Work Done by a Gas (CIE A Level Physics)

• When a gas expands, it does work on its surroundings by exerting pressure on the walls of the container it's in
• This is important, for example, in a steam engine where expanding steam pushes a piston to turn the engine
• The work done when a volume of gas changes at constant pressure is defined as:

W = pΔV

• Where:
  • W = work done (J)
  • p = pressure of gas in piston (Pa)
  • V = volume of gas (m³)

• For a gas inside a cylinder enclosed by a moveable piston, the force exerted by the gas pushes the piston outwards
• Therefore, the gas does work on the piston

The gas expansion pushes the piston a distance s

Derivation

• The volume of gas is at constant pressure. This means the force F exerted by the gas on the piston is equal to :

F = p × A

• Where:
  • p = pressure of the gas (Pa)
  • A = cross-sectional area of the cylinder (m²)

• The definition of work done is:

W = F × s

• Where:
  • F = force (N)
  • s = displacement in the direction of force (m)

• Substituting the equation for force in to the equation for work gives:

W = p × A × s

• The displacement of the gas s multiplied by the cross-sectional area A is the increase in volume ΔV of the gas:

W = pΔV

• This is the equation for the work done when the volume of a gas changes at constant pressure, where:
  • ΔV = increase in the volume of the gas in the piston when expanding (m³)

• This is assuming that the pressure of the gas in the piston p does not change as the gas expands
• When the gas expands (V increases), work is done by the gas
• When the gas is compressed (V decreases), work is done on the gas

Step 1: Write down the equation for the work done by a gas

W = pΔV

Step 2: Substitute in values

                        ΔV = final volume − initial volume = 0.030 − 0.015 = 0.015 m³

W = (1.0 × 10⁵) × 0.015 = 1500 J