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Thermal Energy

by Ron Kurtus (revised 5 November 2014)

The thermal energy of an object or system consists of the portion of its internal energy that is responsible for the temperature of the system and is involved in heat transfer. It can also be called the thermal kinetic energy.

The internal energy of a system is the total thermal kinetic energy and thermal potential energy of all its atoms and molecules. The Kinetic Theory of Matter is explains the kinetic energy of particles in an object. Kinetic energy can be translational, rotational, or vibrational. The kinetic energy of an atom can influence the kinetic energy of other atoms.

Temperature is an approximate measure of the average kinetic energy of the atoms and molecules of an object. Heating is the transfer of thermal energy between objects due to their temperature difference.

Questions you may have include:

• What is the role of the Kinetic Theory?
• What is the temperature consist of?
• What is heating?

This lesson will answer those questions. Useful tool: Units Conversion

Based on Kinetic Theory

The concept of thermal kinetic energy is based on the Kinetic Theory of Matter (also called Kinetic Molecular Theory of Matter or simply Kinetic Theory).

According to the theory, matter is composed of a large number of atoms or molecules that are in constant motion. This motion may be translational (as in a gas), rotational (as in a liquid) or vibrational (as in a solid). Most of the motion of these particles comprises their kinetic energy. The kinetic energy of an atom can then influence the kinetic energy of other atoms.

The total kinetic energy of all the particles is called the thermal energy or thermal kinetic energy of the object. This is opposed to the kinetic energy of the whole object (like a moving object) and the kinetic energy of a single particle.

However, the particles also have their own potential energy. Thermal potential energy is potential energy at the atomic and molecular levels, where it has the potential of becoming kinetic energy or related forms of energy. Common types of thermal potential energy are chemical bonds, electrostatic forces, and nuclear bonds.

(See Thermal Potential Energy for more information.)

The combination of the total kinetic energy and potential energy can be called the internal energy of the object. Certain internal reactions can convert some potential energy to kinetic energy.

Note: Some sources and Physics textbooks say that thermal energy is total kinetic and potential energy and is the same as internal energy. Unfortunately, this is a confusing concept. For one thing, potential energy does not affect temperature or heat until it become kinetic energy. Check with your instructor on what is used in your class.

Temperature

Whereas thermal energy primarily consists of the total kinetic energy in an object, temperature is a measure of the average kinetic energy of an object.

The kinetic energies of individual atoms vary in an object. However, you can find the average kinetic energy of them, which is the temperature.

For example, if the temperature of two kilograms of steel is the same as one kilogram of steel, the average kinetic energy of the molecules of each is the same. However, the two kilogram object would have twice the thermal energy as one kilogram of steel.

(See the lesson on Temperature for more information.)

Heating

To heat something means to transfer thermal energy from an object or system of higher temperature to one of a lower temperature.

Since temperature is the average kinetic energy of an object, the transfer of thermal energy not only involves the kinetic energy but also the potential energy of the object.

Heating is defined as the flow of thermal energy from an object of one temperature to an object of another temperature. You feel the heating process when warm air from a furnace reaches you.

(See Heat for more information on that subject.)

Summary

The thermal energy (or thermal kinetic energy) of an object or system consists of the portion of its internal energy that is responsible for the temperature of the system and is involved in heat transfer.

The internal energy of a system is the total thermal kinetic energy and thermal potential energy of all its atoms and molecules. The Kinetic Theory of Matter is explains that the kinetic energy of particles can be translational, rotational, or vibrational.

Temperature is an approximate measure of the average kinetic energy of the atoms and molecules of an object. Heating is the transfer of thermal energy between objects due to their temperature difference.

Observe and learn

Resources and references

Ron Kurtus' Credentials

Websites

Heat and Thermodynamics - HyperPhysics flowchart

Thermal Energy - HyperPhysics

Internal Energy - HyperPhysics

Thermal Energy - Wikipedia

Internal Energy - Wikipedia

Thermal Energy Facts - SoftSchools.com

Physics Resources

Questions and comments

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