| 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 | Calculate | 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.
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.
State why knowledge of quantities such as resistivity is useful to scientists.
The current direction is now changed so that charge flows vertically through the film.
Deduce, without calculation, the change in the resistance.
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.
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»
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
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
as area is larger and length is smaller
resistance is «very much» smaller
Award [1 max] for answers that involve a calculation
complete functional circuit with ammeter in series with resistor and voltmeter across it
potential divider arrangement correct
eg:
