Back to Daily Question archive

A2 Daily A Level Physics question

2026-04-29 OCR A High level GCSE electricity Module 4.3.3 Internal resistance and electromotive force (OCR Physics A) Module 4.2 Energy, power and resistance — I–V basics (OCR Physics A)

In a lab test, a student varies the load on a single fresh cell with small but non-negligible internal resistance and records the terminal p.d. V for a range of steady currents I, avoiding heating effects. Which graph best represents V against I for this cell over that range?

  1. A A straight line through the origin with positive gradient, showing V increasing in direct proportion to I.
  2. B A straight line with a positive V-intercept equal to the emf at I = 0 and a constant negative slope as I increases. (correct)
  3. C A horizontal line close to the emf, showing almost no change of V with I across the range.
  4. D A curve starting at V = 0 when I = 0, then rising and approaching the emf at higher I.

Answer

The correct answer is B.

Correct: B — A straight line with a positive V-intercept equal to the emf at I = 0 and a constant negative slope as I increases. A treats the cell like a simple resistor (V ∝ I), giving V = 0 at I = 0 and the wrong (positive) slope, ignoring the open-circuit limit that V must equal the emf. B is correct because a real cell’s terminal p.d. is the emf minus a drop that increases proportionally with current, so V starts at the emf when I = 0 and decreases linearly as I grows. C assumes an ideal source with zero internal resistance, giving nearly constant V regardless of I, which contradicts the observed voltage sag under load. D suggests V = 0 when no current flows and that V rises with I, both opposite to the open-circuit limit and the expected decrease due to internal resistance.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews