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Rosshall Academy Physics Department
Standard Grade Physics
Unit 2: Using Electricity
Notes
Standard Grade Physics
Using Electricity
Notes
1
Section 1: Circuit Electricity
G1.
State that electrons are free to move in a conductor.
G2.
Describe the electric current in terms of the movement of charges around
the circuit.
G3.
Use correctly the units: ampere and volt.
G4.
Draw and identify the circuit symbol for a battery, fuse, lamp, switch,
resistor, capacitor, diode, and variable resistor.
C12.
Carry out calculations involving the relationship between charge, current, and time.
C13.
Use correctly the unit: coulomb.
C14.
State that voltage of a supply is a measure of the energy given to the charges in a circuit.
Electric Charge
All matter is made up of tiny particles known as ___________. When two or more of these particles
join together they form a ______________. Atoms themselves are made up of smaller, sub-atomic,
particles known as ____________, _________________ and ______________.
Protons and electrons carry an electric charge – protons are ___________________ charged and
electrons are ________________ charged.
Electricity is carried by _________________.
Units: Charge, symbol Q, is measured in coulombs (C).
Standard Grade Physics
Using Electricity
Notes
2
Conductors and Insulators
A conductor is defined as a material that allows _______________ to flow through it. An insulator
does not allow electricity to flow through it easily.
Conductors are mostly metals and metals are structured in such a way as to allow electrons to flow
easily through the material. This is because the electrons are not bound to an individual nucleus they
are free to move around the metal lattice. This makes it easy for these electrons to flow from the
negative terminal of a supply to the positive terminal.
Insulators on the other hand have electrons that are tightly bound to a nucleus and are not therefore
free to move.
Electric Circuits
An electric circuit is a path around which electricity can flow. For this to happen the circuit must be
complete i.e. it cannot contain any gaps.
When we draw electrical circuits we use symbols. These make the diagrams neater and are like the
words in a language – any physicist anywhere in the world will be able to recognize these symbols.
+
Draw a simple electric circuit containing two batteries, a switch and a lamp.
Explain how the switch works.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
3
Electric Current
When a battery is connected to a conductor the electrons in the conductor flow from the
_____________ terminal to the __________________ terminal. This is known as an electric
______________.
An electric current is the amount of charge flowing in a circuit per second.
Mathematically this can be written as:
Charge
Q
=
Current x time
=
It
Units: Electric current is measured in amperes, usually shortened to amps (A).
And Henry scores to give
Arsenal the Premiership!
Current and Charge – Worked Example
A portable radio draws a current of 2A.
If it is used for 2hrs what charge flows through the radio.
Voltage
The voltage in a circuit is a measure of the amount of energy given to each coulomb of charge by the
electricity supply.
Units: Voltage is measured in volts (______).
Standard Grade Physics
Using Electricity
Notes
4
G5.
State that in a series circuit, the current is the same at all points.
G6.
State that the sum of currents in parallel branches is equal to the current
drawn from the supply.
G7.
State that the sum of voltages across components in series is equal to
the voltage of the supply.
G8.
State that the voltage across components is parallel is the same for each component.
G9.
Draw and identify the circuit symbols for an ammeter and voltmeter.
G10. Draw circuit diagrams to show the correct position for an ammeter and voltmeter in a circuit.
G11. State a practical application in the home which uses two or more switches used in series.
C15. Draw circuit diagrams to describe how the various car lighting requirements are achieved.
Series and Parallel Circuits

Series Circuits
A series circuit is one which has only ______ path for the electricity to take.
Draw a series circuit containing two batteries, two bulbs and a switch.

Parallel Circuits
A parallel circuit is one which has _________ _______ one path for the current to take.
Draw a parallel circuit containing two batteries, two bulbs and a switch in a position that allows it to
control one of the bulbs.
Standard Grade Physics
Using Electricity
Notes
5
Current in Series and Parallel Circuits
An ammeter is a device used to measure the _____________ flowing in a circuit. To measure the
current in the circuit the ammeter must be connected in ______________ with the components in
the circuit.

Current in a Series Circuit
I
Any current that leaves the batteries
must flow round the one path to get
back to the battery.
I
A
A
I
The current in a series circuit is
_______ ____________ at all
points in the circuit.
A

Current in Parallel Circuits
I
The current leaving the battery is the
I
___________ as the current entering the battery.
A
A
I1
I2
At the junction of the branches the current has a
choice of paths and so __________ ___.
A
I = I1 + I2
The current in a parallel circuit _________ __.
A
Standard Grade Physics
Using Electricity
Notes
6
Voltage in Series and Parallel Circuits
A voltmeter is a device used to measure the voltage across components. To measure the voltage
across a component the voltmeter must be connected in ____________ with the component.

Voltages in Series Circuits
Vs
V1
V2
The voltage in a series circuit _________ ___.
Vs = _____ + _____

Voltages in Parallel Circuits
The voltage is a parallel circuit across all the
Vs
branches is the __________ and is equal to the
_____________ voltage.
Vs
=
______ = ______
V1
V2
Standard Grade Physics
Using Electricity
Notes
7
Practical Circuits
Household lights are wired in parallel. Give two reasons why.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
When you plug in a hairdryer you have to switch it on at the mains and then
switch on the hairdryer. Are these switches in series or in parallel? Explain
your answer.
____________________________________________________________________________
____________________________________________________________________________
Give two more examples of a device at home which uses two or more switches wired like this.

_____________________________________________________________________

_____________________________________________________________________
Car Lighting Circuits
Use the space below to draw a car lighting circuit. The circuit must perform to the following
specifications.


The four sidelights lights can only be switched on if the ignition switch is on.
The two headlights can only be switched on if the sidelights have already been switched on.
Standard Grade Physics
Using Electricity
Notes
8
Section 2: Resistors in Circuits
G1.
State that an increase in resistance of a circuit leads to a decrease
in the current in that circuit.
G2.
Carry out calculations involving the relationship between resistance,
current and voltage.
G3.
Use correctly the unit: ohm.
G4.
Give two practical uses of variable resistors.
G5.
State that when there is an electric current in a wire, there is an energy transformation.
G6.
Give three examples of resistive circuits in the home in which electrical energy is transformed
into heat.
C15. State that V/I for a resistor remains approximately constant for different currents.
C18. Carry out calculations involving resistors in series and parallel.
Resistance
The resistance of a component is a measure of its ________________________ to the flow of
electric current.
Electrical conductors have a _________ resistance but _____________________ will have a _____
resistance.
If the resistance in a circuit increases then the amount of current flowing in the circuit will
______________.
Units: Resistance is measured in ________ ().
Standard Grade Physics
Using Electricity
Notes
9
Ohm’s Law
This circuit was used to investigate what
happened to the current through a resistor
when the voltage across the resistor was varied.
The results are shown on the graph below.
A
V
Voltage/V
Voltage/V
Voltage against
Voltage
againstCurrent
Current
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
Current/A
Current/A
The voltage and current values are shown in the table below. Complete the table by calculating the
values in the final column.
Voltage/V
10
20
30
40
50
60
70
80
Current/A
1
2
3
4
5
6
7
8
Voltage/Current
What do you notice about the values of voltage divided by current?
_________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
10
In the previous experiment we discovered that for a given resistor when the voltage across the
resistor is divided by the current through the resistor the answer is always the same. This value is
known as the resistance of the resistor.
Mathematically we can write this relationship as:
Resistance
=
Voltage across resistor
Current through resistor
R
=
V
I
This is more commonly written as
V
=
IR
This is known as Ohm’s Law.
Ohm’s Law – Worked Example
A hairdryer is connected to a power supply that can supply 230 V.
If the current passing through the hairdryer is 0.5 A what is its resistance?
Standard Grade Physics
Using Electricity
Notes
11
Combining Resistors in Series and Parallel Circuits

Resistors in Series Circuits
To find the total resistance of a number of resistors in a series circuit, the resistance of the
individual resistors in the circuit should be ____________ together.
R1
R2
R3
The resistors in the circuit above could be replaced by a single resistor that would produce the same
effect.
To find the value of this resistor, the total or resultant resistance, the following relationship should
be used:
RT
=
R1 + R2 + R3 + …………
where RT is the total resistance.
The above circuit could therefore be redrawn as follows:
RT
Standard Grade Physics
Using Electricity
Notes
12

Resistors in Parallel Circuits
To find the total resistance of a number of resistors in a parallel circuit the following relationship
should be used:
1
RT
=
1
+
R1
1
+
R2
1
R3
+ …………
Using the above relationship these
resistors could be replaced by a
single resistor that would produce
the same effect.
R1
RT
R2
R3
Combining Resistors – Worked Example
Calculate the value of the single resistor that could be used to replace the following resistors.
10
100
1000
Standard Grade Physics
Using Electricity
Notes
13
If the resistors in the previous example are now connected in parallel, calculate the value of the
single resistor now needed to replace them.
10
100
1000
In the series circuit, which resistor plays the most important role in the value of the total resistance?
_________________________________________________
In the parallel circuit, which resistor plays the most important role in the value of the total
resistance?
_________________________________________________
This result is important.
In a series circuit the ________________ resistance is the most important resistor. As more
resistors are added in series the total resistance will ________________.
In a parallel circuit the _______________ resistance is the most important resistor. The total
resistance can never be larger than the smallest resistance in the circuit. As more resistors are added
in parallel the total resistance will ___________________.
Standard Grade Physics
Using Electricity
Notes
14
Ohm’s Law – Worked Examples
1.
Calculate the ammeter and voltmeter readings in the following circuits.
(a)
6V
A
18
12
V
(b)
12V
6
A
12
9
V
Standard Grade Physics
Using Electricity
Notes
15
Variable Resistors
A variable resistor is one whose resistance can be made to ___________.
In its simplest form a variable resistor consists of a length of wire with a sliding contact that can ‘tap’
into the wire at various positions along its length.
A variable resistor allows the amount of current flowing in a circuit to be changed.
Both circuits shown above show a variable resistor connected to a battery.
Variable resistors have many uses in electrical circuits. Give two examples of practical uses of variable
resistors.

_______________________________________________________________________

_______________________________________________________________________
Resistive Heating
When a current is passed through a wire the wire _______________ __.
Sometimes this effect is useful and the heat energy can be used by an electrical device. On other
occasions the fact that wires heat up when electricity is passed through them can cause problems and
it needs to be overcome.
The conversion of electrical energy into _________ energy in a wire carrying an electric current is
known as resistive heating and it is used in a number of common household devices.
Give three examples of common electrical appliances that use resistive heating.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________
Electrical Heaters
In an electric heater the electrical energy is
transformed into heat energy in the resistance wire
(also known as an element).
Standard Grade Physics
Using Electricity
Notes
16
G7.
State that the electrical energy transformed each second = IV.
G8.
State the relationship between energy and power.
G9.
Use correctly in context, the terms energy, power, joule and watt.
G10. Carry out calculations involving the relationship between power, current and voltage.
G11. State that in a lamp, electrical energy is transformed into heat and light.
G12. State that the energy transformation in an electrical lamp occurs in the resistance wire
(filament lamp) or a gas (discharge tube).
G13. State that a discharge tube lamp is more efficient than a filament lamp (i.e. more of the energy
is transformed into light and less into heat).
G14. State that the energy transformation in an electric heater occurs in resistance wire (element).
C16. Explain the equivalence of IV and I2R.
C17. Carry out calculations using the relationships between power, current and resistance.
Power
The power of a device is a measure of how much energy it uses per second. Mathematically this can be
written as:
Power
=
P =
Energy
Time
Time
E
t
Units: The units of power are watts (___).
Power – Worked Example
An electric drill has a power rating of 650 W.
How much energy does it uses if it is on for 5 minutes?
Standard Grade Physics
Using Electricity
Notes
17
Electrical Power
Power is a measure of the amount of __________ used by a device each second.
The current through a device is a measure of the __________ flowing each second and the voltage is
a measure of how much ___________ is carried by each coulomb of charge. This means that the
current through a device and the voltage across it can be used to work out the power of the device.
Power
=
current x voltage
P
=
IV
Electrical Power – Worked Examples
1.
A CD player has a power rating of 80 W.
If it is connected to a voltage of 230 V, what current flows through it?
2.
Calculate the size of the resistor in the circuit shown. The bulb is working at its correct
rating.
12V
A
9V 12 W
Standard Grade Physics
Using Electricity
Notes
18
Power and Resistance
Electrical power is given by
P =
IV
V =
IR
According to Ohm’s Law
By combining these equations we can arrive at a third:
P =
IV
but
P =
I x IR
P =
I2R
V = IR
One final equation that allows us to calculate electrical power is:
2
P = V
R
Power and Resistance – Worked Example
A vacuum cleaner draws a current of 4.8 A from the power supply.
If the power rating of the vacuum cleaner is 1200 W what is its resistance?
Standard Grade Physics
Using Electricity
Notes
19
Electrical Bulbs (Lamps)
There are two main methods of generating light in a bulb. The traditional way involves allowing a
current to flow through a very thin wire known as a filament. A more modern method is to pass a
current through a gas.
Both methods convert electrical energy to _________ and __________ energy. Discharge tubes are
more efficient because they produce more ____________ energy and less _________ energy.

Filament Lamps
When a current passes through a wire ________ energy is
Filament
produced.
If the wire is very thin then it has a very high
_________________ and this means that the wire gets so
hot that ___________ energy is produced.

Discharge Tubes
Gas
Neon lamps are examples of discharge tubes.
An electric current is passed through a gas and this causes
light energy to be released by the gas.
Type of Lamp
Where energy change takes place
Filament bulbs
Discharge Tube
What is the main disadvantage of using filament bulbs to produce light?
____________________________________________________________________________
What is the advantage of using discharge tubes?
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
20
Section 3: Alternating and Direct Current
G1.
State that the mains supply is a.c. and a battery supply is d.c.
G2.
Explain in terms of current the terms a.c. and d.c.
G3.
State that the frequency of the mains supply is 50 Hz.
G4.
State that the mains voltage is 230 V.
C5.
State that the declared value of an alternating voltage is less than its peak value.
Alternating and Direct Current

D.C. Electricity
Direct current (d.c.) is current that flows in one direction only. A battery is a source of d.c.
electricity.
The electric charges (____________ electrons)
leave the negative terminal of the battery and
flow round to the positive terminal.
The polarity of the terminals (whether they are
positive or negative) is fixed.

A.C. Electricity
Alternating current (a.c.) is current that flows in two directions. The mains is a source of a.c.
electricity.
+
-
-
+
½ a cycle later
A.c. electricity is generated in cycles i.e., it has a frequency. After one ½ of a cycle the polarity of
the terminals is reversed and the direction of the electricity also reverses.
Note: Although the direction of the electricity reverses it always flows from negative to positive.
Standard Grade Physics
Using Electricity
Notes
21
‘Looking’ at Electricity
It is possible to ‘look’ at electricity using an oscilloscope. When an electrical signal is applied to an
oscilloscope the oscilloscope produces a trace that allows us to ‘look’ at the signal.
Every oscilloscope has a control known as a time-base and this controls how quickly the beam in the
oscilloscope sweeps across the screen. The time-base can be switched on and off.
d.c. trace with time-base off
d.c. trace with time-base on
a.c. trace with time-base off
a.c. trace with time-base on
The oscilloscope can be used as a voltmeter when an electrical signal is applied. The vertical
displacement of the beam is a measure of the size of the voltage of the supply.
Standard Grade Physics
Using Electricity
Notes
22
Mains Electricity
Mains electricity is a.c. We have seen that a.c. electricity produces a wave trace on an oscilloscope.
Peak Voltage Vp

Peak Voltage, Vp
The peak voltage is the ___________ voltage available from the supply. It is measured by measuring
the __________ of the a.c. trace on the oscilloscope.

Frequency
The frequency of the a.c. supply is the ___________ of cycles per ____________. It is measured in
____________ (__).
The peak voltage is not a very useful voltage to know – it occurs only very briefly twice in each cycle.
Another voltage associated with an a.c. voltage is known as the declared voltage (it is a sort of
complicated average voltage) and it is always less than the peak voltage.
In the United Kingdom the mains supply has a declared voltage of ________ V and a frequency
of _______ Hz.
Standard Grade Physics
Using Electricity
Notes
23
Section 4: From the Wall Socket and Beyond
G1.
Describe the mains supply/battery as a supply of electrical energy and
describe the main energy transformations occurring in household appliances.
G2.
State approximate power ratings of different household appliances.
G3.
Select an appropriate flex given the power rating of an appliance.
G4.
Identify live, neutral and earth wires from the colour of their insulation.
G5.
State to which pin each wire must be connected for a plug, lamp-holder and extension socket.
G6.
State that household wiring connects appliances in parallel.
G7.
State that kWh is a unit of energy.
C22. Describe, using a circuit diagram, a ring circuit.
C23. State advantages of using the ring circuit as a preferred method of wiring in parallel.
C24. Give two differences between the lighting circuit and the power ring circuit.
C25. Explain the relationship between kilowatt-hours and joules.
Energy Transformations
The mains or batteries are supplies of electrical ___________. When a device is connected to a
supply of electrical energy they generally convert this energy into another form of energy.
Washing Machine
Energy Change:
_________________________________________________________
Television
Energy Change:
_________________________________________________________
Lamp
Energy Change:
_________________________________________________________
Food Mixer
Energy Change:
_________________________________________________________
Standard Grade Physics
Using Electricity
Notes
24
Power Rating of Electrical Appliances
All of the appliances listed above will have different power ratings. The power rating of an appliance is
shown on its rating plate.

230V
This rating plate from a kettle gives us some important
information about the kettle.
2400W
50 Hz
The kettle should be connected to 230 V, the  sign
indicates that a.c. electricity should be used and the
frequency of the supply should be 50 Hz.
In addition the power rating of the kettle is 2400 W.
Made in UK
Use the following power ratings to complete the table below:
1600 W;
2200 W;
Device
Hairdryer
Television
Kettle
Toaster
700W;
Power Rating
Flexes
Flexes come in various thicknesses and
it is important that the correct thickness
of flex be attached to each device.
If too thin a flex is used then the electricity
flowing through it can cause it to overheat and
possibly melt which could be a fire hazard.
85W
Earth Wire
(Green and Yellow)
Neutral Wire
(Blue)
Live Wire
(Brown)
The thickness of flex needed by a device is determined by its power rating – the greater the power
rating the _____________ the flex required.
A flex consists of 2 or 3 cores of thin, stranded copper wire within an insulated sheath. Each core is
coated with a coloured insulated layer.
Standard Grade Physics
Using Electricity
Notes
25
3-Pin Plugs
When trying to remember which wire should be
connected to which pin think of the colour of the
insulation.
The live wire is covered with bRown insulation and
should be connected to the Right hand pin.
The neutral wire is covered with bLue insulation
and should be connected to the Left hand pin.
Household Wiring
Electricity is distributed round the country by the National Grid and it enters our home by the service
cable. The diagram below shows the different connections and circuits in a normal house.
Electricity
co. fuse
box
Electricity
Meter
Ring
Main
Circuit
Ring Main
Circuit
Cooker
Immersion
heater
Lighting
Circuit
Consumer unit
(fuse box)
Standard Grade Physics
Using Electricity
Notes
26

Electricity Company Fuse Box
This contains a fuse that protects the mains wiring. If a serious fault develops then this fuse will cut
off the electricity supply to the house. It should only be accessed by electricians from the electricity
company.

Electricity Meter
This measures how much electrical energy is being used by the household.

Consumer Unit (Fuse Box)
This houses the mains switch that cuts the house wiring from the electricity supply and the fuses or
circuit breakers for the various household wiring circuits.
Compare the lighting and ring main circuits in the diagram. There are two noticeable differences
between them.

The fuse in the lighting circuit is _____________ than the fuse in the ring main circuit.
The lighting circuit supplies electricity to the lights only and as such does not require a large
____________. The ring circuit supplies the power sockets and therefore carries much more
_____________.

The branches in the lighting circuit are connected in __________________.
In the ring circuit the arrangement is slightly different – the wiring forms a ring.
Ring Main Circuit
A ring circuit adds to the advantages of a parallel circuit. Like a parallel circuit the ring main circuit
allows individual power sockets to be switched on and off without affecting any of the other sockets.
The other advantages concern the amount of current carried by the ring circuit.
Look at the circuits below:
A
1.6 A
A
1.6 A
0.8 A
A
0.8 A
A
The current in the ring main circuit is half that in the parallel circuit. This is because the current has
two possible routes round the circuit. The smaller current means that there is less power loss in the
cables (P= I2R) and because the current carried by the wires is less they can be thinner and therefore
cheaper.
Standard Grade Physics
Using Electricity
Notes
27
The Kilowatt Hour (kWh)
Electrical energy is normally measured in ________________. When we are measuring the amount of
energy used the ___________ is too small. The electrical energy used in your home is measured in
units known as kilowatt hours.
1 kWh would be the amount of electrical energy used by a 1 kW heater in 1 hour.
We know that the power of a device is a measure of how much energy it uses each second.
P =
Rearranging this equation gives
E =
E
t
P x t
=
1kW x 1 hr
=
1000W x (60 x 60)s
=
3 600 000 J
=
3.6 MJ
1 kWh = 3.6 MJ
It is clear from this that the kilowatt hour is a much more convenient unit for electricity companies!
To calculate the number of kilowatt hours used (sometimes referred to as units) the following should
be used:
Number of kilowatt hours
= Power in kilowatts x time in hours
Kilowatt Hours – Worked Example
If a 100 W light bulb is left on for 1 hour and 20 minutes, how much energy does it use in
kilowatt hours?
Standard Grade Physics
Using Electricity
Notes
28
G8.
State that the human body is a conductor of electricity and that moisture
increases its ability to conduct.
G9.
State that fuses in plugs are intended to protect flexes.
G10. State that mains fuses protect mains wiring.
G11. Select an appropriate fuse given the power rating of an appliance.
G12. State that a circuit breaker is an automatic switch which can be used instead of a fuse.
G13. State that the earth wire is a safety device.
G14. State that electrical appliances which have the double insulation symbol do not require an
earth wire.
G15. Draw the double insulation symbol.
G16. Explain why situations involving electricity could result in accidents (to include: proximity to
water, wrong fuses, wrong, frayed or badly connected flexes, short circuits and misuse of
multiway adaptors).
G17. State that connecting too many appliances into one socket is dangerous because a large
current can be drawn from the supply.
G18. Describe how to make a simple continuity tester.
G19. Describe how a continuity tester may be used for fault finding.
C24. Explain why fuses and switches must be in the live lead.
C25. State one reason why a circuit breaker may be used in preference to a fuse.
C26.
Explain how the earth wire acts as a safety device.
Electrical Safety
Electricity can be dangerous and for this reason great care must be exercised when dealing with it.
There are safety procedures that can be used to try and make electricity as safe as possible.

Human Conductivity
Human bodies conduct electricity and this conductivity can be increased if the body is ________.
______________ increases the conductivity of the human body. For this reason there are no light
switches or power sockets in a bathroom – the possibility of someone touching either with wet hands
makes it too dangerous.

Fuses
All plugs are fitted with a fuse. This is generally a small glass cartridge that contains a very thin piece
of wire.
Circuit Symbol
The purpose of a fuse placed in a plug is to protect the ____________________.
Mains fuses protect _________ ____________________.
Standard Grade Physics
Using Electricity
Notes
29
Fuses protect wiring from excessive ________________. If too large a _____________ flows
through the fuse wire it melts and breaks into two pieces (it ‘blows’). There is now a ________ in the
circuit and the __________________ will stop flowing.
Fuses come in different sizes e.g., 3 A and 13 A. The number (rating) tells us how much current can
flow through the plug safely. If the current exceeds this value the fuse will ‘blow’ cutting the device
from the electricity supply and making sure that the wires do not carry too much current.
What could happen if a wire carried more current than it was designed to take?
____________________________________________________________________________
____________________________________________________________________________
Different fuse ratings are required because different appliances require different currents.
Most household devices are designed to run off the mains voltage (_______ V in the UK). Given that
the power of a device is given by P = IV then, if all devices are connected to the same voltage, devices
with different power ratings will require different currents and therefore different fuse ratings.
In general the following guidelines should be followed:
Devices under 700 W should be fitted with a ______ A fuse.
Devices over 700 W should be fitted with a ______ A fuse.
The exception to this is any device containing an electric motor. All such devices should be fitted with
a 13 A fuse regardless of its power rating.
Fuses and switches must be connected into the ___________ wire. This is to make sure that when
the fuse ‘blows’ or the switch is off the device is isolated from the power supply.

Circuit Breakers
A circuit breaker is an automatic switch which switches off (trips) when too
large a current flows through it. Although circuit breakers are more expensive
than fuses there are advantages to using circuit breakers that outweigh the
disadvantage of the excess cost.
List two advantages of using circuit breakers rather than fuses.

_______________________________________________________________________
_______________________________________________________________________

_______________________________________________________________________
_______________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
30

The Earth Wire
The earth wire is a safety device that protects the user of an electrical appliance that has developed
a fault. The earth wire must always be connected to the metal casing of an appliance.
Fault
Path of
Current
Fault
Live
Live
Earth
connection
Cable
Neutral
Neutral
Explain how the earth wire protects a user from electrocution should a fault develop. Your description
should include:
What the fault is;
Why it is dangerous;
What the earth wire does and
What happens as a result of the earth wire being connected correctly.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
31

Double Insulation
Occasionally appliances are fitted with flexes that contain only two cores – live and neutral. These
devices do not require an earth wire because they are said to be double insulated.
What does it mean when we say that an appliance is double insulated?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Any appliance that is double insulated will have the symbol shown below on its rating plate:
Dangerous Situations
There are various situations, some found often in houses, which can be dangerous and cause accidents.

Use of the Wrong Fuse
If too big a fuse is used then the flexes may carry too great a
current. This is dangerous because ____________________
_______________________________________________
_______________________________________________

Cracked Plugs and Frayed Wires
If the plug casing is cracked or the wires are frayed then there is
a danger of a person coming into contact with bare wires.
This is dangerous because ________________________________
____________________________________________________
____________________________________________________

Proximity to Water
Care should be taken around water. Kettles should
be unplugged before being filled and switches should
not be used with wet hands. This is dangerous because
________________________________________
________________________________________
________________________________________
Standard Grade Physics
Using Electricity
Notes
32

Misuse of Multiway Adaptors
Connecting too many appliances into a multiway adaptor may cause too much
current to be drawn from the supply. This is dangerous because ________
________________________________________________________
________________________________________________________
Circuit Faults
There are two main types of circuit fault – short circuits and open circuits.

Short Circuits
A short circuit occurs when a wire comes loose and connects across a component.
Short circuits are dangerous because the bare wire has practically
zero resistance and this causes a large current to flow. When the
current flowing in the circuit is larger than the flexes are
designed to carry then this can cause the wire to ____________
_________________________________________________
This bulb has been
short circuited.
_________________________________________________
If the resistance of the this circuit was to be measured it would be ______ .

Open Circuits
An open circuit occurs when something happens to cause a gap to be created within the circuit.
An open circuit is dangerous because the gap in the circuit means
that someone could come into contact with the bare leads.
This is dangerous because _____________________________
_________________________________________________
_________________________________________________
This circuit is on open
circuit.
If the resistance of an open circuit was measured it would be ____________________.
Standard Grade Physics
Using Electricity
Notes
33
Continuity Testers
Continuity testers allow us to test for an open circuit i.e. they test for continuity. If a wire has a
break in the copper core then it is not continuous and this is what the continuity tester can detect.
The simplest continuity testers consist of a battery, a bulb and a resistor connected as shown below:
A
B
The item to be tested is placed between A and B. If there is continuity, i.e. there is no break in the
item, then the bulb will _______________. If there is a break, the bulb will not _____________.
The purpose of the resistor is to ______________ the bulb should a large current flow in the circuit.
Standard Grade Physics
Using Electricity
Notes
34
Section 5: Movement from Electricity
G1 Identify on a simple diagram of an electric motor, the rotating coil, field coil
[magnet], brushes, and commutator.
G2 State that a magnetic field exists around a current-carrying wire.
G3 Give two examples of practical applications which make use of the magnetic effect of a current.
G4 State that a current-carrying wire experiences a force when the wire is in a magnetic field.
C5 State that the direction of force on a current-carrying wire depends upon the direction of current
and of the field.
C6 Explain the operation of a d.c. electric motor in terms of forces acting on the coil and the purpose
of brushes and commutator.
C7 State the reasons for the use in commercial motors of carbon brushes, multi-section commutators
and field coils.
Magnetism
We live in the Earth’s gravitational force field – it is what causes any object lifted above the ground
to fall when it is released. There are other types of force field one of which is a magnetic force field.
There are three materials that are naturally magnetic – iron, nickel and cobalt.
Permanent magnets have a magnetic force field (usually shortened to magnetic field) around them at
all times. If we place a piece of paper above the magnet and dust it with iron filings, these filings will
show the shape of the magnetic field.
Magnetic field due to a bar magnet.
Magnets have two poles – the _____________ pole and the ____________ pole.
If two like poles are brought together the magnets will _________________ each other.
If two unlike poles are brought together the magnets will _________________ each other.
The Earth also has a magnetic field and in fact acts like a giant bar magnet.
Magnetic
north pole
South pole
North pole
Magnetic
south pole
Standard Grade Physics
Using Electricity
Notes
35
Electromagnetism
When a current is passed through a wire it sets up a magnetic field around the wire. The strength of
the magnetic field can be increased by ______________________________________________
___________________________________________________________________________.
Reversing the direction of the electric current reverses _________________________________
___________________________________________________________________________.
A simple electromagnet can be made from a length of wire coiled around an iron core. The iron core
strengthens any magnetic effects produced in the wire. If a current is passed through the wire the
core becomes magnetized and the set up is said to be an electromagnet. When the current is switched
off the core loses its magnetism.
The strength of the magnetic field produced by an electromagnet can be increased by
 ________________________________________________________________________
 ________________________________________________________________________
What are the two main advantages that electromagnets have over permanent magnets?
 ________________________________________________________________________
 ________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
36
The Electric Bell
An electric bell is a device that makes use of electromagnetism.
Look at the diagram below.
Wire coiled round
an iron core
When the door bell switch is closed a _______________ flows through the circuit. This magnetises
the ________________________ which in turn pulls the ___________________________
towards the iron core. The movement of the armature causes the _________________ to hit the
bell.
When the armature moves towards the core it breaks contact at T which breaks the circuit and causes
the ____________________ to stop. This de-magnetises the ______________________ which
then releases the iron armature. The springy metal strip causes the iron armature to return to its
original position, moving the striker away from the bell and allowing contact to be re-made at T. The
whole process is repeated as long as the ______________________________________________
_____________________.
Standard Grade Physics
Using Electricity
Notes
37
The Magnetic Relay
The diagram below shows a magnetic relay. This device is used as a remote switch. By switching on
switch S another circuit, perhaps in a different location, can be activated.
Explain how the relay works.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Standard Grade Physics
Using Electricity
Notes
38
Movement from Magnetism
We know that when two permanent magnets are brought close to one another they either attract or
repel.
We also know that when a current flows through a wire a magnetic field is set up around the wire.
?
What would happen if we placed a wire inside the magnetic field due to
a permanent magnet and the switched on the current.
?
North Pole
I
South Pole
When the current is switched on a magnetic field is formed around the wire. This magnetic field
interacts with the magnetic field from the permanent wire and the wire experiences a force and moves
as a result.
The direction of the movement depends on two factors:
__________________________________________________________________
__________________________________________________________________
Wire moves upwards
North
Pole
South
Pole
I
North
Pole
I
South
Pole
Wire moves downwards
South
Pole
I
North
Pole
Which direction does the wire move in this case?
________________________________________
Standard Grade Physics
Using Electricity
Notes
39
The Electric Motor
An electric motor makes use of this fact.
A very simple electric motor is shown below.
The main features to note are:
 The coil of wire is free to rotate.

Each side of the coil experiences a force due to the presence of the current in the wire.

The current in each side flows in the opposite direction and so each side experiences a force in
the opposite direction – upwards on one side and downwards on the other. This causes the coil
to rotate.

The split-ring commutator causes the current direction to reverse every half cycle. This
means that the force along each side always acts in the same direction and this in turn means
that the coil can spin through 360.

The carbon brushes maintain good electrical contact between the commutator and the power
supply. Carbon is used because it moulds itself to the shape of the commutator, it can
withstand high temperatures and it reduces wear on the commutator.
Standard Grade Physics
Using Electricity
Notes
40
Practical Motors
The motor shown above is a very simple device. Practical motors vary from this version in a number of
ways.
Rotor coils
Simple Motor
Single coil
Permanent magnet
Practical Motor
Several hundred coils (rotor
coils)
Electromagnet (stator coils)
Split-ring commutator
Multi-segmented commutator
Reason for difference
Greater turning force produced
More compact and produces
stronger magnetic field in
comparison with a permanent
magnet of similar size.
Produces a much smoother
rotation. Split-ring commutator
produces a jerky rotation.
The Finishing Line
Standard Grade Physics
Using Electricity
Notes
41