Thermal Physics: An Overview

You’re likely familiar with thermometers and taking temperatures, feeling hot and cold, and what it takes to boil water. Now it's time to expand upon your intuitive understanding of heat and temperature and learn how the physicists do it.

In this introduction to thermal physics you will learn what heat and temperature are as well as what phenomena this branch of physics applies to.

The Study of Heat and Temperature

Thermal physics is the study of heat and temperature. Heat is defined as energy that transfers between two objects of different temperatures – moving from the warmer object to the cooler object.

Heat is a type of thermal energy. Thermal energy is the energy associated with the molecular motion within an object. Inside any object, the molecules do not simply stand still; even though you can’t visibly see the motion, they are all jiggling around and bouncing into each other.

Temperature is a measure of the average kinetic energy per molecule. You might be familiar with measuring it in degrees Fahrenheit or even Celsius, but the SI unit that scientists prefer is the Kelvin.

The total internal energy an object has depends on its mass, temperature and specific heat capacity. Specific heat capacity is a measure of how much heat energy is required to raise the temperature of a unit mass by 1 degree. Different materials have different specific heat capacities, and the heat capacity of any particular material can usually be looked up in a table.

Transfer of Heat

Heat can transfer from one object to another in three primary ways. These are:

In conduction, the two objects are in physical contact, and the heat energy moves from the warmer object to the cooler object by direct collisions between molecules in the objects.

In convection, heat is transferred by convection currents. This happens when you boil water on the stove. The water at the bottom of the pan warms up first, and as it warms, it expands, becoming less dense. Being less dense, it rises to the top of the pan as the cooler water sinks and then warms up.

In radiation, heat energy is transferred via electromagnetic radiation. This is how you get energy from the sun. That energy travels through the vacuum of space as radiation, that then warms the Earth when it reaches us.

Phase Changes

As heat energy is added to materials, they increase in temperature. At certain points, called phase transitions, the material changes phase. Materials can change from solid to liquid and from liquid to gas, and even from gas to plasma.

The temperatures at which a phase change occurs depends on the material itself and conditions of pressure. This is studied by using a phase diagram.

The amount of energy required to change the phase of a material depends on that material's latent heat. The latent heat of fusion of a material is the amount of heat energy required to change a unit mass of that substance from solid to liquid. The latent heat of vaporization of a material is the amount of heat energy required to change it from a liquid into a gas.


Thermal physics eventually leads to the study of thermodynamics, which is the branch of physics that studies changing thermal systems by using kinetic theory and statistical mechanics.

There are three laws of thermodynamics that govern thermodynamic processes. These are called, simply, the first law of thermodynamics, the second law of thermodynamics and the third law of thermodynamics. When you first learn about these laws, you will typically learn how they apply to an ideal gas and make use of the ideal gas law.

Thermodynamics can help you understand how steam engines, refrigerators, heat pumps and other similar items work.