A-LevelPhysicsThermodynamics9702/42 · May/June 2019 · Q211 Marks

The first law of thermodynamics may be expressed in the form ΔU = q + w.

(a)(i)State, for a system, what is meant by: 1. +q 2. +w.[2]
(a)(ii)State what is represented by a negative value of ΔU.[1]
(b)(i)State and explain the total change in internal energy of the gas for one complete cycle ABCA.[2]
(b)(ii)Calculate the external work done on the gas during the expansion from point C to point A. work done = ................................................................ J[2]
(b)(iii)Complete Fig. 2.2 for the changes from: 1. point A to point B 2. point B to point C 3. point C to point A.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

2(a)(i) 1. energy transfer to the system by heating B1 2. (external) work done on the system B1
2(a)(ii) decrease in internal energy B1
2(b)(i) no change (in internal energy) B1 (because) no change in temperature B1
2(b)(ii) work done = PΔV C1 = (-)1.6 × 105 × (2.4 – 0.87) × 10-3 = (-)240 J A1
2(b)(iii) first row all correct (0, 480, 480) A1 second row all correct (-1100, 0, -1100) A1 final column of third row calculated correctly from the two values above it, so that the final column adds up to 0 A1 second column in final row correct, with correct negative sign A1 and first column in final row calculated correctly so that it adds to the second column to give the third column (fully correct table is: 0 480 480 -1100 0 -1100 860 -240 620 )

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