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AQA · GCSE · Combined Science · Higher

Changing an LED circuit

Q05.1 2 marks

The camera in a mobile phone uses an LED to provide light when taking a photograph. A student investigated how the potential difference across an LED varies with current in it. Figure 9 shows the circuit used.

Circuit diagram for an LED investigation. A cell, switch, ammeter and variable resistor are in series with an LED. A voltmeter is connected in parallel across the LED. The cell positive terminal is on the right; the LED is shown with light arrows.

Figure 9

Enlarged question image

Circuit diagram for an LED investigation. A cell, switch, ammeter and variable resistor are in series with an LED. A voltmeter is connected in parallel across the LED. The cell positive terminal is on the right; the LED is shown with light arrows.
Figure 9

The student closed the switch. The voltmeter gave a reading of 5.0 V

The ammeter gave a reading of 0 mA

The LED did not emit any light.

Explain how the student should have changed the circuit to make the LED emit light.

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Marking points

  1. 1 Say that the student should reverse the connections to the LED or to the battery.
  2. 2 Explain that an LED or diode only allows current through in one direction.

Full-mark answer

The student should reverse the connections to the LED or the battery, because an LED only allows current through in one direction.

Why this answer loses marks

The student should increase the voltage so the LED lights.

The voltmeter already reads 5.0 V across the LED but the current is 0 mA, so the issue is the LED direction, not simply the size of the potential difference.

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