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CAIE A-Level Physics 16.2 First Law of Thermodynamics

Practise calculating gas work, applying the first law with signs and constructing thermodynamic energy balances for heat and internal-energy changes.

Syllabus
2028–2030
Course
Physics 9702
Level
A2

Exam points

  • calculate work during constant-pressure volume changes and distinguish work by and on the gas
  • apply the first law ΔU = q + W with correct signs to calculate energy changes, heat or work
  • construct and interpret thermodynamic energy balances

Covered syllabus points

16.2.1—W = p∆V for the work done when the volume of a gas changes at constant16.2.2—The first law of thermodynamics ∆U = q + W expressed

16.2 The first law of thermodynamics question 1

[Maximum number: 5]

Question (a)

(a)

A fixed mass of an ideal gas at a temperature of 20∘C20^{\circ} \mathrm{C} is sealed in a cylinder by a piston, as shown in Fig. 2.1.

Fig. 2.1

Fig. 2.1

The initial volume of the gas is 1.24×10−4 m31.24 \times 10^{-4} \mathrm{~m}^{3}.
Thermal energy is supplied to the gas and its volume increases by 5.20×10−5 m35.20 \times 10^{-5} \mathrm{~m}^{3}.

[ 2 ]

Question (i)

(i)

The piston is freely moving so that the gas is always at atmospheric pressure.

Atmospheric pressure is 1.01×105 Pa1.01 \times 10^{5} \mathrm{~Pa}.
Calculate the work done by the gas.
work done by gas = J

[ 2 ]

Question (b)

(b)

The gas in (b) is allowed to return to its starting temperature. The piston is now fixed in position.

Thermal energy is supplied to increase the temperature to the same final temperature as in (b).
Use the first law of thermodynamics to suggest and explain how the specific heat capacity of the gas for this situation compares with the value in (b)(iii).

[ 3 ]
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