Increasing the pressure of a gas

Work transfers energy

In physics, work is the transfer of energy by a force. Work is done only when the force causes movement through a distance. For example, when a person pushes down the piston of a bicycle pump, the applied force moves the piston. The person therefore does work on the air trapped inside the pump.

This transfers energy from the person’s chemical energy store, through the moving piston, to the gas. A larger force or a greater movement of the piston means that more work is done.

What happens to the gas

A gas consists of particles in constant random motion. Its internal energy is the total kinetic energy and potential energy of all its particles. When work is done on an enclosed gas, energy is transferred to this internal energy store.

As the piston moves down, it compresses the gas into a smaller volume. Gas particles collide with the inward-moving piston and gain energy. An increase in their average kinetic energy means an increase in the gas’s temperature. The pump and the air near its outlet may consequently feel warm after several rapid strokes.

The explanation is an energy-transfer chain:

force moves piston → work is done on gas → internal energy increases → average particle kinetic energy can increase → temperature can increase

Why the pressure also increases

Compression brings the particles into a smaller volume. They therefore collide with the container walls more frequently. If their average kinetic energy has also increased, their collisions can be more forceful. These effects increase the gas pressure, so the compressed air can be forced through the valve and into the tyre.

Pressure and temperature are related here, but they are not the same quantity. Pressure concerns the force produced by particle collisions on surfaces; temperature is related to the particles’ average kinetic energy.

Why the temperature may not remain high

The specification says that doing work on a gas can increase its temperature. During rapid compression, there is little time for energy to leave the gas, so the temperature rise is easier to notice. Afterwards, energy is transferred from the warmer gas to the pump and surroundings by heating, and the gas cools again. If compression occurs slowly enough for substantial energy to escape while the piston moves, the temperature rise may be smaller.

Energy pathway

A force moves the piston, so work is done on the enclosed gas. Energy is transferred to the gas’s internal energy store. Its particles can gain average kinetic energy, so its temperature can rise.

$\text{work on gas} \rightarrow \text{greater internal energy} \rightarrow \text{higher temperature}$

Bicycle pump

Pushing the piston compresses the air. The smaller volume produces more frequent particle collisions with the walls, increasing pressure. Rapid compression also warms the air because little energy escapes during the stroke.

Keep distinct

• Internal energy: total kinetic and potential energy of all the gas particles.
• Temperature: related to their average kinetic energy.
• Pressure: caused by particle collisions exerting force on surfaces.
• The temperature can rise; transferred energy may subsequently leave by heating the surroundings.