What is potential difference in the context of Cambridge International A Levels Physics?

In Cambridge International A Levels Physics, potential difference, often referred to as voltage, is the measure of the work done to move a unit charge between two points in an electric field. It is measured in volts (V) and is a fundamental concept in understanding how electric circuits operate.

How is electrical power related to potential difference and current?

Electrical power, measured in watts (W), is the rate at which electrical energy is transferred by an electric circuit. It is directly related to potential difference and current through the formula P = VI, where P is power, V is potential difference, and I is current. This relationship is crucial for solving problems in Cambridge International A Levels Physics.

Why is potential difference important in electric circuits?

Potential difference is essential in electric circuits because it drives the flow of electric charge, or current, through the circuit. Without a potential difference, there would be no movement of charge, and thus no current, making it a key concept in the study of electricity at the Cambridge International A Levels.

How can potential difference be measured in a circuit?

Potential difference can be measured using a voltmeter, which is connected in parallel across the component whose voltage you wish to measure. This ensures that the voltmeter does not significantly alter the circuit's operation, a technique often practiced in Cambridge International A Levels Physics experiments.

What is the relationship between potential difference, resistance, and current in a circuit?

The relationship between potential difference, resistance, and current in a circuit is described by Ohm's Law, which states that V = IR, where V is the potential difference, I is the current, and R is the resistance. This fundamental law is a cornerstone of electrical studies in the Cambridge International A Levels Physics curriculum.

Why is "Confusing EMF and PD" a common mistake in Potential difference and power?

Why it happens: Both measured in volts. How to avoid it: EMF: energy converted FROM other forms TO electrical (battery, generator). PD: energy converted FROM electrical TO other forms (resistor, motor). Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Potential difference and power

0% Complete

3 tasks remaining

Short Notes - Potential difference and power

Detailed Study Notes

Detailed notes on Electricity for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

PD and Power Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

PD vs EMF. Electrical power in three forms. Energy consumption.

At a glance

PD = energy converted to other forms / charge.
EMF = energy supplied / charge.
$P = VI$ .
$P = I^2R = V^2/R$ .
kWh = practical energy unit.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

9.2.1 — Distinguish PD and EMF.
9.2.2 — Use $P = VI$ and variants.
9.2.3 — Compute energy consumption.

PD and EMF

Both volts but opposite roles.

PD. Energy converted FROM electrical TO other forms per unit charge (resistor, motor). $V = W/Q$ .

EMF. Energy converted FROM other forms TO electrical per unit charge (battery, generator).

Both measured in volts.

Cambridge tip. Mark scheme rewards clear distinction.

Figure 1 — e.m.f. and potential difference compared

The direction of the energy conversion is what separates them: e.m.f. puts energy into the circuit, potential difference takes it out.

Figure 2 — Potential difference is energy transferred per unit charge

Voltage is never "energy" on its own — it is always energy per unit charge, which is why a small charge at high voltage can carry little total energy.

PD: from electrical.
EMF: to electrical.
Both in volts.

See the full worked example for potential difference and power →

Power

Three equivalent forms.

Power. Rate of energy transfer.

Three forms via Ohm's law $V = IR$ .

$P = VI$ (general).
$P = I^2 R$ (current and resistance).
$P = V^2/R$ (voltage and resistance).

Pick form based on which quantities you know.

Example. 60 W bulb at 240 V: $I = 60/240 = 0.25$ A; $R = 240/0.25 = 960$ Ω.

Energy.

$E = Pt$ in joules.
$E$ in kWh: $P$ in kW × $t$ in hours.
1 kWh = $3.6 \times 10^6$ J.

Cambridge tip. kWh is a unit of ENERGY, not power.

Figure 3 — Electrical power: P = VI = I²R = V²/R

The current is the same all the way round this single loop. The three power expressions are equivalent — pick whichever uses the quantities you already know.

Figure 4 — Choosing the right power formula

Start from whichever two quantities the question gives you. Working out a third quantity first is the usual way to waste time and introduce rounding errors.

Figure 5 — Energy in joules and in kilowatt-hours

A kilowatt-hour is just a large joule: it is a power multiplied by a time, so it belongs on the energy side of every equation.

$P = VI = I^2R = V^2/R$ .
Pick form based on knowns.
kWh = energy unit.

See the full worked example for potential difference and power →

Quick recap

PD vs EMF.
Three forms for $P$ .
Energy = power × time.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

PD and EMF definitions.
$P = VI = I^2R = V^2/R$ .
1 kWh = $3.6 \times 10^6$ J.

How it’s examined

Power on every Paper. Most-tested: $P = VI$ (5 marks), energy consumption (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — Potential difference and power

Step-by-step solutions to past-paper-style questions on potential difference and power, written exactly the way a tutor would explain them at the board.

1Energy from PD (5 marks)
Extended• Adapted from 9702/22 May/Jun 2024• PD
Question
A 12 V battery passes 5.0 C of charge. Find energy transferred. (5 marks)
Step-by-step solution
1. Step 1
  $W = VQ$ .
  $W = 12 \times 5.0 = 60 \text{ J}$
Answer
$W = 60$ J.
2Bulb power (5 marks)
Extended• power
Question
A bulb operates at 60 W with 240 V. Find the current and resistance. (5 marks)
Step-by-step solution
1. Step 1
  $P = VI \Rightarrow I = P/V$ .
  $I = 60/240 = 0.25 \text{ A}$
2. Step 2
  $R = V/I$ .
  $R = 240/0.25 = 960 \text{ }\Omega$
Answer
$I = 0.25$ A; $R = 960$ Ω.
3Energy consumed (5 marks)
Extended• energy
Question
Heater rated 2.5 kW runs for 3.0 h. Find energy used in J and kWh. (5 marks)
Step-by-step solution
1. Step 1
  $E = P \times t$ . In kWh: $2.5 \times 3.0 = 7.5$ kWh.
2. Step 2
  In J: $E = 2500 \times 10800 = 2.7 \times 10^7$ J.
Answer
7.5 kWh = $2.7 \times 10^7$ J.

Key Formulae — Potential difference and power

The formulae you need to memorise for potential difference and power on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Potential difference
$V = \dfrac{W}{Q}$
When to use
Energy per unit charge transferred. Units: volt (V) = J/C.
Electrical power
$P = VI = I^2 R = \dfrac{V^2}{R}$
When to use
Rate of energy transfer. Three equivalent forms via Ohm's law.

Key Definitions and Keywords — Potential difference and power

Definitions to memorise and the exact keywords mark schemes credit for potential difference and power answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Potential difference (PD)
Examiner keyword
Energy converted from electrical to other forms per unit charge. $V = W/Q$ . Units: V.
Electromotive force (EMF)
Examiner keyword
Energy converted FROM another form INTO electrical form per unit charge. e.g. chemical → electrical in battery.
Volt
Unit of PD or EMF. 1 V = 1 J/C.

Common Mistakes and Misconceptions — Potential difference and power

The traps other students keep falling into on potential difference and power questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Confusing EMF and PD
9702 Examiner Reports 2022-2024
Why it happens
Both measured in volts.
How to avoid it
EMF: energy converted FROM other forms TO electrical (battery, generator). PD: energy converted FROM electrical TO other forms (resistor, motor).