaction. 5. Molecule : A molecule is the smallest particle of an element or a compound

Oasis School Science and Technology - 8 245 which can exist freely. 6. Protons : Protons are positively charged sub-atomic particles located in the nucleus of an atom. 7. Neutrons : Neutrons are charge-less sub-atomic particles located in the nucleus of an atom. 8. Electrons : Electrons are negatively charged sub-atomic particles that revolve around the nucleus in elliptical orbits called shells. 9. 1 amu : 1 amu is defined as 1 12 th part of a carbon-12 atom. 10. Atomic number : An atomic number of an atom is defined as the number of protons present in the nucleus of that atom. 11. Atomic mass : The atomic mass or atomic weight of an atom is the sum of the total number of protons and neutrons. 12. Electronic configuration : The systematic distribution of electrons in various orbits or energy levels around the nucleus of an atom is called electronic configuration. 13. 2n² rule : The rule that determines the maximum number of electrons that can be accommodated in different shells of an atom is called the 2n² rule. 14. Symbol : A symbol is the abbreviation of the full name of an element which is represented by one or two English letters. 15. Radicals : Radicals are charged atoms or group of atoms having a common charge which act as a single unit during a chemical reaction. 16. Electropositive radicals : The atoms or group of atoms which have a positive charge in them are called electropositive radicals or basic radicals. 17. Electronegative radicals : The atom or group of atoms which have a negative charge in them are called electronegative radicals or acidic radicals. 18. Valance shell : The outermost shell of an atom from where the loss or gain of electrons takes place is called the valence shell. 19. Valance electrons : The total number of electrons present in the valence shell (outer shell) are called valence electrons. 20. Valency : The combining capacity of an element or a radical with another element or radical to form a compound or molecule is called valency. 21. Duplet state : The arrangement of two electrons in the K-shell of an atom is called a duplet state. 22. Duplet rule : The tendency of elements by which they try to maintain two electrons in their valence shell (last shell) either by transferring or sharing electrons is called the duplet rule. 23. Octet state : The state of having eight electrons in the valence shell (last shell) of an atom is called the octet state. 24. Octet rule : The tendency of elements by which they try to maintain eight electrons in their valence shell (last shell) either by transferring or sharing electrons is called the octet rule. 25. Molecular formula : The molecular formula of a molecule is the symbolic representation of the molecule of an element or a compound in molecular form.

246 Oasis School Science and Technology - 8 26. Molecular weight : The sum of atomic weights of the atoms present in a molecule is called molecular weight. 27. Mendeleev’s periodic law: According to Mendeleev's periodic law, "The physical and chemical properties of elements are a periodic function of their atomic weights." 28. Mendeleev's periodic table : The table which is obtained after arranging elements based on increasing atomic mass is called Mendeleev's periodic table. 29. Modern periodic law : Modern periodic law states that "Physical and chemical properties of the elements are a periodic function of their atomic numbers." 30. Modern periodic table : The table which is obtained after arranging elements based on increasing atomic numbers is called the modern periodic table. 31. Period : The horizontal rows in the periodic table are called periods. 32. Groups : The vertical columns in the periodic table are called groups. 33. Chemical reaction : The combination, decomposition or displacement that occurs in the molecules of matter during a chemical change is called a chemical reaction. 34. Word equation : The chemical reaction expressed by writing the full names of reactants and products is called a word equation. 35. Chemical equation : The chemical reaction expressed by writing symbols and molecular formulae of reactants and products is called a chemical equation. 36. Reactants : The chemical substances which take part in a chemical reaction are called reactants. 37. Products : The chemical substances which are produced after a chemical reaction are called products. 38. Balanced chemical equations : The chemical equation written by balancing the total number of atoms of each element in reactants and products is called a balanced chemical equation. 39. Unbalanced chemical equation: The chemical equation in which the total number of atoms of each element in reactants and products are not equal is called an unbalanced or skeleton chemical equation. Introduction A variety of substances having mass and volume are found in our surroundings. These substances are called matter. Examples: Brick, stone, air, soil, water, plastic, etc. Anything which has mass and volume is called matter. Matter can exist in three different physical forms, viz. solid, liquid and gas. Among a variety of matter, some are simple and others are complex. We can decompose complex matter into simple form. The substances that can be broken down into simple substances are called compounds. Sodium chloride (NaCl), water (H2 O), limestone (CaCO3 ), carbon dioxide (CO2 ), etc. are some examples of compounds. A compound is a substance made up of two or more elements (atoms) in a fixed proportion by weight. Compounds are formed by chemical reaction between any two or more elements. The smallest particle of a compound is called molecule. The pure substances that cannot be broken down into two or more simpler substances are