The current through a diode flows in one direction only. The diode has a…

The idea

A diode only lets current pass one way. Connected in the forward direction it has a very low resistance once the potential difference is above a small threshold, so the characteristic rises steeply. Reverse the connections and its resistance is very high, so the current stays at effectively zero however far the potential difference is turned up, which is why an ammeter in that circuit reads 0.0 A at every setting. The graph is therefore flat along the reverse side and sharply rising on the forward side. Light emitting diodes behave the same way and emit light only when connected the right way round, which is why they are used as indicators and why diodes are used to protect circuits from reversed batteries.

What earns the marks

• Stating that the diode has a very high resistance in the reverse direction.
• Concluding that the current is therefore zero when the connections are reversed.
• Stating that current passes in one direction only.
• Describing the forward characteristic as a low resistance above a small threshold potential difference.

Common mistakes and how to put them right

• Saying the diode blocks the potential difference rather than passing almost no current.

• Claiming the ammeter reads zero because the circuit is broken, when it is complete but high in resistance.

• Drawing a symmetrical characteristic, so showing current in both directions.

• Assuming an LED lights whichever way round it is connected.

Exam tips

• For the reversed LED question, the second mark depends on the first, so lead with the very high reverse resistance.
• An answer of LEDs only allow current in one direction earns one mark on its own, so include it as well.
• Label both halves of a sketched diode characteristic, forward and reverse, so the asymmetry is obvious.