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# Thermal Energy is Kinetic Energy

by Ron Kurtus (revised 17 January 2019)

The thermal energy of an object consists of the total kinetic energy of all its atoms and molecules. It is a form of energy related to heat and temperature.

Thermal energy can be created internally by chemical, nuclear and electrical reactions. It can also be created or increased from external effects, such as mechanical motion, radiation, and thermal conduction.

Questions you may have include:

• How can thermal energy be created?
• What are internal reactions?
• What are external effects?

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

## Creation of thermal energy

Thermal energy is the total internal kinetic energy of an object due to the random motion of its atoms and molecules. It is sometimes confused with internal energy or thermodynamic energy. They consist of the sum of the internal kinetic energy (thermal energy) and the potential energy of an object. You may need to make sure which definition a teacher or book is using.

### Kinetic Theory of Matter

The Kinetic Theory of Matter states that matter consists of atoms or molecules in random motion. Those moving particles can transfer their kinetic energy to other nearby particles. The total kinetic energy of all the particles in an object make up the thermal energy of that object.

### Temperature and heat

Temperature and heat are related to thermal energy.

• Temperature is defined as the average kinetic energy of all the atoms or molecules in an object.
• Heat is defined as the flow of thermal energy from an object of one temperature to an object of another temperature. You feel the flow of heat when warm air from a furnace reaches you.

## Internal reactions

The thermal energy of an object can be created or increased by chemical and nuclear reactions, as well as electrical effects. Each releases or transfers energy that cause an object's internal particles to increase their motion and thus their kinetic energy.

### Chemical

For example, some chemical reactions cause nearby molecules to accelerate, thus increasing the total thermal energy of the object. Burning is a common form of a heat-producing chemical reaction.

### Nuclear

Nuclear reactions, such as nuclear fission or nuclear decay, give off high-speed particles that increase the thermal energy of a material.

### Electrical resistance

The resistance to the motion of electrons in an electrical circuit cause the wire's molecules to increase their kinetic motion, thus increasing the thermal energy of the wire. Often you can feel the wire get warm when electricity is flowing through it.

## External effects

External sources of energy such as mechanical motion, radiation and thermal conduction can also increase the thermal energy of an object.

### Mechanical

Mechanical sources of heat are primarily external. When objects rub together, the friction causes molecules to increase their energy, resulting in heat. Likewise, bending or pounding on a piece of metal will cause it to get warmer.

Light from the sun radiating on an object can transfer energy to the object's molecules, causing them to move faster. In other words, the object heats up. Radiation is considered a form of heat transfer.

### Conduction of heat

The Kinetic Theory of Matter shows how the kinetic energy of a material's particles can be increased though collisions with faster nearby particles. This explains how a material can be heated through conduction heat transfer.

## Summary

Thermal energy consists of the total internal kinetic energy of an object due to the random motion of its atoms and molecules. It is related to heat and temperature. Thermal energy can be created internally with chemical, nuclear and electrical reactions. It can also be created or increased from external effects, such as mechanical, radiation and conduction effects.

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.

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.

## 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.

## 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

### Books

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