Purpose

Distinguish voltage from current and use potential difference correctly when describing a circuit.

Learning objectives

• Define potential difference
• Relate one volt to one joule per coulomb
• Measure voltage in parallel
• Explain source voltage and component voltage drop

Voltage describes an energy difference between two points. It is not a substance flowing through a wire: it tells us how much energy each coulomb could transfer moving between those points.

Core theory

A source separates charge and establishes an electric potential difference. A load converts electrical energy into heat, light, motion, or another form.

Potential is defined at a point relative to a reference; voltage is the difference between two potentials. That is why a voltmeter always uses two probes.

Across a closed-loop circuit, voltage rises supplied by sources balance voltage drops across loads when stored energy is not changing.

Assumptions: The stated energy is electrical energy transferred by the lamp; Values refer to the same interval.

1. Identify: E = 24 J and Q = 2 C.
2. Substitute: V = E ÷ Q = 24 J ÷ 2 C.
3. Calculate: V = 12 J/C = 12 V.

The potential difference across the lamp is 12 V.

Reasonableness check: Each coulomb transfers 12 joules, so two coulombs transfer 24 joules.

Knowledge check

Answer: 12 V. Use V = E/Q: 60 J ÷ 5 C = 12 J/C.

Practical exercise

On a de-energised battery circuit or simulator, label the two points for each voltage reading before calculating or measuring it.

Summary

• Voltage is an energy difference, not a flow
• It is measured between two points
• One volt equals one joule per coulomb