GCSE Standard Questions: Electric Circuits

Q9. Here is an LED torch.

(a) The torch contains one LED, one switch and three cells.

Which circuit diagram shows the correct circuit for the torch?

Circuit C

[1 mark]

(b) Write down the equation which links charge flow (Q), current (I) and time (t).

charge flow = current × time

Q = It

[1 mark]

The current in the LED was 50 mA.

Calculate the total charge flow through the cells.

Q = It

I = 50 x 10^-3

Q = 50 x 10^-3 x 14,400

Q = 720 C

[3 marks]

(d) When replaced, the cells were put into the torch the wrong way around.

Explain why the torch did not work.

No current flows through a diode when connected in reverse because a diode has a very high resistance in the reverse direction.

[2 marks]

(e) Write down the equation which links efficiency, total power input and useful power output.

Efficiency = (Useful power output)/(Total power input)

[1 mark]

(f) The total power input to the LED was 0.24 W.

The efficiency of the LED was 0.75

Calculate the useful power output of the LED.

Useful power output = Efficiency x Total power input

Useful power output = 0.75 x 0.24

Useful power output = 0.18 W

[3 marks]

(Total 11 marks)