GCSE Physics revision

Direct and alternating potential difference

Domestic uses and safety

AQA 4.2.3.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain the difference between direct and alternating potential difference.

MOST 🎯🎯

Apply the scientific explanation of direct and alternating potential difference to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving direct and alternating potential difference and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

The V = IR Equation

The equation V = IR states that potential difference (voltage) equals current multiplied by resistance, making it the most important equation in the electricity topic.
Potential difference is measured in volts (V), current in amps (A), and resistance in ohms (Ω).
To find resistance, rearrange the equation to R = VI ; for example, a 24 V battery producing 8 A gives a resistance of 3 Ω.

1. The V = IR EquationAQA

Notes

The equation V = IR states that potential difference (voltage) equals current multiplied by resistance, making it the most important equation in the electricity topic.
Potential difference is measured in volts (V), current in amps (A), and resistance in ohms (Ω).
To find resistance, rearrange the equation to R = VI ; for example, a 24 V battery producing 8 A gives a resistance of 3 Ω.

Relationship Between Voltage, Current, and Resistance

As long as resistance remains constant, increasing the potential difference causes the current to increase proportionally.
This proportional relationship means that if you double the voltage, the current also doubles, provided resistance stays the same.
Temperature must remain constant for this proportional relationship to hold, as increasing temperature increases resistance.

2. Relationship Between Voltage, Current, and ResistanceAQA

Notes

As long as resistance remains constant, increasing the potential difference causes the current to increase proportionally.
This proportional relationship means that if you double the voltage, the current also doubles, provided resistance stays the same.
Temperature must remain constant for this proportional relationship to hold, as increasing temperature increases resistance.

Current–Potential Difference Graph for Resistors and Wires

A current vs. potential difference (I–V) graph for a wire or resistor produces a straight line through the origin, indicating constant resistance.
A steeper gradient on the I–V graph indicates a lower resistance, as less potential difference is needed to drive a given current.
A shallower gradient indicates a higher resistance; for example, a 10 Ω resistor produces a less steep line than a smaller resistor.
The negative region of the graph shows the same linear relationship but with the battery connected in reverse, giving negative values of both current and voltage.

3. Current–Potential Difference Graph for Resistors and WiresAQA

Notes

A current vs. potential difference (I–V) graph for a wire or resistor produces a straight line through the origin, indicating constant resistance.
A steeper gradient on the I–V graph indicates a lower resistance, as less potential difference is needed to drive a given current.
A shallower gradient indicates a higher resistance; for example, a 10 Ω resistor produces a less steep line than a smaller resistor.
The negative region of the graph shows the same linear relationship but with the battery connected in reverse, giving negative values of both current and voltage.

Why Real Wires Don't Always Give Perfectly Straight Lines

In practice, higher currents cause wires to heat up over time, which increases resistance and causes the I–V graph to deviate from a perfectly straight line.
This heating effect means the assumption of constant temperature is an idealisation that does not always hold in real circuits.

4. Why Real Wires Don't Always Give Perfectly Straight LinesAQA

Notes

In practice, higher currents cause wires to heat up over time, which increases resistance and causes the I–V graph to deviate from a perfectly straight line.
This heating effect means the assumption of constant temperature is an idealisation that does not always hold in real circuits.

Filament Lamps

A filament lamp contains a very thin metal wire (filament) that heats up as current flows through it until it emits light.
As the filament heats up, its resistance increases significantly, causing the I–V graph to curve and become less steep at higher voltages.
The curved shape of the filament lamp's I–V graph shows that less current flows per unit of potential difference as temperature and resistance rise.

5. Filament LampsAQA

Notes

A filament lamp contains a very thin metal wire (filament) that heats up as current flows through it until it emits light.
As the filament heats up, its resistance increases significantly, causing the I–V graph to curve and become less steep at higher voltages.
The curved shape of the filament lamp's I–V graph shows that less current flows per unit of potential difference as temperature and resistance rise.

Diodes

A diode is a component that only allows current to flow in one direction, known as the forward direction.
In the reverse direction, a diode has an extremely high resistance, meaning effectively no current flows when the potential difference is negative.
On an I–V graph, a diode shows no current for negative potential differences and a rapid increase in current once a threshold positive potential difference is reached.

6. DiodesAQA

Notes

A diode is a component that only allows current to flow in one direction, known as the forward direction.
In the reverse direction, a diode has an extremely high resistance, meaning effectively no current flows when the potential difference is negative.
On an I–V graph, a diode shows no current for negative potential differences and a rapid increase in current once a threshold positive potential difference is reached.

Direct Potential Difference (DC)

A direct potential difference produces a direct current (DC), which flows in one direction only and never reverses.
Batteries are a common example of a DC source, always fitted into a socket in one way to ensure current flows in the correct direction.

Alternating Potential Difference (AC)

An alternating potential difference produces an alternating current (AC), which constantly changes direction between positive and negative.
The frequency of an AC supply describes how many times per second the current changes direction, measured in Hertz (Hz).

Mains Electricity in the UK

Mains electricity in UK homes and schools runs on an alternating current (AC) supply, meaning the current is always changing direction.
The frequency of UK mains electricity is 50 Hz, meaning the current changes direction 50 times per second.
The voltage of UK mains electricity is 230 V, which is far higher than the 2–8 V typically used in simple circuit diagrams.

9. Mains Electricity in the UKAQA

Notes

Mains electricity in UK homes and schools runs on an alternating current (AC) supply, meaning the current is always changing direction.
The frequency of UK mains electricity is 50 Hz, meaning the current changes direction 50 times per second.
The voltage of UK mains electricity is 230 V, which is far higher than the 2–8 V typically used in simple circuit diagrams.

Why Mains Electricity is Dangerous

The high voltage of 230 V in mains electricity makes it significantly more dangerous than the low voltages used in battery-powered circuits.
It is important to remember both key values — 50 Hz and 230 V — as these will not be provided in AQA exam questions.

Exam Tips: AC vs DC

In an exam question asking for the difference between AC and DC, you must fully define both: AC constantly changes direction, whilst DC flows in one direction only.
Always use precise scientific language and include both definitions to secure full marks on a two-mark question.