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Pentecostal School
F.2 Science 2013-2014
Chapter 8 Summary
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After the lessons, you should understand the following key ideas:
8.1 Conditions necessary for current flow
1. Electric circuit

Complete path
closed circuit

Incomplete path
open circuit
2. The flow of electric current requires:

a source of electrical energy; and

a closed circuit.
8.2 Electrical conductors and insulators
1. Materials can be classified as electrical conductors and insulators:
Electrical conductors
Electrical insulators


Which one conduct current?
2.
Switch

A device for opening or closing an electric circuit.
8.3 Current, voltage and resistance
1. Free electrons

Large number in electrical conductors; very few in electrical insulators
2. In a closed circuit with a cell:

Net flow of free electrons forms current

Direction of current: positive (+) terminal
negative (–) terminal

However, the flow of free electrons is in opposite direction.
3. The relationship between current, voltage and resistance:
Current
net flow of free
electrons
Meaning
Voltage
Resistance
amount of energy
supplied by a
opposition of a
material to the flow
battery to electrons
of current
Unit
ampere (A) or
milliampere (mA)
volt (V)
ohm (Ω)
Measuring device
ammeter
voltmeter
---



no. of cells connected (in the same direction), voltage across the cells
voltage , current (with a fixed resistance)
resistance , current (with a fixed voltage)
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4.
Resistor

A circuit component with resistance
Rheostat: a resistor that has an adjustable resistance
Resistance of a wire depends on its

length (length , resistance ),

thickness (thickness , resistance ), and

the material used.

5.
8.4 Circuit symbols and diagrams
Circuit component
Circuit symbol
Circuit component
Circuit symbol
Bulb
Battery
Resistor
Rheostat
Connecting wire /
connector
Ammeter
Switch
1.
Voltmeter
To draw a circuit diagram:

Draw the symbol of each component according to the order it appears in the
circuit.

Use a ruler to draw connecting wires as straight lines.

Put a solid dot at each T-shaped junction.
8.5 Series and parallel circuit
1.
Series circuit
2.
Parallel circuit
Connected in series
Connected in parallel
Current flows in one path only
Current flows in more than one path
Same current at different points in the Current in the main loop
circuit
= Sum of the currents in the branches
No current flows if any point in the circuit No current in an open branch, but other
is broken
branches are not affected
Larger current flows in the branch with a
lower resistance
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8.6 The heating effect of current and its potential hazards
1. Heating effect of current


2.
Some electrical energy changes to heat energy when passing through a conductor
Electrical appliances, such as electric kettles, heaters and hair dryers
Fuse
If the current is greater than fuse rating, the fuse blows out and breaks the circuit

Prevent overheating
Circuit breaker

3.
8.7 Household electricity
1.
Three-pin plug
3.
In a three-pin plug:
2.
Mains socket
Pin
Wire connected
Colour of the wire
Earth
Earth
Green and yellow
Live
Live
Brown
Neutral
Neutral
Blue
Electricity is supplied to homes through the live wire and the neutral wire

Fuse is always placed in the live wire
Earthing

Earth wire connects the metal case of an electrical appliance

Protects us from an electric shock
Ring circuit

Each of the live, the neutral and the earth wires branches into two paths and
forms a ring circuit

All sockets connected in parallel to a ring circuit

4.
5.
6.
7.
Overloading

Too many electrical appliances connected to a socket

Dangerous
Short circuit

Accidentally connect a wire across an electrical component in a circuit

A large current may pass through the wire
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