Energy and particles · GCSE Physics
Gas pressure
Teacher-written GCSE Physics revision on gas pressure: particle collisions with container walls, why heating at constant volume raises pressure, and how volume and temperature change the collision rate.
Gas pressure is collisions with the walls. Faster particles, more frequent hits, or a smaller area of wall, mean a larger pressure. Pressure is force over area.
The important bits
What you need to know
- 1
A gas is mostly empty space: particles move randomly at high speed and collide with each other and with the container walls.
- 2
Each collision with a wall exerts a tiny force. Lots of collisions on an area give a pressure. Pressure p = F / A, in pascals (Pa). 1 Pa = 1 N/m².
- 3
At constant volume, raising temperature raises pressure because particles move faster: they hit the walls more often and with more force.
- 4
At constant temperature, decreasing volume raises pressure because particles hit the walls more often in a smaller space.
- 5
Absolute temperature in kelvin is proportional to the average kinetic energy of the particles. 0 °C is 273 K. Never use a negative kelvin value.
- 6
A vacuum has fewer particles, so fewer collisions and lower pressure. Space is a very good approximation to a vacuum.
- 7
Brownian motion of smoke particles is evidence that unseen air particles are moving randomly and colliding.
- 8
On Higher / Physics routes, pV = constant at constant temperature for a fixed mass of gas. If V halves, p doubles, with p in pascals and V in m³.
Quotations worth analysing
Short evidence. Real method.
“The pressure of a gas is caused by particles colliding with the walls of the container.”
Collisions, not particles “wanting more space”. Force on an area is the definition of pressure; the particle story explains where the force comes from.
“Increasing the temperature of a gas at constant volume increases the pressure.”
Faster motion means harder, more frequent collisions. Name both “more often” and “with more force” for the full explain mark.
Go deeper
Gases are collisions with the container
Each bounce on a wall exerts a tiny force. Lots of bounces on an area give pressure. Squeeze the same gas into a smaller volume at constant temperature and the particles hit the walls more often, so pressure rises. Heat the gas at constant volume and they hit harder and more often. Vacuum questions work the same way: fewer particles, fewer collisions, lower pressure. You do not need the full gas laws on every specification, but you do need the particle explanation. If pV = constant appears, keep p in pascals and V in cubic metres, and keep the temperature and the mass of gas the same. A bicycle pump is this idea in the hand: smaller volume, larger pressure, until the tyre’s valve opens.
Go deeper
Temperature is a kinetic-energy story in kelvin
Particle kinetic energy depends on absolute temperature. Doubling the kelvin temperature roughly doubles the average kinetic energy, which is why pressure at constant volume tracks kelvin temperature for an ideal gas. Converting 27 °C to 300 K is not decoration: using 27 in a proportion would be nonsense. Absolute zero is 0 K, where (in the model) particles would have minimum kinetic energy. You will not reach it in a school lab, but the idea explains why “twice as hot” must mean twice the kelvin temperature, not twice the Celsius reading. Brownian motion is the visible clue: pollen or smoke jerks because unseen gas particles punch it from random directions.
See the idea in action
A sealed syringe holds 12 cm³ of air at 100 kPa. The volume is slowly reduced to 8.0 cm³ at constant temperature. For a fixed mass, p₁V₁ = p₂V₂. Convert if you wish: the ratio of volumes is enough here. p₂ = p₁ × (V₁ / V₂) = 100 × (12 / 8.0) = 150 kPa. Pressure rose because the same particles hit the walls more often in the smaller volume. If the syringe were then warmed at that new volume, pressure would rise further because collisions would also become harder and more frequent.
Exam technique
Turn knowledge into marks
Explain gas pressure with collisions, not with particles “wanting more space”. For temperature, say faster particles hit more often and harder. Keep kelvin for any proportion with temperature. Use pV = constant only at constant temperature.
Common mistakes
Do not give these marks away
- 01
Saying particles in a gas do not collide with the walls, or that they “need more space”.
- 02
Using Celsius in a temperature-ratio calculation instead of kelvin.
- 03
Applying pV = constant when the gas is also being heated, or mixing kPa with Pa inconsistently.
Why does the pressure of a gas in a sealed, rigid can increase when it is heated?
AThe particles expand and take up more space
BThe particles move faster, so they collide with the walls more often and with greater force
CThe number of particles increases
DThe volume of the can increases, reducing the collision rate
Show the answer
The particles move faster, so they collide with the walls more often and with greater force. A rigid can keeps volume constant. Heating raises particle kinetic energy, so collisions are more frequent and more forceful, increasing pressure.
Quick questions
If this is the bit you searched
What causes gas pressure?
Particles colliding with the walls of the container. The total force on an area is the pressure: p = F/A.
What happens if you squeeze a gas at constant temperature?
Volume falls, particles hit the walls more often, and pressure rises. For a fixed mass, pV stays constant on Higher-tier treatments.
Why convert Celsius to kelvin?
Average particle kinetic energy is linked to absolute temperature. 0 °C is 273 K; twice the Celsius temperature is not twice the kinetic energy.
What does Brownian motion show?
Random motion of visible particles (smoke, pollen) caused by collisions with unseen, randomly moving air particles.