Thermal Physics

The Kinetic Molecular Model

The simple kinetic molecular model of matter is used to describe the thermodynamic interactions betweens solids, liquids, and gases.

States of Matter

There are 3 states of matter (excluding plasma) with distinct properties. These states of matter can be distinguished by the forces, distances, and motion of the molecules.

Changing State of Matter

Melting, boiling and sublimation absorb thermal energy from the environment while freezing, condensation, and deposition release thermal energy into the environment. The temperature does not change during melting and boiling because the energy is being used to break the bonds between the particles.

• Melting: Solid to Liquid
• Freezing: Liquid to Solid

• Boiling: Liquid to Gas
• Condensation: Gas to Liquid

• Sublimation: Solid to Gas
• Deposition: Gas to Liquid

Evaporation

During evaporation, the most energetic molecules leave the surface of the liquid. This results in the cooling of the liquid because the average kinetic energy of the remaining particles decreases.

The rate of evaporation can be increased by:

• temperature
• surface area
• wind speed

Evaporation and Boiling

Boiling occurs throughout a liquid while evaporation occurs on the surface of a liquid. Unlike evaporation, boiling can only take place when the temperature reaches the boiling point or range of the substance.

Sweating

Sweating cools down the body due to evaporation.

1. Energy is transferred from skin to water molecules
2. Water in sweat evaporates
3. Most energetic particles leave the surface of the skin
4. The average kinetic energy of the remaining particles is reduced

Pressure Changes

Thermal Expansion

Measuring Temperature

Thermometers and thermocouples can be used to measure temperature.

Thermometer

A thermometer is made of a bulb and tube containing liquid. This liquid is usually mercury or alcohol.

The sensitivity of a thermometer is measured by how much the liquid expands for an increase in temperature.

• Increasing the diameter of the tube will decrease the sensitivity
• Increasing the size of the bulb will increase the sensitivity

Thermocouple

A thermocouple is made up of 2 different metals. The junction of these metals is heated. Both metals are connected to a voltmeter by copper wires.

Thermocouples are used as thermometers for measuring high temperatures and temperatures that vary rapidly.

Thermal Processes

Objects can transfer thermal energy in 3 ways:

• Conduction
• Convection
• Radiation

Conduction

Conduction can technically happen in any medium. However it is more common in solids than fluids.

Conudction occurs when particles collide and transfer their kinetic energy.

• Solids transfer thermal energy from particle to particle by vibrating
• Electrons can transfer thermal energy throughout a metal by colliding with other metal atoms

Metals are made up of a lattice of positive ions and a sea of delocalised (free) electrons.

Vacuum flasks are good at reducing heat lost by conduction because their vacuum layer has no particles. Conduction and convection are not possible in a vacuum.

Convection

Convection is the main method of energy transfer in fluids. Heating fluids can cause convection currents.

1. Heated fluid expands and becomes less dense
2. Heated fluid rises
3. Colder fluid sinks to replace the heated fluid

The convection currents will cycle if the heat source is placed at the bottom of the fluid.

Blankets are good at reducing heat lost by convection because they trap a layer of air. Air is a poor conductor of heat.

Radiation

Thermal energy can travel as infrared waves. Infrared waves can travel in a vacuum unlike methods of thermal energy transfer.

Dark and dull objects will absorb the most heat when it is hot and emit the most heat when it is cold.