CHAPTER 1 ef BY UNI DWM20032

Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Energy

• Energy is defined as the capability to do work.

• Energy has different forms.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Energy

• ‘Electrical energy is one form, because electricity is capable of

performing work.

• One of the first types of energy was mechanical energy. Mechanical
energy exists in two forms:

1. Potential Energy
2. Kinetic Energy

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Potential Energy

• Potential energy
is the energy that
a body has virtue
of its position.

• It took a certain
amount of work
to get the box on
the table.

• A vertical force
had to be exerted
through a
distance to
accomplish this.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Kinetic Energy

• Kinetic energy is
the energy of
motion.

• An object that has
motion - whether
it is vertical or
horizontal motion -
has kinetic energy.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Kinetic Energy

• There are many forms of kinetic energy vibrational (the energy due
to vibrational motion), rotational (the energy due to rotational

motion), and translational (the energy due to motion from one

location to another).

Both kinetic and potential energy are forms of energy that represents the

capability to do work.

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ENERGY & POWER IN DC CIRCUITS

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ENERGY & POWER IN DC CIRCUITS

ELECTRICAL WORK

•Electrical work is done if a quantity of

charge (coulombs) is moved between two
points which are at different electrical
potentials.
•The SI unit of work is the ‘joule’.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ENERGY & POWER IN DC CIRCUITS

ELECTRICAL WORK

One joule of work is done when a charge of one coulomb moves through a potential

difference of one volt.

Work (joule) = Charge (coulomb) × Potential Difference (volt)

Work = Q × V joules

Since one coulomb is one ampere second
Q =I ×t

Then, Work = V × It joules

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ELECTRICAL ENERGY

• Electrical energy is the ability of an electrical system to do work.

• When electrical energy is converted into work, some of it may be lost on the form
of heat.

• The electrical energy and the amount of work done are equivalent.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ELECTRICAL ENERGY

• The heat losses are neglected.
• Electrical energy expressed in watt-seconds (Ws) or watt-hours (Wh) is found by

multiplying the voltage times the current times the time

• The units of energy and work are the same, that is joules and the same equation
is used for both.

Energy = Work = VIt joules

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ELECTRICAL ENERGY

ELECTRICAL POWER
• Electrical power (symbol P) is the rate at which work is done or the rate of

conversion of energy by an electrical system.

∴ Power (watts) = Work done (joules) = VIt
Time taken (seconds) t

• The SI unit of power is the watt which is a rate of work of 1 joule per second.

Therefore P = V × I
That is watts = volts × amps

• By substituting V = IR in the above formula, two other expressions for electrical power are
obtained:

P = VI = I2R = V2 watts
R

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Power Rating

• Power is the rate at which energy is being
used.

• When power is used there is a certain
amount of time involved.

• The amount of energy in an electric Power = Energy / Time

circuit is obtained by multiplying the voltage
times the current times the time.

• The amount of electric power is equal to the
amount of energy, per unit of time. the word
‘per’ means to divide.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

cont…Power Rating

• Electrical components are often given a power rating.

• The power rating, in watts, indicates the rate at which the

device converts electrical energy into another form of

energy, such as light, heat, or motion.

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cont…Power Rating

• An example of such a rating is noted
when comparing a 150-watt lamp to a
100-watt lamp.

• The higher wattage rating of the 150-
watt lamp indicates it is capable of
converting more
electrical energy into light energy than
the lamp of the lower rating.

• Other common examples of devices
with power ratings are soldering irons
and small electric motors.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

CAPACITOR

 A capacitor is a passive electrical component that stores
electrical charge and has the property of capacitance.

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CAPACITANCE & CAPACITORS

CAPACITOR

 A capacitor is consisting of a pair of conductors separated
by a dielectric (insulator).

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

CAPACITOR

 When there is a potential difference (voltage) across the
conductors, a static electric field develops in the dielectric
that stores energy and produces a mechanical force
between the conductors.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

CAPACITOR

 A capacitor stores energy in an electric field that is
established by the opposite charges stored on the two
plate.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

BASIC CAPACITOR

• Two metal plates close together, but separated by an insulator or
dielectric (which could be air)

• When voltage are apply across them, electrons are removed from one
plate and applied to the other and each becomes charged.

• The charge held by the combination may be very large because of the
concentration of the electric field between the plates.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

BASIC CAPACITOR

If the voltage between the plates is increase, the
charge increases, but the ratio of charge to
voltage remains the same. This ratio gives the
capacitance (C) of the capacitor.

Charge (Q) / Voltan (V)= A constant called capacitance(C)

When the charge (Q) is in coulombs and the
voltage (V) in volts, then the capacitance (C) is in
farads (F).

(and also Q = VC, V = Q/C )

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

CAPACITOR

• A capacitor has a capacitance of one Farad when a charging
current of one ampere, flowing for one second, causes a change
of voltage of one volt between its plates.

• The Farad is a huge unit and smaller units are used in practice.

i) 1 microfarad (µF) = 10-6 farad
ii) 1 picofarad (pF) = 10-12 farad

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

FACTORS AFFECTING CAPACITANCE

1) Plate Area

Greater plate area gives greater capacitance; less plate area gives less
capacitance.

Less capacitance More capacitance

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cont..FACTORS AFFECTING CAPACITANCE

2) Plate Spacing

Further plate spacing gives less capacitance; closer plate spacing gives greater
capacitance.

Less capacitance More capacitance

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cont…FACTORS AFFECTING CAPACITANCE

3) Dielectric Material

 Greater permittivity of the dielectric gives greater capacitance; less permittivity of the
dielectric gives less capacitance.

(Relative permittivity = 1.0006) (Relative permittivity = 7.0)

Less capacitance More capacitance

 “Relative permittivity” means the permittivity of a material, relative to that of a pure

vacuum. (the ability of a substance to store electrical energy in an electric field)

 The greater the number, the greater the permittivity of the material. Glass, for instance,
with a relative permittivity of 7, has seven times the permittivity of a pure vacuum, and
consequently will allow for the establishment of an electric field flux seven times stronger
than that of a vacuum, all other factors being equal.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

• A capacitor can be made variable rather than fixed in value by varying
any of the physical factors determining capacitance.

• One relatively easy factor to vary in capacitor construction is that of
plate area, or more properly, the amount of plate overlap.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

• An approximation of capacitance for any pair of separated conductors
can be found with this formula:

C = ε A/d

Where,
C = Capacitance in Farads
ε = Permittivity of dielectric (absolute, not relative)
A = Area of plate overlap in square meters
d = Distance between plates in meters

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITANCE & CAPACITORS

• In the case of multi-plate capacitors,
capacitance is calculated using the
formula:

C = (n -1) ε A

d

• Where n is the number of
plates and A is the area of
a single plate.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

CAPACITOR CONSTRUCTION

1) PAPER CAPACITOR

• The cheapest and most common type used for routine circuits.

• The construction consists of two aluminium foil strips
separated by waxed paper acting as the dielectric.

• The foil strips and paper dielectric are rolled tightly into a
tube, which is then sealed inside an outer container made of
plastic, wood or metal.

• The paper capacitor is usually made in the value range of
250pF to 15uF with a working voltage of up to 500V DC.

• For special applications, they can be constructed with much
higher working voltages, up to 150kV DC.

• These are very expensive and at these voltages, must be
encased in an oil-filled metal case.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

2) MICA CAPACITOR

• 2 types of mica capacitors are stacked-foil and silver mica.

• The basic construction of the stacked-foil type consists of
alternate layers of metal foil and thin sheets of mica.

• The metal foils forms the plate, with alternate foil sheets
connected together to increase the plate area.

• More layers are used to increase the plate area, thus
increasing the capacitance.

• The mica/foil stack is encapsulated in an insulating material.

• The silver-mica capacitor is formed in a similar way by
stacking mica sheets with silver electrode material screened
on them.

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• Mica capacitors are generally available with
capacitance values ranging from 1pF to 0.1 µF and
voltage ratings from 100 V dc to 2500V dc and higher.

• Common temperature coefficient range from -20
ppm/oC to +100 ppm/oC.

• Mica has typical dielectric constant of 5.

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3) GLASS CAPACITORS

• Glass capacitors are also used in high quality applications.

• They usually constructed with a capacitance range of 1pF to 0.05uf
and have a greater capacitance to volume ratio than other
construction methods.

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4) CERAMIC CAPACITORS

• Ceramic dielectrics provide very high dielectric constant (1200 is typical).

• As a result, comparatively high capacitances values can be achieved in a
small physical size.

• Commonly available in a ceramic disk form, in a multilayer radial-lead
configuration or in a leadless ceramic chip.

• Ceramic capacitors are typically available with capacitance values ranging
from 1pF to 2.2 µF and voltage ratings up to 6 kV. Typical temperature
coefficient is 200,000 ppm /o C.

•

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

5) ELECTROLYTIC CAPACITORS

• Electrolytic capacitors are polarized so
that one plate is positive and the other
negative.

• These capacitors are generally used for
high capacitance values from 1 µF up to
over 200,000 µF.

• They have relatively low breakdown
voltages (350 V is a typical maximum but
higher voltages are occasionally found)
and high amount leakage.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ELECTROLYTIC CAPACITORS

• Electrolytic capacitors offer much higher
capacitance values than mica or ceramic
capacitors, but their voltage rating are typically
lower.

• While other capacitors use two similar plates, the
electrolytic capacitor consists of one plate made
of aluminum foil and another plate made of a
conducting electrolyte applied to a material such
as plastic film.

• These two “plates” are separated by a layer of
aluminum oxide that forms on the surface of the
aluminum plate.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

ELECTROLYTIC CAPACITORS

• Because of the process used for the insulating
pentoxide dielectric, the metallic (aluminum or
tantalum) plate is always positive with respect to
the electrolyte plate, and thus all electrolytic
capacitor are polarized.

• The metal plate (positive lead is usually
indicated by a plus sign or some other obvious
marking and must always be connected in a dc
circuit where the voltage across the capacitor
does not change polarity. Reversal of the polarity
of the voltage can result in complete destruction
of the capacitor.

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6) VARIABLE CAPACITORS

• Variable capacitors are used in a circuit
when there is a need to adjust the
capacitance value either manually or
automatically.

• These capacitors are generally less than
300 pF but are available in larger values
for specialized application.

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VARIABLE CAPACITORS

• Adjustable capacitors that normally have slotted
screw-type adjustments and are used for very fine
adjustments in a circuit are called trimmers.

• Ceramic or mica is a common dielectric in these
types of capacitors, and the capacitance usually is
changed by adjustmenting the plate separation.

• Generally, trimmer capacitors have values less than
100 pF.

• The varactor is a semiconductive device that exhibits
a capacitance characteristic that is varied by
changing the voltage across its terminals. This
device usually is covered in detail in a course on
electronic devices.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Capacitor Charging

As soon as the switch is closed in position 1 the
battery is connected across the capacitor, current
flows and the potential difference across the
capacitor begins to rise but, as more and more
charge builds up on the capacitor plates, the
current and the rate of rise of potential difference
both fall.

Finally no further current will flow when the p.d.
across the capacitor equals that of the supply
voltage Vo. The capacitor is then fully charged.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Capacitor Discharged

• As soon as the switch is put in position 2 a 'large' current
starts to flow and the potential difference across the
capacitor drops.

• As charge flows from one plate to the other through the
resistor the charge is neutralized and so the current falls
and the rate of decrease of potential difference also falls.

Eventually the charge on the plates is zero and the current
and potential difference are also zero - the capacitor is
fully discharged.

• Note that the value of the resistor does not affect the
final potential difference across the capacitor – only the
time that it takes to reach that value.
The bigger the resistor the longer the time taken.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Resistive-Capacitive Series Circuit Time Constant

• As a capacitor become charged in a dc circuit, the current flow
decrease as the voltage developed by the capacitor increases
over time and opposes the source voltage. Therefore the rate
of charge of a capacitor reduce over time.

• The amount of time taken to charge and discharge a capacitor
is very important factor in the design of electronics circuits.
Resistor/capacitor combinations are often used to control the
rate of charge and discharge of an intended application.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Resistive-Capacitive Series Circuit Time Constant

• As a capacitor become charged in a dc circuit, the current flow
decrease as the voltage developed by the capacitor increases
over time and opposes the source voltage. Therefore the rate
of charge of a capacitor reduce over time.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Resistive-Capacitive Series Circuit Time Constant

• Resistance directly affects the time required to charge a
capacitor in other words as the circuit resistance increases, it
takes more time to charge a capacitor.

• The amount of time of the capacitor in a resistor-capacitor (RC)
circuit to become fully charged depends on the values of the
capacitor and resistor.

• However the rate of change is not constant nor is it linear.

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Topic contents refer as in AN1101/Appendix 1/Issues 1 (01 January 2011)

Resistive-Capacitive Series Circuit Time Constant

• The graph shows rate of charge of a capacitor in a
RC circuit.

• The rate of charge greatly decreases over time and
that the latter stages of its charging time is many
times longer than when the voltage is initially
applied.

• A capacitor reaches 63% of it charge in
approximately one fifth of the time it takes to
become fully charged. Because of this, capacitors
in actual applications are not necessarily charged.

• The time taken for a capacitor to charge to 63% of
its full capacity is known as its resistive-capacitive,
i.e RC Time Constant

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