© oxford university press 2011 unit p2 part 2 upgrade: model answers 11 electric plug higher tier...

© Oxford University Press 2011

Unit P2 Part 2 Upgrade: Model answers 11

Electric plug Higher tier question

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Question

Explanation of question

Answer

Question (part 1)

The diagram shows a cross section through a three-pin electric plug.



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Answer

Question (part 2)

(a) Each wire has a particular function. Match the wire to its function by drawing lines from the wire to its function.

[3]

Wire Colour Function

Live wire Green / yellow Prevents the device becoming live

Neutral wire Brown Carries the high voltage

Earth wire Blue Completes the circuit



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Answer

Question (part 2)

(b) Some electrical devices are ‘double-insulated’. Explain why these devices do not need earthing.

[2]

(c) A fuse wire works as a safety device. Explain how a fuse wire can reduce the risk of a fire starting if the device develops a fault.

[2]



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Answer

Explanation of question (part 1)

This question addresses the specification statement on household electricity. You are required to recall the structure and wiring of a three-pin plug. You should also understand how a fuse wire is used as an electrical safety device and should be aware that some devices are double-insulated.

Question (a) is a straightforward recall question requiring you to match the name, colour and function of each of the three wires in a three-pin plug.



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Answer

Explanation of question (part 2)

Question (b) requires an understanding of earthing and the meaning of double-insulated.

Question (c) asks for an explanation of the function of a fuse wire. Your answer should show an understanding of current flow and the heating effect of a current.



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D–C answer

(a) Live − brown − Carries the high voltage

Neutral − blue − Completes the circuit

Earth − green / yellow − Prevents the device becoming live

comment

This candidate scores 4/7 which is a grade D–C standard answer.

The answer for (a) is correct and so three marks are awarded.

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D–C answer (continued)

(b) Double-insulated means that there are two insulators present in the

device so they cannot become live.

comment

The answer for (b) does not explain that it is the casing that cannot become live. It is made from an insulator which cannot conduct electricity. The answer does not relate this to role of earth wire. No marks can be awarded.

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D–C answer (continued)

(c) The fuse melts if the current is too high. This stops current flowing into the device because there is a gap in the circuit.

comment

The answer for (c) does not explain that the excess current flowing in the fuse is caused by a fault in the device. Only one mark is awarded for stating that the fuse melting stops the current flowing.

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B–A* answer

(a) Live − brown − Carries the high voltage

Neutral − blue − completes the circuit

Earth − green / yellow − Prevents the device becoming live

comment

In (a), the candidate recalls correctly the structure, colour coding and function of each of the three wires and so scores three marks.

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B–A* answer (continued)(b) Double-insulated devices have an

outer casing that is made of an insulator so they cannot become live. As they cannot become live there is no need for an Earth wire to remove any excess current should the device become faulty as the casing cannot cause a shock.

commentIn (b), the answer is correct but rather long-winded. Two points are made:

i) double-insulated devices have insulating outer casing which cannot become live

ii) as the casing cannot become live, there is no need for earth wire to protect against shock if device becomes faulty.

This scores two marks.NextPrevious .



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B–A* answer (continued)(c) If a fault develops in the device too

high a current flows. This high current exceeds the safety rating of the fuse and the fuse melts, creating a gap in the circuit and preventing any current flowing into the device.

comment

In part (c), the candidate understands that excess current causes the fuse wire to melt. The cause is the fault in device. The effect is that no more current flows. Two marks are awarded.

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© oxford university press 2011 unit p2 part 2 upgrade: model answers 11 electric plug higher tier...

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