A diode is connected in a circuit with a battery and an ammeter. Explain what happens to the current measured by the ammeter when the diode is connected in the forward direction and then reversed so that it is connected in the reverse direction.

OCR A-Level Physics A (H556) — 3.4 Materials · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student is investigating how a diode controls the flow of current in a circuit. The student connects a diode in series with a battery and an ammeter, first in the forward direction, then turns the diode around so it faces the reverse direction.

Model answer (5 marks)

In the forward direction the diode conducts, so current flows freely through it and the ammeter shows a measurable, significant current.
In the reverse direction the diode behaves as a very high‑resistance element, so almost no current flows through it and the ammeter reads close to zero.

Examiner tips

  • Use the word ‘conducts’ for forward, ‘high resistance’ for reverse. Mention the ammeter reading for each case. Keep the answer short and to the point.
  • common_mistakes
  • :
  • Saying the diode is ‘open’ in reverse without noting the high resistance. Forgetting to mention the ammeter reading in the forward case.

Mark scheme (5 marks)

  1. In the forward direction, current flows (freely) through the diode
  2. The ammeter shows a (measurable / significant) current reading when the diode is in the forward direction
  3. In the reverse direction, the diode has a very large (high) resistance
  4. In the reverse direction, (almost) no current flows through the diode
  5. The ammeter reads (close to) zero when the diode is reversed, because the diode only allows current to flow in one direction

Key terms in this question

diode · current · forward direction · reverse direction · ammeter

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