Thermal Physics: Evaporation, Gas Laws, and Expansion

Definition: The process by which liquid turns to gas below the boiling point.
Mechanism: Particles near the surface with the highest $K.E$ (kinetic energy) escape the liquid.
Cooling Effect: As high-energy particles escape, the average $K.E$ of remaining particles decreases, leading to a drop in temperature.
Factors Affecting Evaporation Rate:
- Temperature: Increase in temperature increases rate (more $K.E$ for particles to escape).
- Surface Area: Increase in surface area increases rate (more surface molecules able to escape).
- Wind Speed: Increase in wind speed increases rate (gives particles more $K.E$ to escape).
- Humidity: Increase in humidity decreases rate (decreases the capacity of air to hold water).
- Atmospheric Pressure: Increase in pressure decreases rate (particles require more $K.E$ to escape).

Boyle's Law: For a fixed mass at constant temperature, pressure is inversely proportional to volume.
Equation: $P_1 \times V_1 = P_2 \times V_2 = \text{constant}$ .
Mechanism: When volume increases, particles hit the walls less often; therefore, average force and pressure decrease.

General Principle: Heating an object increases temperature and average $K.E$ , causing particles to vibrate and take up more space.
Essential Note: Particles themselves DO NOT get bigger.
Relative Order of Expansion: \text{Gases (expand the most)} > \text{Liquids} > \text{Solids (expand the least)}.
Solids:
- Consequences: Railway tracks need gaps and long bridges need rollers to prevent damage during expansion; wires may snap during contraction.
- Bimetallic Strip: Composed of brass and iron. Brass expands faster than iron, causing the strip to bend when heated. Used in fire alarms to complete a buzzer circuit.
Liquids: Expansion is visible to the naked eye and is utilized in liquid-in-glass thermometers.

Definition: At constant pressure, the volume of a gas is directly proportional to its temperature.
Equation: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$ .
Constant: $\frac{\text{Volume}}{\text{Temperature}} = \text{constant}$ .