O-LevelPhysicsElectricity5054/21 · May/June 2018 · Q57 Marks

A student has three resistors; each resistor is labelled 3.9Ω, 3W. She checks the value of the resistance of one of the resistors.

(a)In the space below, draw a circuit diagram of a circuit that she can use to find this resistance. Include a battery, a variable resistor, a voltmeter and an ammeter in the circuit.[2]
(b)(i)When the circuit is turned on, the potential difference (p.d.) across the resistor is 4.2V and the current in the resistor is 1.2A. Calculate the resistance of the resistor. resistance = ................................................................[2][2]
(b)(ii)Shortly after the circuit is turned on, the resistor overheats and gives off smoke. Explain why it overheats. Include a calculation in your answer.[2]
(b)(iii)The other two resistors are also labelled 3.9Ω, 3W. The first resistor is now removed and the student connects these other two resistors in series to the p.d. of 4.2V. State and explain whether these resistors overheat.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[B1] correct circuit symbols for battery, resistor, variable resistor, ammeter and voltmeter
[B1] ammeter in series with resistor and voltmeter in parallel in correct series circuit
[C1] (R=) V/I in any form numerical or algebraic
[A1] 3.5 Ω
[B1] voltage or current or power too large
[B1] calculation of power as 5(.04) W or 5.6 W or max voltage as 3.4(2) V or 3.2(4) V or max current as 0.88 A or 0.93 A
[B1] no and half voltage (2.1 V across each) or half current (0.54 A through each) or quarter power (1.13 W in each)

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