KIRKCALDY HIGH SCHOOL
Physics Department
Physics
S3 Electricity and Energy
Electrical Circuits
Content Level 4
Through investigation, I understand the relationship between current, voltage and resistance.
I can apply this knowledge to solve practical problems.
SCN 4-09a
By contributing to investigations into the properties of a range of electronic components, I can
select and use them as input and output devices in practical electronic circuits.
SCN 4-09b
Using my knowledge of electronic components and switching devices, I can help to engineer
an electronic system to provide a practical solution to a real-life situation.
SCN 4-09c
I have carried out research into novel materials and can begin to explain the scientific basis of
their properties and discuss the possible impacts they may have on society.
SCN 4-16a
I have researched new developments in science and can explain how their current or future
applications might impact on modern life.
SCN 4-20a
Having selected scientific themes of topical interest, I can critically analyse the issues, and
use relevant information to develop an informed argument.
SCN 4-20b
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Content Statements
Content National 3
Electricity
Domestic Electricity and Safety
Electrical Circuits
Content National 4
Practical electrical and electronic circuits
Measurement of current, voltage and resistance using appropriate meters in series or
parallel circuits.
Identification and use a range of electrical and electronic components to construct
practical electronic circuits and systems.
Current and voltage relationships in a series circuit.
Practical applications of series and parallel circuits.
Qualitative factors that affect resistance.
Use of the appropriate relationships between voltage, current and resistance in
calculations for series circuits.
Electrical power
Electrical power as a measure of the energy transferred electrically by an appliance
every second.
Power consumption of different appliances, qualitative and quantitative.
Use of the appropriate relationship between power, energy and time to justify energy
saving measures.
Energy efficiency as a key factor in energy generation, distribution and use.
Calculation of efficiency given input and output power/energy.
Content National 5
Practical electrical and electronic circuits
Measurement of current, voltage and resistance, using appropriate meters in complex
circuits.
The function and application of standard electrical and electronic components including
cell, battery, lamp, switch, resistor, variable resistor, voltmeter, ammeter, LED, motor,
loudspeaker, photovoltaic cell, fuse, diode, capacitor, thermistor, LDR.
Current and voltage relationships in a parallel circuit
Use of an appropriate relationship to calculate the resistance of resistors in series and
in parallel circuits.
Ohm’s law
Use of a V-I graph to determine resistance.
Use of an appropriate relationship to calculate potential difference (voltage), current
and resistance.
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Content Statements
Learning Outcomes
At National 4 level, by the end of this section you should be able to:
Practical electrical and electronic circuits
 1. Measure current using an appropriate meter in series and
parallel circuits.
 2. Measure voltage using an appropriate meter in series and
parallel circuits.
 3. Measure resistance using an appropriate meter in series and
parallel circuits.
 4. Identify and use a range of electrical and electronic
components to construct practical electronic circuits and
systems.
 5. State the relationship for current in a series circuit.
 6. State the relationship for voltage in a series circuit.
 7. Use the relationship for current to identify values in a series
circuit.
 8. Use the relationship for voltage to identify values in a series
circuit.
 9. Describe practical applications of series and parallel circuits.
 10. Describe qualitative factors that affect resistance.
 11. Use the appropriate relationships between voltage, current
and resistance in calculations for series circuits.
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Learning Outcomes
Learning Outcomes
At National 5 level, by the end of this section you should be able to:
Practical electrical and electronic circuits
 1. Measure current using an appropriate meter in complex circuits.
 2. Measure voltage using an appropriate meter in complex circuits.
 3. Measure resistance using an appropriate meter in complex circuits.
 4. For the following standard electrical and electronic components:
Cell, battery, lamp, switch, resistor, voltmeter, ammeter, LED,
motor, loudspeaker, photovoltaic cell, fuse, diode, capacitor,
thermistor and LDR.
 Identify the correct circuit symbol.
 Describe its function.
 List an application of the component.
 5. State the relationship for current in a parallel circuit.
 6. State the relationship for voltage in a parallel circuit.
 7. Use the appropriate relationship to calculate current in a parallel
circuit.
 8. Use the appropriate relationship to calculate voltage in a parallel
circuit.
 9. Use the appropriate relationship to calculate the resistance of
resistors in series circuits.
 10. Use the appropriate relationship to calculate the resistance of
resistors in parallel circuits.
Ohm’s law
 1. Determine resistance from a V-I graph using
a) Values selected from the graph
b) the gradient of the graph.
 2. Carry out calculations using the appropriate relationship to calculate
potential difference (voltage), current and resistance.
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Learning Outcomes
Circuit Symbols
Circuit symbols are used because they are quicker to draw and are
easy to recognize.
You need to be able to
Name the component represented by the symbol

Draw the symbol for certain components

Put together symbols to represent a circuit in a circuit diagram.
wires

Wires
Wire
joining
Wires
Energy supplies
measurement
crossing
Voltmeter
Ohmmeter
Ammeter
Ω
Ω
Oscilloscope
Cell
Battery
DC power
AC power
supply
supply
Ω
Photovoltaic
Resistors
Cell
Variable
Resistor
Resistor
LDR
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Thermistor
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Practical Electrical and Electronic Circuits N4/5
components
Circuit Symbols
Lamp (bulb)
Switch
Fuse
Capacitor
devices
Semiconductor
Diode
LED
Npn
MOSFET
transistor
Input device
Output device
Microphone
Loudspeaker
Motor
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Practical Electrical and Electronic Circuits N4/5
Series Circuits
The simplest type of circuit is a series circuit.
In a series circuit there is only ONE path for the electricity to flow
around.
The energy in the circuit comes from the cells. They supply the
voltage for the circuit.
The energy is carried round the circuit by the electrons as the current
flows round the circuit.
As each electron flows through each bulb it gives up some of the
energy carried. This energy is converted into light (and heat).
The wires use up a very tiny amount of energy – this is usually ignored.
If the circuit is broken
– the bulbs go out.
- there is no path for the energy to flow.
If more cells are added – the bulbs get brighter.
- the total energy in the circuit has gone up.
If more bulbs are added – the bulbs get dimmer.
-
the energy available is being shared
among
more components.
We need to be able to measure these changes so we can calculate what is
happening in the circuit.
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Practical Electrical and Electronic Circuits N4
Measuring Current - I
Current is measured using an ammeter.
+
-
The ammeter is connected in SERIES.
A
Break the circuit where you want to
measure the current. Join in the ammeter.
This will need one extra wire.
Start off with the highest current reading, then work down.
If the meter gives a negative reading the wires need to be swopped
round.
Measuring Voltage - V
Voltage is measured using a voltmeter.
The voltmeter is connected in parallel.
Connect the voltmeter on either side of
the component you wish to measure the
voltage across.
Start off with the highest voltage reading, then work down.
If the meter gives a negative reading the wire need to be swopped
round.
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Practical Electrical and Electronic Circuits N4/5
Using a Multimeter
Display
On/Off switch
Resistance (Ω)
Voltage DC
Current DC
Current AC
Voltage AC
10A
A
COM
VΩ
A multimeter is able to measure multiple types of electrical value. The
ones we are most interested in are current, voltage and resistance.
The meter can do other things, but they are less important.
CURRENT
VOLTAGE
RESISTANCE
1. Turn the dial so
1. Turn the dial so
1. Turn the dial so
that it points
that it points
that it points
towards ac current
towards ac voltage
towards resistance.
or dc current as
or dc voltage as
2.
appropriate.
appropriate.
2. Use COM and A
2. Use COM and VΩ
or COM and 10A
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2. Use COM and
VΩ
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Practical Electrical and Electronic Circuits N4/5
Series Circuit Rules
Vs
I1
I2
B1
B2
B3
V1
V2
V3
The current in a series circuit is the same at all points.
I1 = I2
The supply voltage is equal to the sum of the voltage drops.
VS = V1 + V2 + V3
12V
The cell supplies 12V and
the current is 2A at point
B
V
A
5V
A.
The voltage across R1 is 8V.
What is the current at B
and the voltage across R2?
R1
R2
Current is same at all points – so current at B = 2A.
Voltage across R2 = 12 – 5 = 7V
A series circuit has a supply voltage of 24V. There are 4 identical
lamps in the circuit. What is the voltage across each lamp?
Each lamp will have the same voltage across it, so 24/4 = 6V
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Practical Electrical and Electronic Circuits N4
Parallel Circuit Rules
In a parallel circuit there is more than one path for the electricity to
flow around.
The energy in the circuit comes from the cells. They supply the
voltage for the circuit.
The energy is carried round the circuit by the electrons as the current
flows round the circuit.
As each electron flows from the cells it reaches a junction. Some
electrons flow through the first loop – as these electrons pass through
the bulb they give up all the energy carried.
The rest of the electrons travel through the wire until the reach the
next junction. Some electrons flow through the second loop and the
rest flow through the third loop. Again as each electron passes
through the bulb in its loop it gives up all the energy carried.
This energy is converted into light (and heat).
The wires use up a very tiny amount of energy – this is usually ignored.
If the circuit is broken at the cells
- all the bulbs go out.
If a bulb is disconnected - the other bulbs stay on.
If more cells are added – the bulbs get brighter.
If more bulbs are added – the bulbs stay the same brightness as one
another.
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Practical Electrical and Electronic Circuits N5
Parallel Circuit Rules
Vs
Is
Is
I1
I2
I3
B1
V1
B2
V2
B3
V3
The current from the supply is equal to the sum of the current in all
the branches.
Is = I1 + I2 + I3
The voltage across each component in parallel with the supply is the
same.
Vs = V1 = V2 = V3
Vs
R1 and R2 are identical. If
the voltage across R1 is 10V
I4
I1
R1
I3
R2
I4
is 0.4A, what are the values
for I1, I2 and I4 and what is
the voltage across the
2
I2 =I3 so I2 = 0.4A.
and the current through R2
supply and R2?
I1= I4 = I2 + I3 = 0.4 + 0.4 = 0.8A
Vs = V1 = V2 = 10V
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Practical Electrical and Electronic Circuits N5
Circuit Examples
The two switches are in series with
the motor. It will not operate unless
both switches are closed.
This is an example of a safety circuit
on an electric saw. One switch is the
on switch, the other will only be closed
M
if the safety screen is in place.
This parallel circuit allows
a light to be switched on
from either the top or
A
the bottom of the
B
staircase to make the
stairs safe to use in the
dark.
If the connection is
C
moved from A to B (or
from D to C) the circuit is
D
complete and the light will
come on.
Ring main circuit – this is
a special type of parallel
circuit used for the
sockets in a house.
There are two paths to
each socket meaning the
current can be halved and
the wiring can be thinner.
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Practical Electrical and Electronic Circuits N4
Resistance - R
Resistance is the opposition to current flow.
Resistance is measured in ohms – symbol Ω.
There are four things which can be altered about the wire
1. Length
The longer the wire the higher the resistance.
2. Thickness
The thicker the wire the lower the resistance.
3. Temperature
The higher the temperature the higher the resistance.
4. Material
Different materials have different resistances.
Measuring Resistance
We can measure the resistance of a component using an ohmmeter.
Ω
The component should not have any current flowing through it.
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Practical Electrical and Electronic Circuits N4
Using Ohm’s Law – By Calculation
To create resistance in circuits we use components called resistors.
This is more practical than using a long piece of wire.
Variable voltage supply
V
The current through and voltage across a fixed value resistor are
measured when the value of the supply voltage is altered.
Voltage (V)
A graph of current against voltage is plotted
0
Current (A)
There is a direct relationship between current and voltage. This is the
resistance. We can write this as an equation
R=V
Resistance = Voltage
I
Current
This is Ohm’s law, more often written as V = IR.
The gradient of the graph is the resistance of the resistor.
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Ohm’s Law N5
Using Ohm’s Law – By Calculation
V = voltage – volts (V)
V = IR
I = current – Amperes (A)
R = resistance – Ohms (Ω)
A torch bulb has a resistance
A 12V car battery supplies
of 25Ω. When it is operating
current for the starter motor
at the correct voltage the
in a car. The current is 2A –
current through it is 0.24A.
what is the resistance of the
What is the voltage?
wire?
V = IR = 25 x 0.24 = 6V
R = V/I = 12/2 = 6 Ω
A lightbulb is designed to work
A 4kΩ resistor has a current
at 230V. The resistance of
of 2.5mA flowing through it.
the bulb is 200 Ω. What is the
What is the voltage across the
current?
resistor?
I = V/R = 230 /200 = 1.15A
V = IR = 4 x 103 x 2.5 x 10-3
= 10V
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Ohm’s Law N5
Using Ohm’s Law – from Graphs
The values for voltage and current can be plotted on a graph like the
one above. Resistance can be calculated in two ways
1. Choose a point from the graph and use the values for V and I to
calculate R
2. Use the gradient of the graph to calculate R.
Method 1
Method 2
V = 0.6V
Gradient = y2 – y1
I = 6A
x2- x1
= 0.6 – 0.2
6–2
R = V/I = 0.6/6 = 0.1 Ω
= 0.4
= 0.1 Ω
4
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Ohm’s Law N5
Non-Ohmic Devices
A non-ohmic material is one which does not obey Ohm’s Law.
If you plot a graph of current against voltage it will not give a straight
line. The devices below are examples of non-ohmic components.
A filament lamp (bulb) is a non-ohmic device. When the bulb is switched
on current flows through the wire. The wire has resistance and heats up.
When it is glowing white-hot it emits both light and heat.As the
temperature changes the resistance also changes.
The graph for the
current and voltage
in a filament lamp
looks like this.
Different lamps will
have different curves.
Other non-ohmic devices include
light emitting diodes, thermistors
and light dependent resistors.
The graph shows the change in
resistance for a light emitting
diode.
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Ohm’s Law N5
Resistors in Series
If resistors are joined in series with one another the total resistance
increases..
The total resistance can be calculated using
RT = R 1 + R 2 + R 3
The total resistance is always greater than the largest resistance.
A circuit contains two resistors connected in series. The resistors are
15Ω and 27Ω respectively. What is the total resistance of the
circuit?
RT = R1 + R2 = 15 + 27 = 90Ω
A circuit contains three resistors connected in series. The resistors
are 10Ω, 30Ω and 50Ω respectively. What is the total resistance of
the circuit?
RT = R1 + R2 + R3 = 10 + 30 + 50 = 90Ω
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Practical Electrical and Electronic Circuits N5
Resistors in Parallel
If resistors are joined in parallel with one another the total resistance
decreases. The total is always less than the smallest resistor being
added
The total resistance can be calculated
using
To calculate total resistance in parallel
either
1. Use fractions
OR
2. Use x-1 button on the calculator
cUsing Fractions
If R1 = 6Ω, R2 = 12 Ω and R3 = 24 Ω.

Find the lowest common denominator

Remember that this is
 Rt =
= 3.4 Ω
Using the x-1 button on the calculator.
Remember that you should write down the calculation as you go along.

Key 6
Enter
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x-1
x
-1
-1
+ 12 x
-1
+ 24 x
= 7/24
Remember that this is
= This gives the answer.
Practical Electrical and Electronic Circuits N5
Resistors in Parallel
Three resistors are connected
in parallel. Their resistances
are 4Ω, 6Ω and 12Ω, what is
the total resistance?
=>
Rt = 2Ω
A circuit has two resistors connected in parallel. The resistors are
both 20Ω. What is the total resistance?
=
=
Rt = 10Ω
How could you use only 20 Ω resistors to make 5Ω?
Connect four 20 Ω resistors in parallel with one another.
A 25Ω resistor is connected in parallel with a 15Ω and a 30Ω resistor.
What is the total resistance?
1/Rt = 0.14 => Rt = 7.1Ω
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Practical Electrical and Electronic Circuits N5
Mixed Series and Parallel Circuits (Complex Circuits)
Vs
R1 = 10Ω
R2 = 20Ω
R4 = 10Ω
R3 = 20Ω
First you need to get rid of the resistors in parallel (R2 and R3) by
replacing them with a single resistor.


Rt =
= 10Ω
Now the circuit has three 10 Ω resistors in series. These can be
added together to give a total resistance of 30 Ω.
Add R1 and R2 to
find the total
R1 =15Ω
resistance on that
R2 = 15 Ω
side.
Next add R3 and
this value in
parallel to get the
R3 = 10 Ω
overall resistance.
R1
R1 + R2 = 15 + 15 = 30Ω

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
23
Rt = 7.5Ω
Practical Electrical and Electronic Circuits N5
Mixed Series and Parallel Circuits (Complex Circuits)
R1=8Ω
R2=16Ω
R3=8Ω
R4=12Ω
R5=12Ω
R6=6Ω
R7=6Ω
Add together each pair of resistors in series.
R1 + R2 =8 + 16 = 24Ω
R4 + R5 =12 + 12 = 24Ω
R6 + R7 =6 + 6 = 12Ω
This means the circuit can be redrawn as
Next tackle the
24 Ω
24 Ω
8Ω
12 Ω

two sets of
resistors in
parallel.



Rt = 6Ω
Rt = 8Ω
Finally add the two resistors together to give a total of 14 Ω as a
replacement for the seven resistors in the original circuit.
Any circuit can be broken down into series and parallel sections. These
can be rearranged until you end up with one resistor which is the
equivalent to all the others put together.
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Practical Electrical and Electronic Circuits N5
Complex Circuits
20Ω
10Ω
20Ω
10Ω
The resistance of the two 20Ω resistors in parallel is 10Ω.
The resistance of the two 10Ω resistors in parallel is 5Ω.
Total resistance = 10 + 5 = 15Ω
12Ω
36Ω
6Ω
12Ω
6Ω
1. 36 + 12 = 48Ω
2. 12Ω in parallel to 48Ω is the equivalent of 9.6Ω
3. 6Ω in parallel with 6Ω is the equivalent of 3Ω
4. 9.6Ω and 3Ω in series is the equivalent of 12.6Ω
For each section the equations need to be set out properly with all the
steps shown .
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Practical Electrical and Electronic Circuits N5
Voltage Divider Circuit
Vs
R1
R2
The supply voltage is split between the two resistors. This is a very
useful circuit because the correct choice of resistor allows you to
deliver a precise output voltage.
The current in the circuit can be calculated using Ohm’s Law.
I = V = VS
R
R1+R2
To calculate the voltage across each resistor use Ohm’s Law again.
V1 = I R1 = VS R1
V2 = I R2 = VS R2
R1+R2
R1+R2
Voltage divider circuits are often drawn like this.
VS
The calculations are
exactly the same as
R1
above.
If you prefer the
R2
VOUT
0V
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circuit to look like the
first one turn the page
on its side.
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Practical Electrical and Electronic Circuits N5
Voltage Divider Circuit
VS
If Vs is 9V and R1 and R2
are 10Ω and 20Ω
R1
respectively, calculate
the value for Vout.
R2
VOUT
0V
Vout = Vs
R2
= 9 x 20 = 9 x 2 = 6V
R1+R2
10+20
3
VS
A student wants Vout to
R1
be 5.5V. If R1 is 100Ω
and Vs is 6V, calculate the
value needed for R2 .
R2
VOUT
0V
Vout = Vs
R2
= > 5.5 = 6 x
R1+R2
R2
=> 5.5 (100+ R2 ) = 6 x R2
100+ R2
 550 + 5.5R2 = 6R2
 550 = 6 R2 – 5.5 R2
 R2 = 550/0.5 = 1100Ω
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Practical Electrical and Electronic Circuits N5