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Date November 2017 Marks available 1 Reference code 17N.2.SL.TZ0.3
Level Standard level Paper Paper 2 Time zone Time zone 0
Command term State Question number 3 Adapted from N/A

Question

Electrical resistors can be made by forming a thin film of carbon on a layer of an insulating material.

A carbon film resistor is made from a film of width 8.0 mm and of thickness 2.0 μm. The diagram shows the direction of charge flow through the resistor.

The resistance of the carbon film is 82 Ω. The resistivity of carbon is 4.1 x 10–5 Ω m. Calculate the length l of the film.

[1]
a.i.

The film must dissipate a power less than 1500 W from each square metre of its surface to avoid damage. Calculate the maximum allowable current for the resistor.

[2]
a.ii.

State why knowledge of quantities such as resistivity is useful to scientists.

[1]
a.iii.

The current direction is now changed so that charge flows vertically through the film.

Deduce, without calculation, the change in the resistance.

[2]
b.

Draw a circuit diagram to show how you could measure the resistance of the carbon-film resistor using a potential divider arrangement to limit the potential difference across the resistor.

[2]
c.

Markscheme

«l = \(\frac{{RA}}{\rho } = \frac{{82 \times 8 \times {{10}^{ - 3}} \times 2 \times {{10}^{ - 6}}}}{{4.1 \times {{10}^{ - 5}}}}\)»

0.032 «m»

a.i.

power = 1500 × 8 × 10–3 × 0.032 «= 0.384»

«current ≤ \(\sqrt {\frac{{{\text{power}}}}{{{\text{resistance}}}}}  = \sqrt {\frac{{0.384}}{{82}}} \)»

0.068 «A»

 

Be aware of ECF from (a)(i)

Award [1] for 4.3 «A» where candidate has not calculated area

a.ii.

quantities such as resistivity depend on the material

OR

they allow the selection of the correct material

OR

they allow scientists to compare properties of materials

a.iii.

as area is larger and length is smaller

resistance is «very much» smaller

Award [1 max] for answers that involve a calculation

b.

complete functional circuit with ammeter in series with resistor and voltmeter across it

potential divider arrangement correct

eg:

c.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
a.iii.
[N/A]
b.
[N/A]
c.

Syllabus sections

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