1. AQA Physics Thermal Physics Section 3 Molecular Kinetic Theory

2. Macroscopic Phenomena Macroscopic phenomena is concerned with bulk matter, that which is observable with the naked eye. Macroscopic variables are measurable such as the temperature, pressure and volume of a gas. Microscopic Phenomena Microscopic phenomena concerns that of very small particles such as atoms and molecules. On an individual scale this is not measurable. Molecular Kinetic Theory Molecular kinetic theory of gases attempts to explain the macroscopic properties of gases, such as pressure, temperature, or volume, by considering the behaviour of microscopic partciles.

3. Solids, Liquids and Gases The macroscopic behaviour of a solid (strength, rigidity, elasticity etc.) can be described in terms of the microscopic model of a solid. A solid consists of a lattice where particles are allowed to vibrate but not move relative to each other. The macroscopic behaviour of a liquid (ability to flow, uncompressible etc.) can be described in terms of the microscopic model of a liquid. A liquid consists of particles that have the ability to move relative to each other, but have sufficient proximity that the forces that exist between them are not negligible. The macroscopic behaviour of a gas (ability to flow, compressible etc.) can be described in terms of the microscopic model of a gas. A gas consists of particles that have the ability to move relative to each other, but have sufficient proximity that the forces that exist between them are almost negligible.

4. Ideal Gases Kinetic Theory makes four assumptions about an ideal gas: 1.The particles of gas have zero volume. 2.There are no intermolecular forces between the particles. 3.Particles undergo perfectly elastic collisions. 4.The time between collisions is much greater than the time during a collision.

5. Pressure Macroscopic Observation: An enclosed gas exerts a pressure on the side of the container which it is held. Microscopic Explanation: 1. A particle travels towards a wall of the container. The momentum (P) of the particle is mv

6. Pressure Macroscopic Observation: An enclosed gas exerts a pressure on the side of the container which it is held. Microscopic Explanation: 2. The particle rebounds off the side of the wall elastically and travels in the opposite direction. The momentum (P) of the particle is now –mv The change in momentum (  P) of the particle is therefore 2mv

7. Pressure Macroscopic Observation: An enclosed gas exerts a pressure on the side of the container which it is held. Microscopic Explanation: The force exerted on the particle can be calculated by Newton’s Second Law: Newton’s Third Law states an equal and opposite force is exerted on the walls of the container. The pressure on the walls of the container is proportional to this force.

8. Boyle’s Law Macroscopic Observation: The pressure of a gas is inversely proportional to the volume it occupies when kept at constant temperature. HYPERLINK Microscopic Explanation: With less space to move the particles collide with each other and the walls of the container more frequently. The net force on the walls and hence the pressure therefore increases.

9. Gay-Lusaac’s Law Macroscopic Observation: The pressure of a gas is proportional to its temperature when kept at constant volume. Microscopic Explanation: Particles move with a higher speed and therefore the change in momentum when they strike the walls of the container is greater. Change in momentum is equal to force, and force is proportional to pressure.