A student sets up a series circuit containing a battery, a fixed resistor, and a light-emitting diode (LED). The student then reverses the LED so that it faces the opposite direction. Explain what happens to the current in the circuit when the LED is reversed, and why this occurs.

Edexcel GCSE Physics (1PH0) — 10.1 Electricity and circuits · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When the LED is reversed the current in the circuit falls to zero.
1. A diode only allows current to flow in one (forward) direction.
2. In the reverse direction the LED presents a very high resistance.
3. Because the resistance is so great, the opposition to current flow is enormous and no current can flow through the circuit.

Examiner tips

  • Use the word ‘drops to zero’ or ‘no current flows’ to show full marks for the first point.
  • Mention that a diode is a one‑way device and that reverse bias gives a very high resistance.
  • Explain that high resistance blocks current, linking resistance to current via Ohm’s law.

Common mistakes

  • Saying the LED simply ‘turns off’ without noting the current is zero.
  • Confusing the LED with a resistor and giving a numerical resistance value.
  • Failing to mention that the diode only conducts in the forward direction.

Mark scheme (4 marks)

  1. The current (in the circuit) drops to zero / no current flows when the LED is reversed
  2. A diode / LED only allows current to flow in one (forward) direction
  3. In the reverse direction the LED has a very high / very large resistance
  4. Because resistance is very high, (the opposition to current flow is so great that) current cannot flow / is prevented from flowing through the circuit

Key terms in this question

light-emitting diode · current · series circuit · diode

Related

More Electricity and circuits questions

▶ Try answering this question with AI marking (free) →