Work on a Gas (AQA GCSE Physics)

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Ashika

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Ashika

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Work on a Gas (HT 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

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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

Examples of Doing Work on a Gas (HT only)

  • 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

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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

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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

Ashika, Physics Project Lead

Teacher tip

Ashika

Physics Project Lead

When teaching work done on gases to GCSE students, I would often find that they struggled to visualise the concepts. So I would get the students to act out the scenario. They would be the gas particles moving around, (safely and sensibly) bumping into one another, and bumping into the walls of the room. I would then walk towards them with my arms stretched out to the sides, forcing them into a smaller and smaller section of the room. This exercise models work being done on a gas by decreasing the volume of the container. Every time they bump into another particle, they gain kinetic energy from that particle, so they move faster and faster, hence the kinetic energy and therefore the temperature of the gas increases.

Then we reverse it. The students are now allowed to (safely and sensibly) push me back so that the section of the room they move around in increases. This time the particles lose kinetic energy and begin to move more slowly as the gas does work on the container. The kinetic energy of the gas decreases so the temperature of the gas decreases. Although the students were often reluctant at first to act out being a gas particle, they always understood it better once they had given it a try.

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Ashika

Author: Ashika

Expertise: Physics Project Lead

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.