Basic Electricity Summary Notes
Circuit Symbols
Device
Energy Transformation
Loudspeaker
Microphone
Hair straighter
TV
Lift going up
Heater
Ordinary Lamp
Low energy lamp
Resistor
Electrical Sound
Sound Electrical
Electrical Heat
Electrical Sound + Light
Electrical Kinetic + Potential
Electrical Heat
Electrical Light + Heat
Electrical Light
Electrical Heat
1
Series & parallel circuits
There are two types of circuit we can make, called series and parallel.
The components in a circuit are joined by wires.
 if there are no branches then it's a series circuit
 if there are branches it's a parallel circuit
2
Measuring potential difference
 Potential difference is
measured in volts, V.
 The potential difference
across a component in a
circuit is measured using a
voltmeter.
 This must be connected in
parallel with the component.
Voltage across the
bulb
Measuring current
 Current is measured in
amperes (which is often
abbreviated to amps or A).
 The current flowing through a
component in a circuit is
measured using an ammeter.
 An ammeter must be
connected in series with other
components in a circuit.
 This means we need to break into the circuit when we add an
ammeter.
current in the bulb
3
Series circuits
In a television series, you get several episodes, one after the
other. A series circuit is similar. You get several components one
after the other.
 If you follow the circuit diagram from one side of the cell to
the other, you should pass through all the different
components, one after the other, without any branches.
 If you put more lamps into a series circuit, the lamps will be
dimmer than before.
 In a series circuit, if a lamp breaks or a component is
disconnected, the circuit is broken and all the components
stop working.
 Series circuits are useful if you want a warning that one of
the components in the circuit has failed. They also use less
wiring than parallel circuits.
 The current is the same all the way around a series circuit.
 If two components are connected in series the current will be
the same in each.
 The voltages across all the components in a series circuit
add up to battery voltage.
4
As = A1 = A2
The current is
the same in
components
connected in
series
As
A2
The flow of
current into a
component is
always the same
as the current
that flows out
A1
Vs
Vs = V1 + V2
The supply
voltage is shared
out between the
components
V1
5
V2
Parallel circuits
In parallel circuits different components are connected on
different branches of the wire. If you follow the circuit diagram
from one side of the cell to the other, you can only pass through
all the different components if you follow all the branches.
 In a parallel circuit, if a lamp breaks or a component is
disconnected from one parallel wire, the components on
different branches keep working.
 And, unlike a series circuit, the lamps stay bright if you add
more lamps in parallel.
 Parallel circuits are useful if you want everything to work,
even if one component has failed.
 This is why our homes are wired up with parallel circuits.
 The sum of currents in parallel branches is equal to the
current drawn from the supply.
 if two identical components are connected in series, the
supply voltage divides equally across them
 If one component has twice as much resistance as the other,
the voltage across the higher resistance component is
double the voltage across the lower resistance component
 The voltage is the same across components connected in
parallel
6
As = A1 + A2
As
Current splits in
a parallel circuit
The current from
the supply is
equal to the sum
of the currents in
the branches
A1
A2
Vs
Vs = V1 = V2
Voltage is the same
across components
connected in
parallel
V1
The voltages (p.d.s)
across the resistors
connected in
parallel are the
same.
V2
7
Resistance
The resistance of an electrical component is a measure of its opposition
to current.
The two main ways of increasing the current in an electrical circuit are by
increasing the voltage or by decreasing the resistance.
The most common use for a resistor is simply to control the size of the
current flowing in part of a circuit.
The larger the resistance the smaller the current.
The smaller the resistance the larger the current.
Resistance is measured in units of ohms (Ω)
Symbol for resistor:
Symbol for variable resistor:
Uses of Variable resistors







Dimmer Switch
Fan Speed
Scaletrix controller
Petrol Gauge
Heating Element
Radio volume control
Skin Resistance – Lie Detector
8
Measuring Resistance using and Ammeter and a Voltmeter
The quantities voltage, current and
resistance are linked by the
relationship:
resis tan ce 
voltage
Current
The symbol for resistance is R, it
is measured in ohms (Ω)
The symbol for voltage is V, it is measured in volts (V)
The symbol for current is I, it is measured in amperes (A)
Example
A torch lamp takes a current of 0.3 amperes from a 3 volt battery.
Calculate its resistance.
resis tan ce 
voltage
Current
resis tan ce 
3
0.3
resis tan ce 10
9
Ohm’s Law Calculations
Calculating Voltage
A torch bulb hasof resistance 100 Ω has a current of 0.05A flowinh in it.
Calculate the voltage across the bulb.
V  IR
V  0.05 x100
V  5V
Calculating Resistance
An electric fire has a voltage of 230V across it and a current of 5A flowing
in it Calculate the resistance.
V  IR
230  5 xR
230
R
5
R  46
Calculating Current
An electric motor of resistance 200 Ω has a voltage of 5V across it.
Calculate the current flowing in it.
V  IR
5  Ix 200
5
I
200
I  0.25 A
10
1.
a) Describe what is meant by a series circuit.
b) Describe what is meant by a parallel circuit.
2.
Copy out this circuit and show how an ammeter and a voltmeter
should be connected.
What unit would the ammeter
measure in?
What unit would the voltmeter measure in?
3.
4.
5
a) If the current through A1is 5A, what is the current through A2?
a) What is the current from the battery?
A1
6.
In the following circuit the supply voltage is 12V.
Lamp 1
Lamp 2
a) If both bulbs are identical, what is the voltage across lamp 1?
b) If the current through lamp 1 is 3A, what is the current through lamp 2?
11
A2
7.
In this circuit there are 3A going through
the resistor and 4A through the bulb.
a) What current is being drawn from the supply?
b) If the supply is 12V, what is the voltage across i) the resistor and ii)
the bulb?
8.
Complete the following table
Current (A)
Voltage (V)
Resistor
Supply
M
6V
Lamp
3A
Motor
1.5A
4V
Resistor
9. Draw the circuit symbols for a resistor and a variable resistor.
10. How is the variable resistor different from an ordinary resistor?
11. If the resistance of a variable resistor is increased, what happens to
the current?
12. Give two practical uses of a variable resistor.
13 State the energy changes in a resistor.
14 If the voltage across a bulb is 6V and the current through it is 2A.
What is the resistance of the bulb?
15 Calculate the missing values – lay your works as shown on page 10
Voltage/V
Current /I
Resistance / Ω
2
3
100
4
230
3
2
1000
12