Pressure in gases:

The particles in a gas are constantly moving at high speed, and so when they collide with a solid surface, such as the wall of a container or even your own skin, they exert a small force on it; the total of all of these forces together is called gas pressure. Like all forms of pressure, gas pressure is measured in units called Pascals (Pa) where 1 Pa is equal to a force of 1 N acting over an area of 1 m²; the pressure of the air around you right now is 100 kPa (100,000 Pa), which sounds like a lot, but you cannot feel it because it acts on us equally in all directions (not just downwards), and the inwards push from the air pressure is exactly balanced by the outwards push of the pressure of the gases and fluids in our bodies.

Changing the temperature:

When the temperature increases, the gas particles move faster, which means they hit the walls of their container more often and with more force, which increases the gas pressure. This relationship is directly proportional so doubling the temperature (in Kelvin) will double the pressure. Note: This relationship only holds true if the volume and number of particles are kept constant.

Changing the number of particles:

When there are more particles in the same-sized container, they collide with the walls of the container, which increases the gas pressure. Note: The temperature stays the same, so the force of each collision stays the same; it is just that there are more particles to collide. This relationship is directly proportional, so doubling the number of gas particles will double pressure. Note: This relationship only holds true if the volume and temperature are kept constant.

Changing the volume:

As the volume of the container decreases, particles are forced towards the walls of the container, causing them to collide with the walls more often, which increases the pressure. This relationship is inversely proportional so halving the volume doubles the pressure, whilst doubling the volume halves the pressure. Note: This relationship only holds true if the temperature and number of particles are kept constant.

Try this in the simulation:

• Visualising pressure: Pressure is exerted each time a particle collides with the walls of its container; in this simulation, this causes a pink flash on the wall of the container; when particles exert more force on the container, it produces a brighter pink flash. You will also see the pressure gauge in the top right giving a measurement of the pressure; the slight fluctuations are not realistic but happen because there are only 100 or so particles in the container so the collision rate goes up and down a little which varies the pressure a little. In reality, this container would have billions of trillions of particles in it, producing a very even collision rate and a pressure that does not fluctuate.
• Increasing the temperature: When you first load the simulation, with the temperature at 298 K (25 °C) the pressure is about 100 kPa. Double the temperature to 596 K using the Temperature slider (and arrow buttons for fine control), the pressure will double to about 200 kPa; halve the temperature to 149 K and the pressure should halve to about 50 kPa.
• Increasing the number of particles: When you first load the simulation, with the number of particles at 100 the pressure is about 100 kPa. Double the number of particles to 200 using the Number of particles slider (and arrow buttons for fine control), the pressure will double to about 200 kPa; halve the number of particles to 50 and the pressure should halve to about 50 kPa.
• Decreasing the temperature: When you first load the simulation, with the volume at 100% the pressure is about 100 kPa. Halve the volume to 50% using the Volume (container area) slider (and arrow buttons for fine control), the pressure will double to about 200 kPa.
• Understanding pressure: When a particle collides with a surface, the pressure is always exerted at 90 degrees to the surface, no matter the angle of the surface. Switch the Track particle toggle to On and look carefully at what happens when the particle strikes the wall of the container: if it hits faster it produces a brighter pink flash, but also if it hits head-on rather than at an angle it produces a brighter pink flash. This happens because only the component of the particle's velocity at 90 degrees to the surface exerts any force on it.