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 its container
• The work done when a volume of gas changes at constant pressure is defined as:

W = pΔV

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

 

• This is important, for example, in a steam engine where expanding steam pushes a piston to turn the engine
• For the 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

Gas in a Cylinder Doing Work on a Piston

The gas expansion pushes the piston a distance s

  

Work Done on a Gas

• Doing work on a gas involves a transfer of energy
  • This increases its internal energy and can also cause an increase in the temperature

• Work can be done on a gas by compression
  • A force is used to push a piston by a certain distance
  • This decreases the volume of the gas
  • The molecules move around faster and therefore have a higher kinetic energy
  • This increase in kinetic energy increases its temperature

 

Work Done on a Gas

To compress the gas, a force must be used to move the piston a certain distance. This involves doing work.

 

Differences Between Work Done By or On a Gas

• When a gas expands, work is done by the gas (−W)

When a gas expands, work done W is negative

• When the gas is compressed, work is done on the gas (+W)

When a gas is compressed, work done W is positive

 

Positive and Negative Work

Positive or negative work done depends on whether the gas is compressed or expanded