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Practical Electricity
This unit of work is concerned with examining electrical circuits,
electrical components and how we use electricity in the home.
If we had to draw pictures of electrical components then anyone
who was not a good artist would have difficulty. To get around this
problem we use simple symbols to represent the components.
Instead of drawing a lamp
we use the symbol
You need to recognise and be able to draw the following symbols.
battery
lamp
switch
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
In any electrical circuit some energy source must be used. In the
circuits that we examine in class the battery is used as a source of
electrical energy. There are many different types of battery but
they all supply energy.
Electric Circuits
We can combine our symbols to represent electric circuits. The
actual circuit is shown on the left and our simpler circuit diagram
is shown on the right.
Actual Circuit
Circuit Diagram
The circuit shown above has only one path around it. Going from
the negative terminal on the battery to the positive it is only
possible to have one path. A circuit that has only one path is called
a SERIES CIRCUIT.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
X
X
Actual Circuit
Circuit Diagram
The circuit shown above has more than one path around it. Going
from the negative terminal on the battery to the positive it is
possible to have more than one path.[a choice can be made at point
X] A circuit that has more than one path is called a PARALLEL
CIRCUIT.
Measuring circuit conditions
There are two quantities that can be measured in an electrical
circuits, CURRENT and VOLTAGE.
CURRENT is the measure of the number of charges passing a
point in a circuit each second.
VOLTAGE is the measure of energy given out by a battery or the
energy used by a component.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
If we want to measure the current then we must place our
measuring device so that the charges can pass through it. This
means that any circuit already built up must be broken and the
measuring device placed into the gap.
To measure current we use a device called an AMMETER.
The current is measured in the unit called the AMPERE
[AMP for short] Symbol A
Example: current = 4.5 A
A
Original circuit
Broken circuit
Ammeter in place
When we wish to measure voltage the device used is placed in the
circuit differently. This device measures the difference in the
energy carried by the charges either side of the component. The
circuit does not have to be broken to measure voltage.
To measure voltage we use a device called an VOLTMETER.
The current is measured inthe unit called the VOLT
Symbol V
Example: voltage = 6 V
V
Original circuit
Voltmeter in place
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Circuit Rules
There are rules that allow you to find out currents and voltages in
series and parallel circuits.
Series Rules:
6V
The charges have only one way to
go around the circuit. This means
that all the charges must pass every
part of the circuit. In a series circuit
this means that the current is the
same at all points in the circuit.
6V
6 Volts
supplied
4 Volts
used
2 Volts
used
The battery supplies energy to the
charges in the circuit. The
components use up this energy so
that the charges have no energy left
when they get back to the battery. In
a series circuit the voltages across
the components in the circuit add up
to the supply voltage.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Parallel Rules:
6A
3A
1A
2A
4.5V
In a parallel circuit the charges
have a choice of paths to take.
They can only go down one of
the paths at any time.
This means that the currents
through the different paths must
add up to the main current.
6A
4.5V
4.5 V
supplied
4.5 V
used
4.5 V
used
4.5 V
used
In a parallel circuit each charge
gains the same quantity of
energy from the battery. Each
charge can only go down one of
the paths at any time.
The charges give up this energy
to the components in the
branches. This means that the
voltage across each component
is the same.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Worked Examples
SERIES
.
5V
2A
3V
?V
Find the current through the 3V
lamp and the voltage across the
other lamp.
Voltage: The voltage across both
lamps must add up to 5V[the
supply voltage]
So 5V = 3V + ?V
?V = 5V – 3V
?V = 2V
Current: The current is the same at all points in the circuit.
So the current through the 3V lamp is 2A.
PARALLEL
Find the voltage across the
resistor R1 and the current
flowing in the main circuit.
Voltage: The voltage across
components connected in parallel
is the same.
So voltage across R1 is 10V.
10 V
?A
1.2 A
R1
1.6 A
R2
Current: the current in the
branches adds up to the main current.
So ?A = 1.2A + 1.6A
?A = 2.8A
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Resistance
A resistor is acomponent which tries to stop [resist] the flow of
charges through a circuit.
There are two types of resistor:
(i) Fixed resistors,normally just called resistors, that have a
particular value.
(ii) Variable resistors that can have their values altered.
These resistors are shown below along with their circuit symbols.
Resistor
Variable resistor
Resistors are used in many different ways in electrical circuits.
Any time a current passes through a resistor the resistor changes
electrical energy into heat energy. This property is used in many
heat producing electrical devices.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Measuring resistance
The resistance of a component can be measured directly using a
meter.
The device used to measure resistances is called an OHMMETER.
The unit of resistance is called the OHM.
Symbol Ω
Ω
The meter is simply
connected across the
resistance with nothing else in
the circuit.
It is possible to find the value of a resistor if you do not have an
ohmmeter. The current through the resistor and the voltage across
the resistor need to be measured.
The circuit below shows how this could be done.
ammeter to
measure current
through resistor
A
V
voltmeter to measure voltage
across resistor
If the resistance is increased the current flowing will decrease. This
is because the resistor tries to stop the charges flowing.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Calculating the resistance.
Once the voltage and current have been measured we must use an
equation to find the resistance.
resistance =
A
voltage
current
3A
V
9V
To find the resistance of the resistor in the circuit above:
Current = 3 A
Voltage = 9 V
Resistance = ?
resistance =
voltage 9
=
=3Ω
current
3
The resistance is 3 Ω.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Uses of variable resistors
Variable resistors change the current flowing in a circuit. This
allows them to change the output from electrical devices.
Speed control
on a fan
Heat control on
an iron
Speed control
on a toy train
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Dimmer switch
for a lamp
Mains electricity
For most of us our most common use of electricity is in the home,
TV, hi-fi, Playstation, computer, microwave, the list is endless.
It is important then to have some knowledge of how the electricity
is supplied in the home and how we can keep ourselves safe.
The mains supply voltage is 230 volts. All electrical appliances
that use the mains voltage directly are designed to work from a
voltage of 230 volts. The only way that this can happen is if the
wiring connects the appliances in PARALLEL.[ see circuit rules]
In order to use most electrical appliances we plug them into the
mains socket using a plug.
Earth
green/yellow
Fuse
Neutral
blue
Live
brown
Cable
Grip
Flex
Diagram of household plug
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Safety
The plug contains two parts that are involved in safety. These are
the fuse and the earth wire.
The Fuse
The fuse is normally a thin wire inside some type of casing. If the
current flowing in the circuit becomes too large the thin wire melts
and breaks. We normally say that the fuse has ‘blown’. This is
important because the flex connecting the appliance to the socket
could overheat and even start a fire if the current flowing is too
large.
The earth wire
The earth wire gives an easy path for the current if a fault makes
the casing of an appliance live. This easy path produces a large
current and the fuse blows making the appliance safe.
Double insulation
Some appliances have plastic insulating coverings [hairdryers,
power tools, etc.] and you cannot make the casing live. These
appliances do not need an earth wire. They have the symbol above
on their casing, normally on the ratings plate.
Circuit Breakers
5A
Very often circuit breakers are used instead of fuses in the mains
fuse box in the home. Circuit breakers are much easier to reset and
are more difficult to tamper with than fuses.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
How much will it cost?
You probably don’t think too much about the cost of energy at the
moment, mainly because you don’t have to pay for it!!
Every time you switch on an electrical appliance it costs money. It
is fairly easy to work out roughly how much it costs to run an
appliance. The steps set out below show how you can do this at
home.
1. Find the ratings plate on the appliance. It normally looks
something like this.
230 V ~ 50 Hz
800W
made in UK
2. Find out the power rating of the appliance. This is the number
followed by the symbol W. The symbol stands for watts, the unit
of power. On the ratings plate above the power is 800W.
3. Multiply the power by the number of hours the appliance is
switched on for.
4. The higher your answer to number 3. the more money you will
have to pay.
Cost = power x time
Extra
Actual cost is calculated by dividing your answer to number 3 by
1000 and multiplying this by 6p.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Safety in the Home
Many people in Scotland are injured or even killed by problems
involving household electrical appliances.
Some knowledge of the dangers can help you to keep yourself safe.
Danger - humans conduct electricity
Faulty Iron
Humans conduct electricity so if you
touch a faulty appliance the electric current
will pass through your body. This will cause
serious injury or even death.
If you are wet then you conduct electricity even
better. This is why there are no electrical sockets
and special light switches in a bathroom.
Danger – overloading sockets
Another common danger is overloading
sockets. If too many high power
appliances are connected to one socket the
socket can overheat and could even start a
fire. Appliances that produce heat are
usually high powered.
Danger – Stupidity
A cause of many accidents involving
electricity in the home is thoughtlessness.
Sticking a fork into the switched on toaster
to free a stuck piece of bread can lead to
serious injury.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Testing for faults
You switch on an electrical appliance and it does not work.
It may be that the fuse is blown.
In some situations a wire may have broken. If this break is inside
the insulation on a flex then you will not be able to see it.
This type of fault is known as an open circuit.
It would be useful to have some device that would allow us to test
if the fault is simple like the two mentioned above, or whether
there is a more serious problem with the appliance itself.
The device we use to test for broken wires is called a continuity
tester. The actual circuit and the circuit diagram are shown below.
Probes
The wire to be tested is connected between the probes. If the wire
is broken the lamp stays out. If there is no break the lamp lights.
Before the tester is used it must be checked to see if it is working
properly. The probes are touched together and the lamp will light if
the tester is working.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
Electrical Circuits
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Draw the circuit symbols for a battery, lamp and switch.
A battery is a source of electrical energy.
Lamps convert electrical energy into light energy.
Heaters convert electrical energy into heat energy.
Motors convert electrical energy into Kinetic energy.
Describe a series circuit.
An ammeter is used to measure electric current.
Draw the circuit symbol for an ammeter.
Draw a circuit diagram showing an ammeter connected in series.
Current is measured in amperes.
In a series circuit the current is the same at all points.
Describe a parallel circuit.
Adding the currents in two parallel branches is equal to the current drawn
from the supply.
A voltmeter is used to measure voltage.
Draw the circuit symbol for a voltmeter.
Draw a circuit diagram showing a voltmeter connected in parallel.
Voltage is measured in volts.
Adding voltages across components in a series circuit is equal to the
voltage of the supply.
The voltage across two parallel branches is the same as the supply
voltage.
RESISTANCE
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Draw the circuit symbol for a resistor and a variable resistor.
Resistors convert electrical energy into heat energy.
An ohmmeter is used to measure resistance.
Resistance is measured in ohms.
An increase in resistance of a circuit leads to a decrease in the current in
the circuit.
 Calculate the resistance using: resistance 
 Give two uses of a variable resistor.
voltage
current
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
MAINS ELECTRICITY
 Household wiring connects appliances in parallel so that they receive the
same voltage.
 The value for mains voltage in the UK is 230 V.
 Draw the circuit symbol for a fuse.
 Identify live, neutral and earth wires from the colour of their insulation.
 Identify the live, neutral and earth terminals in a plug.
 Explain how a fuse acts as a safety device.
 Electrical energy costs for the home increase as:
(a) the power of the appliance increases
(b) the time of use of the appliance increases.
 Calculate the current using: current 
power
to determine fuse values.
voltage
 A circuit breaker is an automatic switch, which can be used instead of a
fuse.
 The human body is a conductor of electricity and that moisture increases
its ability to conduct.
 The earth wire is a safety device.
 Electrical appliances, which have the double insulation symbol, do not
require an earth wire.
 Explain why connecting too many appliances to one socket is dangerous.
 Explain why situations involving electricity could result in accidents.
 Describe how to make a simple continuity tester.
 Describe how a continuity tester may be used to identify an open circuit.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
1. Draw the circuit symbols for
(a) a battery, (b) a lamp, (c) a switch
2. What does a battery supply to an electrical circuit?
3. Write down the energy changes for the following devices
(a) Lamp
(b) Heater
(c) Motor
4. Draw a series circuit including a battery, a switch and a lamp.
5. Draw the circuit symbol for an ammeter and state what it is used
for.
6. What quantity is measured in amperes.
7. Redraw the circuit below to include
(a) an ammeter.
(b) a voltmeter to measure the voltage
across the lamp
8. Draw a parallel circuit that includes a battery and two lamps.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
9.Two identical 3V bulbs are connected to a supply as shown.
What is the voltage of the supply?
3V
3V
10. Two resistors are connected in parallel to a 10 V battery.
10 V
(a) What is the voltage across R1?
0·4 A
R1
(b) What is the voltage across R2?
0·6 A
R2
(c) What size of current is drawn
from the battery?
11. Draw the circuit symbol for a resistor and a variable resistor.
12. What is the energy change in a resistor?
13. What device could be used to measure resistance?
14. What is the unit of resistance?
15. What happens to the value of the current in a circuit if the
resistance of the circuit is increased?
16. A power drill is operated at mains
voltage 230 V and has a current of 0.1 A
through it. Calculate the resistance of
the drill.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
17. Give two uses of a variable resistor.
18. What is the value for the mains voltage?
19. (a) Copy and complete the table.
Wire A
Wire
X
Wire B
name of
terminal
colour of
wire
A
B
C
Wire C
(b) What is the name of component X?
(c) How does component X act as a safety device?
20. What two factors affect the cost of running an electrical
appliance?
21. Give examples of common dangers associated with electricity
in the home.
(What should you not do around electrical appliances or power
points.)
22.(a) Draw a simple circuit for a continuity tester.
(b) Explain how it could be used to identify an open circuit.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft