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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.
we use the symbol
Instead of drawing a lamp You need to recognise and be able to
draw the following symbols. battery lamp switch
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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.
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XActual Circuit
X
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.
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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
Original circuit
A
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
Original circuit Voltmeter in place
V
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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 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.
6 Volts supplied
4 Volts used
2 Volts used
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Parallel Rules:
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.
4.5V
6A
6A
1A 3A 2A
4.5V
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. 4.5 V
supplied 4.5 V used
The charges give up this energy to the components in the
branches. This means that the voltage across each component is the
same.
4.5 V 4.5 V used used
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Worked Examples .
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. 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. Current: the current in the
branches adds up to the main current. So ?A = 1.2A + 1.6A ?A =
2.8A
3 V ? V
5 V 2A
R2
10 V
R1 1.2 A
1.6 A
?A
SERIES
PARALLEL
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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.
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.
Resistor Variable resistor
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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 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. If the
resistance is increased the current flowing will decrease. This is
because the resistor tries to stop the charges flowing.
V
A
ammeter to measure current through resistor
voltmeter to measure voltage across resistor
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Calculating the resistance. Once the voltage and current have
been measured we must use an equation to find the resistance.
resistance = currentvoltage
V
A 3A
9V
To find the resistance of the resistor in the circuit above:
Current = 3 A Voltage = 9 V Resistance = ?
resistance = currentvoltage =
39 = 3
The resistance is 3 .
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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
Speed control on a toy train
Heat control on an iron
Dimmer switch for a lamp
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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
Live brown Neutral
blue Cable Grip
Flex
Fuse
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 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.
5A
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How much will it cost? You probably dont think too much about
the cost of energy at the moment, mainly because you dont 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.
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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 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
Faulty Iron
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.
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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.
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Electrical Circuits 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 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:currentvoltage resistance =
Give two uses of a variable resistor.
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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: voltagepower current = to determine
fuse v
A circuialues.
t breaker is an automatic switch, which can be used instead of
a
uman body is a conductor of electricity and that moisture
increases
ety device. the double insulation symbol, do not
g too many appliances to one socket is dangerous.
entify an open circuit.
fuse. The h
its ability to conduct. The earth wire is a saf Electrical
appliances, which have
require an earth wire. Explain why connectin 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 id
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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.
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9.Two identical 3V bulbs are connected to a supply as shown.
What is the voltage of the supply?
10. Two resistors are connected in parallel to a 10 V
battery.
3 V 3 V
(a) What is the voltage across R1? (b) What is the voltage
across R2?
R2
10 V
R1 04 A
06 A
(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.
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17. Give two uses of a variable resistor. 18. What is the value
for the mains voltage? 19. (a) Copy and complete the table.
Dick Orr 2002 Intermediate 1 Physics Revision Book Draft
(b) What is the name of component X?
Wire A
X Wire B
Wire C
Wire name of terminal
colour of wire
(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.
A B C
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.Electrical CircuitsRESISTANCEMAINS
ELECTRICITY