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GCSE Standard Questions: Electricity in the Home

Q10. The lighting in a home may be provided by LED bulbs, filament bulbs or halogen bulbs.

(a)

(i) In the box below, draw the circuit symbol for an LED.

[1 mark]

(ii) The graph shows how the power output from one type of LED varies with the potential difference across the LED.

The LED is connected in series with a battery and resistor.

The potential difference across the LED is 2.0 V.

Use data from the graph to calculate the energy transferred by the LED in 5 minutes.

P = 40 mW (from graph)

P = 40 x 10-3W (or J/s)

t = 5 minutes = 5 x 60 = 300 s

P = E/t

E = Pt

E = 40 x 10-3 x 300

E = 12J

[6 marks]

(b) The resistance of a filament bulb increases as the temperature of the filament increases.

Explain, in terms of the electrons and ions inside the filament, why the resistance increases.

As the temperature increases the ions in the metal lattice of the filament gain energy and vibrate faster (and/or with a bigger amplitude)

If you say 'ions start to vibrate' or that they 'move around' you do not get the mark. They are fixed in the lattice - they just vibrate with more energy than they were already vibrating with as they get hotter.

The electrons collide more frequently with the ions this makes the 'drift velocity' of the electrons decrease.

Saying the electrons 'start to collide' will not get you the mark - they were already colliding with the ions - also just saying there are more collisions is insufficient to get the mark, unless it is clear that electron collision with ions is meant,

[2 marks]

(c) Why are LED bulbs cheaper to use than filament bulbs or halogen bulbs that produce
the same brightness of light?

Any one from:

they use a smaller current (for the same p.d.)

they are more efficient or less energy/power is wasted

they have a lower power input

They 'use less electricity' was ignored as too vague - also 'they last longer' was insufficient to get a mark.

[1 mark]

(Total 10 marks)