Testing a low-voltage heater

8 marks · A-level · original practice case

Write your working and explanations below or on paper. Your responses stay in this page; PhysicsHelp does not submit or save them. Copy anything you want to keep before closing it. Check calculations separately from method marks. Use unrounded working between parts.

The situation

A heater is tested at two steady operating points. Each reading is simulated and rounded to the displayed instrument resolution. Use the displayed values for calculations. At the second operating point it runs for 120 s at constant voltage and current.

Steady heater readings
V / V	I / A
2.00	0.400
4.00	0.600

Part (a)

Calculate the resistance at the lower voltage. [2 marks]

Give the numerical answer to 3 significant figures.

Final value / Ω

Working

Part (b)

Calculate the energy transferred in 120 s at the higher voltage. [2 marks]

Give the numerical answer to 2 significant figures.

Final value / J

Working

Part (c)

Deduce whether the heater behaves as an ohmic resistor over these operating points. [2 marks]

Your explanation

Part (d)

Suggest why resistance changes, and state a condition needed for a valid Ohm’s law test. [2 marks]

Your explanation

Marking rubric and worked answers

Indicative practice marks. Method marks assess valid working; accuracy marks require the associated valid method. Independent points stand on their own. Tick only points supported by your response. Do not award the same explanation twice. These ticks record your judgement, not an examiner’s result.

Part (a) marking points

Final answer: 5.00 Ω. Numerical checks assess the final value only; they do not check working or the number of digits typed.

Use R = V/I. (1 method mark)
Obtain 5.00 Ω. (1 accuracy mark)

No earlier numerical result is required. Award method credit for valid working and accuracy credit for the stated answer. Accept a defensible equivalent method; wrong physics cannot earn dependent accuracy credit.

Common errors: This uses I/V rather than V/I.

Part (b) marking points

Final answer: 290 J. Numerical checks assess the final value only; they do not check working or the number of digits typed.

Use E = VIt = 4.00 × 0.600 × 120. (1 method mark)
Obtain 2.9 × 10² J. (1 accuracy mark)

Common errors: This is power in watts; multiply by the operating time for energy.

Part (c) marking points

Resistance increases from 5.00 Ω to about 6.67 Ω. (1 independent mark)
Current is therefore not proportional to voltage across these points. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.

Part (d) marking points

Greater heating can raise the temperature and resistance of a metallic element. (1 independent mark)
Hold temperature constant when testing proportionality of current and voltage. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.

Acceptable alternatives: A temperature-controlled bath or sufficiently short low-power measurements can be suggested, with their purpose stated.

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Related skills

Scope and reference sources

AQA 3.5 and selected 3.7.3/3.7.4; OCR A Modules 4/6 (selected). These are shared original exercises; wording, indicative marks and difficulty have not been endorsed by either board.

AQA specification · OCR A specification

Case reference: heater-test · version 1.1.0 · seed 0. Choose another case