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GCSE Standard Questions: Electricity in the Home

Q7. (a) Complete the sentence:

The current through an ohmic conductor is directly proportional to the potential difference across the component, provided that the temperature remains constant.

[2 marks]

(b) The graph shows a 'current – potential difference' graph for a filament lamp.

Explain how the resistance of a filament lamp changes as the potential difference across it increases.

The current through the filament increases when the potential difference across the lamp is increased. The current causes the temperature of the filament to increase so the resistance of the filament increases as more current passes through it.

[3 marks]

(c) Many householders are replacing their filament lamps with LED lamps which are more energy efficient. What does more energy efficient mean?

Energy effiecient means a higher proportion/percentage of the total power/energy input is usefully transferred to the desired form (in this case light energy).

If you just wrote that it wastes less energy that was considered insufficient to get you the mark.

[1 mark]

(d) A Light Dependent Resistor (LDR) is used to turn on an outside lamp when it gets dark.

Part of the circuit is shown below:

 

(i) The light intensity decreases. What happens to the potential difference across the LDR and the current in the LDR?

The resistance of an LDR is high in the dark and low in bright light - so as it gets darker the LDR will get a bigger percentage of the potential difference from the battery and the pd across the LDR will increase. As the total resistance in the circuit has increased the current flowing will decrease.

[2 marks]

(ii) What is the resistance of the LDR when the potential difference across it is 4 V?

Give a reason for your answer.

If the PD across the LDR is 4V it has 1/3 of the battery's voltage.

It therefore must have 1/3 of the resistance of that strand of the circuit.... and the 2000Ω resistor has 2/3 of the resistance.

Its resistance is therefore 1000Ω.

[2 marks]

(iii) Calculate the current through the LDR when the resistance of the LDR is 5000 Ω. Give your answer to 2 significant figures.

Total resistance of the circuit is given by the equation for resistors in series:

RTOTAL = R1 + R2 + ....

2000Ω + 5000Ω = 7000Ω

V = 12V

R = 7000Ω

V = IR

I = V/R

I = 12/7000

I = 1.7 x 10-3 A

I = 1.7 mA

[4 marks]

(Total 14 marks)