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Date May 2019 Marks available 1 Reference code 19M.2.SL.TZ2.4
Level Standard level Paper Paper 2 Time zone 2
Command term State Question number 4 Adapted from N/A

Question

Three identical light bulbs, X, Y and Z, each of resistance 4.0 Ω are connected to a cell of emf 12 V. The cell has negligible internal resistance.

The switch S is initially open. Calculate the total power dissipated in the circuit.

[2]
a.

The switch is now closed. State, without calculation, why the current in the cell will increase. 

[1]
bi.

The switch is now closed. Deduce the ratio power dissipated in Y with S open power dissipated in Y with S closed .

 

 

[2]
bii.

Markscheme

total resistance of circuit is 8.0 «Ω» ✔

P =  12 2 8.0 =18 «W» ✔

 

a.

«a resistor is now connected in parallel» reducing the total resistance

OR

current through YZ unchanged and additional current flows through X ✔

bi.

evidence in calculation or statement that pd across Y/current in Y is the same as before ✔

so ratio is 1 ✔

bii.

Examiners report

Most candidates scored both marks. ECF was awarded for those who didn’t calculate the new resistance correctly. Candidates showing clearly that they were attempting to calculate the new total resistance helped examiners to award ECF marks.

a.

Most recognised that this decreased the total resistance of the circuit. Answers scoring via the second alternative were rare as the statements were often far too vague.

bi.

Very few gained any credit for this at both levels. Most performed complicated calculations involving the total circuit and using 12V – they had not realised that the question refers to Y only.

bii.

Syllabus sections

Core » Topic 5: Electricity and magnetism » 5.2 – Heating effect of electric currents
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