Pressure in Gases

Colliding Gas Particles Create Pressure

1. As GAS PARTICLES move about at high speeds, they BANG INTO each other and whatever else happens to get in the way. When they collide with something, they EXERT A FORCE (and so a PRESSURE) on it. In a SEALED CONTAINER, the outward GAS PRESSURE is the total force exerted by ALL OF THE PARTICLES in the gas on a UNIT AREA of the container walls.
2. FASTER PARTICLES and MORE FREQUENT COLLISIONS both lead to an INCREASE in net force, and so gas pressure. INCREASING TEMPERATURE will INCREASE THE SPEED, and so the PRESSURE (if VOLUME is kept constant).
3. Alternatively, if TEMPERATURE is constant, increasing the VOLUME of a gas means the particles get more SPREAD OUT and hit the walls of the container LESS OFTEN. The gas PRESSURE DECREASES.
4. PRESSURE and VOLUME are INVERSELY PROPORTIONAL — when volume goes UP, pressure goes DOWN (and when volume DECREASES, pressure INCREASES). For a gas of FIXED MASS at a CONSTANT TEMPERATURE

A Change in Pressure can Cause a Change in Volume

1. The PRESSURE of a gas causes a NET OUTWARDS FORCE at right angles to the surface of its container.

2. There is also a force on the OUTSIDE of the container due to the pressure of the gas AROUND IT.

3. If a container can easily CHANGE ITS SIZE (e.g. a balloon), then any change in these pressures will cause the container to COMPRESS or EXPAND, due to the overall force.

   E.g., if a HELIUM BALLOON is released, it rises. Atmospheric pressure DECREASES with height, so the pressure OUTSIDE the balloon DECREASES. This causes the balloon to EXPAND until the pressure inside DROPS to the same as the atmospheric pressure.

Doing Work on a Gas Can Increase its Temperature

1. If you TRANSFER ENERGY by applying a FORCE, then you do WORK. Doing work on a gas increases its INTERNAL ENERGY, which can increase its TEMPERATURE.
2. You can do work on a gas MECHANICALLY, e.g., with a BIKE PUMP. The gas applies pressure to the PLUNGER of the pump, and so exerts a FORCE on it. Work has to be done AGAINST THIS FORCE to push down the plunger.
3. This transfers energy to the KINETIC ENERGY STORES of the gas particles, increasing the TEMPERATURE. If the pump is connected to a TYRE, you should feel it GETTING WARMER.