Kinetic Theory (Edexcel IGCSE Physics: Double Science)
Revision Note
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Katie MExpertise
Physics
Kinetic Theory of Gases
- Molecules in a gas are in constant random motion at high speeds
- Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
- With the walls of its container
- With other molecules
- The random motion of tiny particles in a fluid is known as Brownian motion
Random motion of gas molecules in a container
- Brownian motion provides evidence that air is made of small particles
- This is because when larger particles, such as smoke particles or pollen, are observed floating in the air:
- The larger particles move with random motion
- This is a result of the larger particles colliding with smaller particles that are invisible to the naked eye
Pressure
- A feature of gases is that they fill their container
- The pressure is defined as the force per unit area
- As the gas particles move about randomly they collide with the walls of their containers
- These collisions produce a net force at right angles to the wall of the gas container (or any surface)
- Therefore, a gas at high pressure has more frequent collisions with the container walls and a greater force
- Hence the higher the pressure, the higher the force exerted per unit area
Gas molecules bouncing off the walls of a container
- It is possible for someone to experience this force by closing their mouth and forcing air into their cheeks
- The strain on the cheeks is due to the force of the gas particles pushing at right angles to the cheeks
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