AQA AS Level Physics Paper 2, June 2022: Question 34

1 mark · Medium difficulty · Multiple Choice

Identify the correct definition of electromotive force (emf) in terms of energy dissipated and charge passed.

Practise this question

Question

Multiple-choice question 34 asking to identify what the emf of a practical power supply providing a current I for time t is equivalent to, with four options A, B, C, and D.
Question text

34 A practical power supply provides a steady current I for a time t to an external circuit.

The emf of the power supply during t is equivalent to

[1 mark]

A the energy dissipated in the external circuit.

B the energy dissipated in the whole circuit.

C the energy dissipated in the whole circuit, divided by the product It.

D the potential difference across the terminals of the power supply.

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C, which corresponds to 'the energy dissipated in the whole circuit, divided by the product It'.

34 C (AO2) the energy dissipated in the whole circuit, divided by the product It.

How to answer it

Defining Electromotive Force (EMF)

AQA AS Level Physics • Multiple Choice Question

What this question tests

This question assesses your fundamental understanding of electromotive force (emf) as an energy transformation per unit charge. It tests your ability to connect the definition of emf (total electrical energy supplied to the whole circuit per coulomb of charge) with circuit equations involving current, time, and total energy dissipation.

Question 34 (1 mark)

Evaluating Power Supply EMF and Energy

✅ Correct Answer: C

The energy dissipated in the whole circuit, divided by the product It.

Mark Scheme: C (AO2)

💡 Key Knowledge

  • Definition of emf (E): Energy converted from other forms into electrical energy per unit charge.
  • Formula linking emf to energy and charge: E = W / Q
  • Charge definition: Q = I × t
  • Combining equations yields: E = W / (It) , where W is the total energy dissipated in the whole circuit (including internal resistance).

🧠 Exam Technique

Don't just memorize formulas—understand what every term physically represents. Emf accounts for the entire circuit, driving charge through both internal and external resistances. Always check whether a question refers to the whole circuit or just the external load.

❌ Common Errors

  • Choosing A: Forgetting that a "practical" power supply has internal resistance, meaning energy is also dissipated inside the supply itself.
  • Choosing D: Confusing emf with terminal potential difference. Terminal p.d. only considers energy transferred to the external circuit.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.