Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Gas Pressure
What is Gas Pressure?
Gas pressure is the force per unit area exerted on a surface by gas particles colliding with it; each collision changes a particle's momentum and so exerts a force, and pressure is the total force from all collisions divided by the area of the surface.
This definition is part of the Thermal Physics chapter in Cambridge O Level Physics.
Gas Pressure in context
Thermal physics runs along a single chain of reasoning. Particles are arranged and move in a way that depends on the forces between them; heating makes them move faster, which raises temperature; faster particles collide harder and more often, which creates gas pressure and drives expansion; energy supplied at a change of state pulls particles apart instead of speeding them up; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.
In a solid, particles are close together in a regular arrangement, held by strong forces, and vibrate about fixed positions. In a liquid they are still close but disordered, and they move around one another. In a gas they are far apart with negligible forces except during collisions, and move rapidly and randomly. Raising the temperature raises the average kinetic energy of the particles; at −273 °C — absolute zero — the particles have their least kinetic energy. Gas pressure is the force per unit area produced by particles colliding with a surface, and for a fixed mass of gas at constant temperature \(p_1V_1 = p_2V_2\).
Common mistakes with Gas Pressure
- 4. “Gas pressure is caused by particles resting against the wall.” Why it is wrong: gas particles are in continuous free motion and touch the wall only briefly. A resting particle produces no momentum change and therefore no force. Corrected model: pressure arises from the momentum changes of particles rebounding during collisions, averaged over an enormous number of impacts. Exam-safe: “The particles collide with the wall; each collision changes their momentum and exerts a force, and the total force per unit area is the pressure.”
Questions students ask about Gas Pressure
Why does a gas's pressure rise when it is heated at constant volume?
Gas pressure is the force per unit area produced by particles colliding with a surface; each collision changes a particle's momentum and exerts a small force. Heating the gas raises the average kinetic energy of its particles, so they move faster and collide with the walls more often and more forcefully. If the volume cannot change, this greater rate and force of collision increases the pressure.