Topic 3: Electric Circuits — Edexcel A-Level Physics
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Topic 3: Electric Circuits explained
Potential difference measures the work done (energy transferred) per unit charge moved between two points in a circuit.
Read the full explanation
The defining equation is V = W/Q, where W is the work done in joules and Q is the charge in coulombs, so the volt is one joule per coulomb (1 V = 1 J C⁻¹). Rearranging gives W = VQ and Q = W/V. If a charge of 5.0 C moves through a potential difference of 12 V, the work done is W = 12 × 5.0 = 60 J. Because V is defined per unit charge, the same p.d. across twice the charge transfers twice the energy. In an MCQ, decide which two of V, W and Q are given, rearrange accordingly, and check that the answer carries the correct unit (V, J or C).
Your focus
- Explain that potential difference is the work done per unit charge.
- Apply V = W/Q, including rearranged forms, to calculate p.d., work done or charge.
- Interpret answers with correct units, recognising 1 V = 1 J C⁻¹.
Topic 3: Electric Circuits exam tips
Marking Points
- States that potential difference is the work done per unit charge moved between two points.
- Uses V = W/Q with work done in joules and charge in coulombs, giving potential difference in volts.
- Rearranges the equation correctly, for example W = VQ or Q = W/V, depending on which quantities are supplied.
- Recognises that 1 V = 1 J C⁻¹ and interprets a p.d. as energy transferred per coulomb.
- Keeps units consistent, converting prefixes such as mJ or mC before substitution.
Examiner Tips
- 💡Identify the unknown first, then write the rearranged equation so the arithmetic direction is fixed.
- 💡Check the unit of the answer against the quantity asked for: volts for p.d., joules for work done, coulombs for charge.
- 💡Use the definition as a check: a larger p.d. for the same charge must mean more energy transferred.
Common Mistakes
- Confusing potential difference with power: the correction is that V = W/Q is energy per charge, whereas power is energy per unit time.
- Multiplying when the question requires division: the correction is to rearrange V = W/Q to the form matching the unknown before substituting.
- Mixing units, for example using millicoulombs with volts and reporting joules without conversion: the correction is to convert all quantities to SI base units first.