PHYSICS
MASTERCLASS
ELECTRICITY
EQUATIONS YOU
NEED TO KNOW
There are no electricity equations given on the
formula sheet. You need to recall all of these
yourself.
CIRCUIT SYMBOLS
You need to be
able to draw all of
these symbols and
also recognise
them in a circuit
diagram.
Note:
Small arrows
indicate light.
Arrows going away
is emission of light
and arrow coming
towards is a
dependence on
light.
ELECTRICAL
CURRENT
When a current flows, there is an overall motion of charge
around the circuit.
Definition:
 Current: the rate of flow of charge
𝑪𝑯𝑨𝑹𝑮𝑬
𝑪𝑼𝑹𝑹𝑬𝑵𝑻 =
𝑻𝑰𝑴𝑬
𝑸
𝑰=
𝒕
 Current (I) is measured in Amperes (A)
 Charge (Q) is measured in Coulombs (C)
 Time (t) is measured in seconds (s)
Note: This equation is often quoted as 𝑸 = 𝑰𝒕
POTENTIAL DIFFERENCE
For a current to flow, there must be a source of Potential
Difference
Definition:

Potential Difference: Energy supplied to each coulomb of
charge
𝑬𝑵𝑬𝑹𝑮𝒀
𝑷𝑶𝑻𝑬𝑵𝑻𝑰𝑨𝑳 𝑫𝑰𝑭𝑭𝑬𝑹𝑬𝑵𝑪𝑬 =
𝑪𝑯𝑨𝑹𝑮𝑬
𝑬
𝑽=
𝑸
 Potential Difference (V) is measured in Volts (V)
 Energy (E) is measured in Joules (J)
 Charge (Q) is measured in Coulombs (C)
Note: This equation is often quoted as 𝑬 = 𝑽𝑸
RESISTANCE
The size of the current in a circuit depends on the potential
difference and the resistance of the circuit.
Definition:

Resistance: Anything in a circuit which opposes the flow of
charge
Current, Potential Difference and Resistance are related:
𝑷𝑶𝑻𝑬𝑵𝑻𝑰𝑨𝑳 𝑫𝑰𝑭𝑭𝑬𝑹𝑬𝑵𝑪𝑬 = 𝑪𝑼𝑹𝑹𝑬𝑵𝑻 × 𝑹𝑬𝑺𝑰𝑺𝑻𝑨𝑵𝑪𝑬
𝑽 = 𝑰𝑹
 Potential Difference (V) is measured in Volts (V)
 Current (I) is measured in Amperes (A)
 Resistance (R) is measured in Ohms (Ω)
EXAM TIP: When giving the unit of current, use either the symbol (A) or the full
name (Amperes), but never write ‘Amps’!
RESISTORS
Notice how all four of these
symbols start with a resistor!
Key points:
 For a resistor made of resistance wire:
resistance increases as the length of wire
increases.
 Some resistors have a fixed value of
resistance. These are called fixed resistors.
 A variable resistor is a resistor where the user
adjusts the value of resistance.
 A Light Dependent Resistor is a resistor whose
resistance increases as light intensity
decreases.
 A Thermistor is a resistor whose resistance
increases as temperature decreases
CURRENT-POTENTIAL
DIFFERENCE (I-V) GRAPHS
Key points:
 I-V graphs are used to show how the current
through a component varies with the potential
difference across it
 We can also tell if the component obeys Ohm’s law
 Ohm’s law: The current through an Ohmic conductor
(at a constant temperature) is directly proportional to
the potential difference across the resistor.
 Most components don’t obey Ohm’s law because
their temperature doesn’t remain constant.
 Components which obey Ohm’s Law are called
Ohmic. Those that don’t are called non-Ohmic.
CURRENT-POTENTIAL DIFFERENCE (VOLTAGE)
GRAPHS
Current
Current
Current
Voltage
1. Resistor (Constant
temp)
Voltage
Voltage
2. Bulb (Filament Lamp)
3. Diode
Non-Ohmic: A diode
Ohmic: Current
Non-Ohmic: As current
only lets current go
increases in
increases the filament
in one direction – it
proportion to PD.
gets hotter. This
has very high
Straight line
increases resistance.
resistance in the
through the origin.
other direction
EXAM TIP: This was a required practical so be ready to answer questions on the
method and analysis of results.
SERIES CIRCUITS
Key points:
 The total resistance is the sum of the resistance of each
component
 There is the SAME current through each component
 The total potential difference (voltage) of the supply is
SHARED between the components
 The potential difference (voltage) provided by cells
connected in series is the sum of the potential difference
of each cell
PARALLEL CIRCUITS
Key points:
 The potential difference across each component is the
same
 The total current through the whole circuit is the sum of
the currents through the separate components
 The total resistance of a parallel circuit is smaller than the
resistance of the smallest resistor
 Adding resistors in parallel decreases the overall
resistance of a circuit.
DIRECT CURRENT
Cells and batteries supply electric current which
always flows in the same direction.
This is called direct current (d.c.).
Direct current always flows the same way
ALTERNATING CURRENT
An alternating current (a.c.) is one which is constantly
changing direction.
a.c. power supply
~
Alternating current constantly changes direction.
The lamp works with a.c. and d.c.
MAINS ELECTRICITY
Key points:
 The electricity supplied to
our homes is called Mains
Electricity.
 It is an alternating current
supply.
 In the UK, mains electricity
is supplied at a potential
of 230V.
 The frequency of mains
electricity in the UK is
50Hz
How electricity arrives
into our homes
THREE CORE CABLE
Electrical cable consists of:
1. A LIVE wire
with BROWN insulation
2. A NEUTRAL wire
with BLUE insulation
3. An EARTH wire
with YELLOW-GREEN striped
insulation.
These are all surrounded by an outer
layer made of rubber or flexible
plastic.
THE EARTH
WIRE
This is a safety feature.
The earth wire is connected to
the metal casing of a device.
The other end of this wire is
connected to a metal rod or
pipe that goes into the ground
below a building.
Appliances that have plastic
cases, for example hairdryers,
do not need the earth wire
connection.
EARTH
THE ACTION OF THE EARTH
WIRE
Appliances with metal cases
metal case of
tumble dryer
fuse
L
insulation
N
heater
such as a tumble dryer are
usually earthed by having the
EARTH wire connected to
their metal case.
Normally current flows to and
fro between the LIVE and
NEUTRAL wires through the
heater of the dryer.
E
earth wire connected
to metal case
The metal case is at zero
volts and is safe to touch.
THE ACTION OF THE EARTH
WIRE
metal case of
tumble dryer
fuse
L
insulation
N
E
earth wire connected
to metal case
heater
If the LIVE wire became
loose inside the dryer it
might touch the metal
case.
The metal case would
now be dangerous to
touch and could give a
fatal electric shock.
However, the EARTH
wire provides a low
resistance path to the
ground.
THE ACTION OF THE EARTH
WIRE
metal case of
tumble dryer
fuse
L
insulation
N
E
earth wire connected
to metal case
A large current now
flows through the
fuse and causes it to
melt.
heater
The dryer’s metal
casing is now
isolated from the
LIVE connection and
is safe to touch.
POWER
Electrical devices are designed to bring about transfers of energy
Definition:

Power: rate of energy transfer
𝑬𝑵𝑬𝑹𝑮𝒀 𝑻𝑹𝑨𝑵𝑺𝑭𝑬𝑹𝑹𝑬𝑫
𝑷𝑶𝑾𝑬𝑹 =
𝑻𝑰𝑴𝑬
𝑬
𝑷=
𝒕
 Power (P) is measured in Watts (W)
 Energy (E) is measured in Joules (J)
 Time (t) is measured in Seconds (s)
Note: This equation is often given in the form 𝑬 = 𝑷𝒕
ELECTRICAL POWER
The electrical power output of a device can be calculated from
the current through the device and the potential difference across
it.
𝑷𝑶𝑾𝑬𝑹 = 𝑪𝑼𝑹𝑹𝑬𝑵𝑻 × 𝑷𝑶𝑻𝑬𝑵𝑻𝑰𝑨𝑳 𝑫𝑰𝑭𝑭𝑬𝑹𝑬𝑵𝑪𝑬
𝑷 = 𝑰𝑽
 Power (P) is measured in Watts (W)
 Current (I) is measured in Amperes (A)
 Potential Difference (V) is measured in Volts (V)
ELECTRICAL POWER
Sometime we might need to calculate the power without knowing
the potential difference across the device.
 However, if we know the Resistance of the device and the
Current through it, we can still calculate the power output.
𝑷𝑶𝑾𝑬𝑹 = 𝑪𝑼𝑹𝑹𝑬𝑵𝑻𝟐 × 𝑹𝑬𝑺𝑰𝑺𝑻𝑨𝑵𝑪𝑬
𝑷 = 𝑰𝟐 𝑹
 Power (P) is measured in Watts (W)
 Current (I) is measured in Amperes (A)
 Resistance (R) is measured in Ohms (Ω)
TRANSFORMERS AND THE
NATIONAL GRID
The National Grid is the system of cables used
to deliver electrical power from power stations
to consumers.
The higher the voltage used, the greater is the
efficiency of energy transmission.
Lower voltages result in higher electric
currents and greater energy loss to heat due to
the resistance of the cables.
At power stations the output voltage of the generators is
stepped up by transformers from 25kV to 132kV.
The voltage may be further increased to up to 400 kV for
transmission over long distance pylon lines.
The voltage is reduced in stages by step-down
transformers to different levels for different types of
consumer.
The lowest level is 230V for domestic use. The final stepdown transformer will be at sub station within a few
hundred metres of each group of houses.
PRACTICE
QUESTIONS
There are five questions to attempt from the AQA specimen
materials.
If you have already completed these, there are also four
questions to attempt from other Edexcel’s specimen
materials.