What Causes Gas Pressure?
Gas particles are constantly moving and colliding with each other and the walls of their container. Each collision pushes against the wall — this is pressure. The more collisions per second, and the harder they are, the higher the pressure.
Pressure is measured in pascals (Pa) or kilopascals (kPa). Atmospheric pressure at sea level is approximately 101 kPa (101,000 Pa).
Temperature and Pressure
When temperature increases:
- Particles gain more kinetic energy and move faster
- More collisions with the walls, and harder collisions
- Pressure increases (if the volume stays the same)
This is why a sealed aerosol can says "Do not incinerate" — heating increases pressure until the can bursts.
Never heat a sealed container of gas. The pressure can increase enough to cause an explosion. Screw caps on pressure cookers and release valves on steam engines exist for this very reason.
Volume and Pressure (Boyle's Law)
If temperature is kept constant:
- Squeezing a gas into a smaller volume → particles hit the walls more often → pressure increases
- Expanding a gas → fewer collisions per unit area → pressure decreases
This relationship is called Boyle's Law:
So if you halve the volume, the pressure doubles.
Everyday Examples
| Situation | Gas pressure at work |
|---|---|
| Car or bicycle tyre | Compressed air supports the vehicle's weight |
| Bicycle pump | Compressing air increases its pressure |
| Mountain altitude | Less atmosphere above → lower air pressure |
| Weather systems | High pressure = often clear; low pressure = often rain |
| Breathing | Lungs expand to lower pressure, drawing air in |
Real world: Inflating a bicycle tyre
When you pump up a bicycle tyre, you force more air particles into the same volume. The particles collide with the inner tyre wall more frequently, increasing pressure. A correctly inflated tyre (typically 6–8 bar) supports the rider's weight. An underinflated tyre is dangerous because the reduced pressure can cause blowouts.
Real world: Aerosol cans and the danger of heat
Aerosol cans (hairspray, deodorant) contain gas under pressure. The warning "Do not expose to temperatures above 50 °C" is there because heating increases the speed of gas particles, increasing the pressure inside the can until it can rupture or explode. This is Boyle's and Charles's Law in a safety context.
Real world: Breathing and lung pressure
When you inhale, your diaphragm moves down and your ribcage expands — increasing the volume of your lungs. This lowers the pressure inside below atmospheric pressure, and air rushes in from high (atmosphere) to low pressure (lungs). When you exhale, the volume decreases and pressure rises, pushing air out.
Where do you see this in real life? Car tyres, bicycle pumps, weather forecasts (high/low pressure), pressure cookers, and breathing all involve gas pressure. The warnings on aerosol cans are real safety physics.