CAIE A-Level Physics 16.2.1 Work in a Constant-Pressure Volume Change

Practise calculating constant-pressure gas work with W = pΔV, distinguishing work by or on the gas and applying the correct sign and units.

Exam points

calculate work done during a constant-pressure volume change using W = pΔV
distinguish work done by the gas from work done on the gas and apply the correct sign convention
interpret pressure–volume changes and units in a work calculation

[Maximum number: 2]

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

Fig. 2.1

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

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

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

$W=p \Delta V$

C1

$\begin{aligned} =1.01 \times 10^{5} \times 5.20 \times 10^{-5} =(+) 5.25 \mathrm{~J} \end{aligned}$

A1

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16.2.1—W = p∆V for the work done when the volume of a gas changes at constant question 1