A student builds a series circuit containing a battery, a diode and a fixed resistor. The student then reverses the connections to the diode so that it faces the opposite direction. Explain what happens to the current in the circuit when the diode is reversed, and why.

AQA A-Level Physics (7408) — 3.5.1 Current Electricity · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

When the diode is reversed the current in the circuit falls to (effectively) zero – no current flows.

This is because a diode only allows current to pass in one (forward) direction. In reverse bias the diode presents a very high resistance, so for the same battery voltage the current is extremely small.

In forward bias the diode has a low resistance, so current flows freely through the resistor and the battery.

Examiner tips

  • Use the phrase ‘reverse bias’ to show understanding of direction. Mention the high resistance in reverse bias and low resistance in forward bias. Explain the effect on current using Ohm’s law (I=V/R).
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Mark scheme (5 marks)

  1. When the diode is reversed, the current (in the circuit) decreases to (effectively) zero / no current flows
  2. Because the diode only allows current to flow in one (forward) direction
  3. In the reverse direction the diode has a very high / very large resistance
  4. The greater the resistance, the smaller the current (for a given potential difference)
  5. In the forward direction the diode has a low resistance, so current flows freely / before reversal current was able to flow

Key terms in this question

diode · current

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