Doing Work on a Gas (Edexcel GCSE Physics)

Higher Tier Only

• 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

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

Example 1: Increasing Temperature

• 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

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

Example 2: Decreasing Temperature

• 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

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