Warming up a gas by speeding up its particles

Demonstration

Apparatus and Materials

• Metal-bodied bicycle pump
• Thermocolour film, cut to suitable size

Health & Safety and Technical Notes

Read our standard health & safety guidance

ALTERNATIVE method: demonstrates how compressing a gas increases its temperature. A small piece of cotton wool is placed into the bottom of a narrow plastic tube. When the air is rapidly compressed by a piston, the air temperature increases and the cotton ignites. The 'fire piston' can be used to illustrate the transfer of energy, kinetic theory and Charles' law.

Procedure

1. Fully extend the pump and block the hole at the bottom with a close-fitting bolt and PTFE tape. Attach the thermocolour film to the sides of the cylinder near the bolt.
2. Make sure that the bicycle pump is cool. The temperature should be at the bottom of the range that the film will indicate.
3. Fully extend the pump and squash the air up rather suddenly with one good push. Leave the piston at the position of maximum compression.
4. Watch for the temperature rise of the cylinder, as shown by the thermocolour film attached to it.

Teaching Notes

• This is another simple experiment that can be explained through kinetic theory. Ask students to explain what an increase in temperature of a gas suggests about the particles of the gas. Follow this up by asking for explanations of the increased particle speed. It may take some time before the class is happy that collisions with the approaching piston increase the velocities of the gas particles. (Consider the momentum changes of an air particle as it rebounds from the approaching piston.)
• This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton:

  AS/A2 Advancing Physics

This experiment was safety-tested in June 2004.

• This video shows how to set up a 'fire piston' to illustrate the transfer of energy, kinetic theory and Charles' law: