Doing Work on a Gas
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)
Examiner 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