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8 Making use of electricity
8
Making use of electricity
8 . 1 Conditions necessary for current flow
1
For a current to flow, a complete path and a source of electrical energy are necessary.
2
A complete circuit is called a closed circuit while an incomplete circuit is called an open
circuit.
8 . 2 Electrical conductors and insulators
1
Materials that conduct current are called electrical conductors. Metals, salt solutions and
graphite are examples.
2
Materials that do not conduct current are called electrical insulators. Wood, cotton and
plastics are examples of electrical insulators.
3
A switch is a device for opening or closing an electric circuit.
8 . 3 Current, voltage and resistance
1
Matter is made up of atoms.
electron
(carries negative charge)
nucleus
(carries positive charge)
 Structure of an atom
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Electrical conductors have a large number of free electrons. Electrical insulators have very
few free electrons.
3
In a closed circuit with an energy source, there is a net flow of free electrons through the
circuit from the negative terminal to the positive terminal.
4
The direction of current is opposite to that of electron flow, i.e. from the positive terminal of
the cell to the negative terminal through the circuit.
current
 Direction of electron flow
5
 Direction of current
Analogy of an electric circuit with a water flow model:
water pump
water
battery
current
water pipe
bulb
wire
fan
6
Parts in the water flow model
pump
water flow
fan
water pipe
Parts in an electric circuit
battery
current
bulb
wire
The voltage of a cell is a measure of the amount of energy supplied by the cell to the free
electrons in a circuit.
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Measuring current and voltage:
Current
Voltage
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Unit
Symbol
Ampere
A
Milliampere
mA (1 mA =
Volt
1
A)
1000
V
Ammeter
Voltmeter
In circuit A, the more the cells, the larger the current in the circuit and the brighter the bulb.
black knob
black knob
ammeter
voltmeter
red knob
 Circuit A
9
Measured with
red knob
 Circuit B
In circuit B, the more the cells, the higher the voltage across the cells and the larger the
current flowing through the circuit.
10 Resistance is a measure of the opposition of a material to the flow of current. Its unit is the
ohm ().
11 Resistor:


has a certain value of resistance.
the higher the resistance of the resistor connected to a circuit, the smaller the current
flowing through the resistor.
 Resistors
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12 Factors affecting the resistance of a wire:
Factor
Effect on the resistance of a wire
Material
A copper wire has a lower resistance than a nichrome wire of the same
length and thickness.
Length
For wires made of the same material, a longer wire has a higher
resistance than a shorter wire of the same thickness.
Thickness
For wires made of the same material, a thinner wire has a higher
resistance than a thicker wire of the same length.
13 A rheostat is a resistor that has an adjustable resistance. It can be used to adjust the size of
the current in a circuit.
current out
current in
 A sliding rheostat
current out
current in
 A rotary rheostat
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8 Making use of electricity
8 . 4 Circuit symbols and diagrams
1
The circuit symbols of some common circuit components:
Circuit
component
2
Circuit
component
Circuit symbol
Battery
Resistor
Connecting wire
/ connector
Rheostat
Switch
Ammeter
Bulb
Voltmeter
Circuit symbol
An example of a circuit diagram:
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8.5
Series and parallel circuits
Comparing the series circuit and the parallel circuit:
Series circuit
Parallel circuit
A1 = A2 = A3
A1 +A2 = A3
Current flows in one path only.
Current can flow in more than one path.
Current is the same at different points in the
The current in the main loop is equal to the
circuit.
sum of the currents in the branches.
Current will not flow in the circuit once it is
broken at any point.
An open circuit in a branch does not affect
other branches.
8 . 6 The heating effect of current and its potential
hazards
1
Heat energy is given out when a current passes through a conductor. The heating effect
becomes greater as the current increases.
2
Applications of the heating effect of current:

Electric kettles, heaters and hair dryers
The resistive wires give out a large amount of
heat when a current passes through them.
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
Incandescent bulbs
The filament becomes very hot and glows when
a current passes through it.
3
The fuse and the circuit breaker are the safety devices for preventing the current in a circuit
from becoming too large.

The fuse
 It consists of a metal wire with a low melting point.
 The fuse rating shows the maximum current the fuse can withstand without
melting.

In electrical appliances, the fuse installed should have a rating slightly higher than
the current passing through the appliances under normal operation.
or
 Circuit symbol for a fuse

8.7
1
 Fuses of different fuse ratings
The circuit breaker
 It automatically ‘switches off’ if the current in a circuit exceeds its rating.
 We can reset the circuit breaker by switching it back on.
Household electricity
In Hong Kong, the mains voltage is 220 V.
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The parts in a three-pin plug and a mains socket:
earth hole
earth pin
switch
neutral pin
live pin
live hole
neutral hole
 A three-pin plug
3
 A mains socket
Internal parts of a three-pin plug:
earth wire
fuse
(green and yellow)
(installed in the live wire)
neutral wire
live wire
(blue)
(brown)
4
The earth wire connects the metal case of an appliance to the earth.
The
importance of
earthing:
If a damaged live wire touches the metal case of the appliance, the
earth wire completes the circuit.
A large current flows to the earth and blows the fuse in the live wire.
This cuts off the current from the mains
and protects us from an electric shock.
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Overloading occurs when too many electrical appliances are connected to a single mains
socket. It can cause electricity failure or even a fire.
6
Short circuits (Bonus)
bulb does not light up
connecting wire
The connecting wire
provides a path with a
very low resistance.
thin nichrome wire
A large current flows
through the nichrome
wire.
The nichrome wire heats
up and glows (overheating
of the circuit).
The current flowing through the bulb reduces
to nearly zero. (The bulb is short-circuited.)
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