WJEC (Wales) Physics GCSE
1.1: Electric Circuits
Detailed Notes
(Content in bold is for higher tier only)
This work by PMT Education is licensed under https://bit.ly/pmt-cc
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Circuit Symbols
Symbols are used to represent the different components of electrical circuits.
Common electrical circuit symbols (studyrocket.co.uk)
Types of Circuit
Series
A series circuit is a closed electrical system with a single path for current to flow. This current is
the same everywhere in the circuit and the sum of voltages across all components in the circuit
is equal to the supply voltage.
A simple series circuit (bbc.co.uk)
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Parallel
A parallel circuit is a branched electrical system with multiple paths (branches) for current to flow
along. The total current at a circuit junction equals the sum of current along each of the branches.
The voltage across each branch is the same.
A simple parallel circuit (bbc.co.uk)
Current, Voltage & Resistance
Current
Current (I) is the flow of electrical charge in a circuit. The greater the rate of flow of charge (Q),
the greater the current:
Q = It
Q is charge flow in coulombs (C), I is the current in amperes (A) and t is the time in seconds (s)
In a single closed loop such as a series circuit, current has the same value at any point and can
be measured in series, using an ammeter. The current through a component depends on both the
resistance (R) of the component and the potential difference (V) across the component.
Voltage
Voltage is also referred to as potential difference (p.d.) and is a measure of the ‘force’ required to
move current around the circuit. It is measured as a change in voltage between two parts of a
circuit (circuit segment), such as before and after a component.
The voltage within a segment is measured in parallel with the segment, using a voltmeter. The
total p.d. across the circuit can be increased by increasing the number of source cells.
Resistance
The components of electrical circuits can restrict the flow of current in a circuit. This effect is
known as resistance. The units of resistance are Ohms (Ω). Current, potential difference and
resistance are related and can be calculated using the equation:
V = IR
V is voltage in volts (V), I is the current in amperes (A) and R is the resistance in ohms (Ω)
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Total resistance in a circuit varies depending on whether the components are connected in series
or parallel. Adding components in series increases the total resistance, as it is the sum of
separate resistances:
RT = R1 + R2 + ...
Adding components in parallel reduces the total resistance in a circuit. This total resistance can
also be calculated as the sum of the reciprocals of each component resistance:
1 = 1 + 1 + …
RT R1 R2
Resistors
Resistance of a component can be investigated by monitoring the current flow through it and
potential difference across it. This is done using a variable resistor within the circuit that can
change the voltage and current.
A circuit to investigate how current changes with voltage for a component (bbc.co.uk)
Characteristic current-voltage curves for common components (bbc.co.uk)
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By taking regular measurements of voltage and current across a component for different applied
resistances in the variable resistor, a current-voltage graph can be produced.
If the current through the component is directly proportional to the potential difference across it,
the resistance of the component (V/I) is constant. This is the case for an ordinary resistor, hence
ordinary resistors can be described as ohmic conductors.
The resistance of components such as lamps, diodes, thermistors and LDRs, however, is not
constant and changes with the current flow through it. This produces a non-linear current-voltage
graph.
A filament lamp has a characteristic curve as its resistance increases with increased current.
When the current is increased, the number of charge carriers (electrons) passing through the
component increases. As electrons pass through the filament lamp, they collide with the atoms in
the filament, transfering energy to these atoms, which becomes converted to heat and light
energy. Therefore, when the current travelling through the filament lamp is increased, more heat
and light energy is produced. At an atomic scale, an increase in heat energy manifests as an
increase in vigour with which atoms vibrate. This restricts the flow of electrons through the
component, otherwise known as an increase in the resistance. This explains why the resistance
(V/I ratio) increases as the current passing through the filament lamp is increased in the graph
above.
Diodes produce a different characteristic curve to filament lamps, since current can only flow in
one direction through them.
There are several different ways that resistance in a circuit can be affected:
Temperature Variations
Even in wires and ordinary resistors (ohmic conductors), the ongoing collision of
electrons with the atoms of the conducting material can result in increased atomic
vibration (heating) over time. As a result the resistance of the material may increase the
longer the circuit is active. This is why it is important to turn the power supply off in
between readings when undertaking circuit-related experiments.
Thermistors are unusual in that their resistance decreases with increasing temperature.
This feature means they are often used in temperature detectors and thermostats.
Length of Circuit
The greater the length, the greater the resistance as electrons have to make their way
through more resistor atoms. Therefore the current flow is reduced.
Light Intensity
LDR (Light Dependent Resistor) have changing resistivity depending on the light level.
The greater the intensity of light, the lower the resistance; therefore the resistance is
greatest when it is dark. These are often used in automatic night lights.
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Characteristic Resistance-Temperature and Resistance-Light-Intensity graphs for Thermisters (left) and LDRs
(right) (bbc.co.uk)
Power
Power is the energy transferred per unit time and it is directly proportional to current and
voltage.
E = Pt
P = IV
E is energy in joules (J) and P is power in watts (W)
Power loss in a component is proportional to resistance, and to the square of the current.
P = I2 R
E is energy in joules (J) and P is power in watts (W)
The energy transferred from chemical potential in batteries to electrical energy in wires depends
on the charge stored and potential difference. This energy is then transferred to any form of useful
energy in the devices they power.
E = QV
E is energy in joules (J) and Q is charge flow in coulombs (C)
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