10. Compressing a gas at a constant temperature


Compressing a gas held at a constant temperature

What happens if you take a gas and squeeze it into a smaller volume at a constant temperature? Can we explain what happens in terms of the gas particles?

Think about a piston such as a bicycle pump, with a gas particle bouncing back and forth between the piston head and the opposite wall.

The gas particle causes an outwards pressure on a container walls because it exerts a force on the walls each time it collides and changes direction. With lots of gas particles, the average force of all the collisions acting over the area of the container's walls causes a pressure (p = F/A).

If the temperature is held constant, then the average speed of all the gas particles is also constant. This is because temperature is a measure of the kinetic energy of the gas particles.

If we now push the piston head inwards, squeezing the gas into a smaller volume, then the gas particles now have less space in which to move. The particles collide more frequently with the walls of the container.

This means that the average force exerted by the gas particles on the container walls increases and therefore the gas exerts a greater pressure.

So when the volume of a gas at a constant temperature is decreased (by reducing the container's volume), then the pressure of the gas increases.

GCSE Physics Keywords: Compress, gas, volume, collisions

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