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Date May 2014 Marks available 1 Reference code 14M.1.HL.TZ1.37
Level Higher level Paper Paper 1 Time zone Time zone 1
Command term Question number 37 Adapted from N/A

Question

A body X of emissivity e is at temperature T1. X is inside a box whose walls act as a black body of temperature T2. T1 is greater than T2.

What is the net intensity of radiation leaving body X?

A. \(\sigma {T_1}^4\)

B. \(e\sigma {T_1}^4\)

C. \(e\sigma {T_1}^4 - \sigma {T_2}^4\)

D. \(e\sigma \left( {{T_1}^4 - {T_2}^4} \right)\)

Markscheme

D

Examiners report

This was the only question that had a negative discrimination index, which means that the weaker candidates tended to prefer the correct response. As e is the emissivity of X and we are asked about the intensity of radiation leaving X, the correct response will be qualified by e. This automatically discounts A and C. The temperature of T2 is clearly relevant as the box is emitting radiation towards X, which X will need to re-radiate. Hence it can only be D.

Syllabus sections

Core » Topic 8: Energy production » 8.2 – Thermal energy transfer
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