AQA GCSE Physics

Revision Notes

3.3.3 Work on a Gas

Higher Tier Only

Work on a Gas

  • Work is the transfer of energy by a force
  • 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

compressing-agas, IGCSE & GCSE Physics revision notes

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

  • If a gas is allowed to expand, the gas will do work instead
    • This causes the gas to lose energy, which results in a decrease in temperature
Higher Tier Only

Examples of Doing Work on a Gas

  • An example of doing work on an enclosed gas that leads to an increase in its temperature is a bicycle pump
  • If a thumb is placed on the end of a bicycle pump and it is quickly compressed several times, it will be able to feel the pump getting very warm
    • This is because work is done on the gas, causing its temperature to rise

bicycle-pump, IGCSE & GCSE Physics revision notes

Holding your thumb over the end of a pump whilst pushing the handle causes it to feel warmer

  • The engines in diesel-powered vehicles work in a similar way
  • A mixture of gas and fuel is compressed very suddenly
    • This causes the gas to heat up and ignites the fuel
  • When pressurised carbon dioxide is released from a high-pressure cylinder, the gas does work, which means it loses energy
    • This can cause the carbon dioxide to freeze, forming dry ice

dry-ice, IGCSE & GCSE Physics revision notes

The sudden expansion of carbon dioxide from a cylinder can rapidly cool it, forming dry ice (solid CO2)

Exam Tip

It is important to remember whether the work is done on the gas or by the gas:

  • When work is done on the gas (i.e. it is compressed), the temperature rises
  • When the work is done by the gas (i.e it expands), the temperature falls

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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