What is Thermal Energy?

Thermal energy is energy possessed by a body or system due to the movement of particles within the body or the system. It is one of the different types of energy, where energy basically refers to the ability to do work. As such thermal energy can also be defined as the ability of something to do work as a result of the movement of its particles.

In other words, thermal energy is the energy possessed by an object or body by virtue of the movement of its constituent particles. It is the total internal kinetic energy of an object due to the random motion of its atoms and molecules. Thermal energy is a type of kinetic energy owing to the fact that it results from the movement of particles. Kinetic energy is the energy possessed by an object due to its motion.


The faster the atoms or molecules or atoms that make up a body move, the higher the thermal energy of the body. More often than not, the concept of thermal energy is often confused with heat. In physics, heat is considered as a transfer of energy from a hotter to a cooler body due to temperature differences.

The term ‘Heat’ in physics refers to thermal energy in transit; it always flows from a substance at a higher temperature to the substance at a lower temperature, raising the temperature of the latter and reducing that of the former substance provided that the volume of the bodies remains constant.

In short, heat is energy in transfer, while thermal energy is the internal property possessed by an object before the transfer of energy as heat occurs (often a sum total of the kinetic energies of the various particles that make up the object in question)

Physicists take thermal energy to be equal to a product of k and T, where:

K= Boltzmann’s constant (1.381 x 10-23m2kgs-2K-1)

T= Absolute temperature

This relationship is usually written as: kT or kBT

Thermal energy forms the foundation of the study of heat energy and thermodynamics. It is one of the oldest forms of energy utilized by mankind. Its usage existed even before petroleum and nuclear power sources were discovered.

Most people simply refer to thermal energy as heat. The thermal energy of every matter will always depend on the speed of the molecules and atoms making up the matter. If the movement of these molecules and atoms is faster, the object is said to have higher kinetic energy.

Similarly, faster movement of the molecules and atoms in an object is known to increase the temperature of the object. Therefore, thermal energy increases with increase in motion, and hence it becomes a form of kinetic energy.

As most people confuse thermal energy to other forms of energy and other terms such as temperatures, several facts are hidden to the knowledge of very many people. Therefore, it is essential to know several examples and underlying facts about thermal energy.

Types of Thermal Energy

When a body’s constituent atoms and molecules vibrate, leading to an increase in the body’s internal energy (Thermal energy), a temperature gradient is established. Thus, thermal energy is often classified into various types on the basis of how this internal energy, in the form of heat, is transferred from one body to another. The following are the various types:

1. Conduction

This is a type of thermal energy which involves the movement of the constituent particles of an object, without movement of the body itself. It can be exhibited by objects in all phases (solid, liquid, and gas). The vibratory movement of the particles of an object results in an increase in thermal energy (a form of internal energy) of each atom or molecule which is transferred by contact to neighboring atoms and molecules within the object.

Thus, the thermal energy associated with conduction results from the transfer of the increased internal energy of each constituent atom or molecule to another until all the atoms or molecules are set in vibration. In this type of thermal energy, contact must be made between neighboring particles before the thermal energy can be passed along the body.

In conduction, only a part of the conductor is exposed to the agitating agent, but the rise or fall in thermal energy is transferred evenly from one constituent particle to another. A typical example lies in the increase in temperature of a stainless steel spoon inserted into a cooking pot for some time, while heating is continued.

The atoms or molecules within the stainless steel spoon in direct contact with the hottest part of the cooking pot become agitated and possess greater internal energy due to its motion. This energy is transferred to neighboring atoms or molecules until the whole atoms and molecules in the spoon are set in vibration with a resultant increase in internal energy, exhibited as an increase in temperature of the spoon.

2. Convection

Whereas, convection occurs not only within a body but also between two bodies brought into contact. If one of the substances is a liquid or a gas, then the fluid motion will most certainly occur. Transfer of energy between a solid surface and a moving liquid or gas is called convection.

A fluid undergoes either natural or forced convection. When a liquid or gas is heated, its mass per unit volume generally decreases. In cases where the liquid or gas is in a gravitational field, the hotter, lighter fluid rises while the colder, heavier fluid sinks. This kind of motion, due solely to non-uniformity of fluid temperature in the presence of a gravitational field, is called natural convection.

Forced convection is achieved by subjecting the fluid to a pressure gradient and thereby forcing motion to occur according to the law of fluid mechanics. A typical example of this type of thermal convection is as follows: water in a pan is heated from below, the liquid closest to the bottom expands and its density reduces; the hot water, as a result, rises to the top and some of the cooler fluid descends toward the bottom, thus setting up a circulatory motion.

Similarly, in a vertical gas-filled chamber, such as the air space between two window panes in a double-glazed, or Thermopane, window, the air near the cold outer pane moves down and the air near the inner, warmer pane rises, leading to a circulatory motion (Referred to as ‘Thermal convection’)

3. Radiation

This type of thermal energy is fundamentally different from both conduction and convection in that the substances exchanging heat need not be in contact with each other. In fact transfer of thermal energy by radiation occurs between two bodies despite being separated by a vacuum.

The term ‘radiation’ generally applies to all kinds of electromagnetic-wave phenomena. Planck’s law in Physics states that: all substances emit radiant energy merely by virtue of having a positive absolute temperature. Thus, higher temperatures yield greater amounts of energy.

In addition to emitting, all substances are capable of absorbing radiation. This is illustrated by the fact that, although an ice cube continuously emits radiant energy it melts when an incandescent lamp is focused on it because it will be absorbing a greater amount of heat than it can emit.

Examples of Thermal Energy

When it comes to examples of thermal energy, there are endless examples. At home, for examples, most objects exhibit thermal energy which is a form of both kinetic and potential energy. We also interact with several instances of thermal energy in our daily lives. The following are some of the examples of thermal energy that you will often come across:

1. Solar Energy

Solar radiation (a form of thermal energy) heats up our atmosphere, that’s why heat is felt on Earth.

2. Geothermal Energy

Geothermal energy, which is a form of energy within the Earth’s crust in intense heat that continually flows outward from deep within Earth. This heat originates primarily in the core. This heat is generated within the earth’s crust mainly from the decay of radioactive elements present in all rocks.

The crust, which is about 5 to 75 km (about 3 to 47 miles) thick, insulates the earth’s surface from the hot interior, which at the core may reach temperatures ranging from 4000°C to 7000° C (approximately 7200°F to 12,600° F). When the heat is concentrated near Earth’s surface, it can be used as a source of energy.

3. Heat Energy From the Oceans

Ocean and seawater surfaces possess massive thermal energy storage potential, thanks to their direct exposure to the sun’s rays for prolonged periods. This thermal energy is exploited by utilizing the difference between the temperatures of the shallow and deep water marine areas.

With the appropriate technology, we can harvest thermal energy from the ocean and sea waters to power various industries, machinery, and applications for economic development. The technology used in harnessing this energy is generally known as ocean thermal energy conversion (OTEC).

4. Fuel Cell Energy

We can also harness the energy produced from thermal energy by using fuel cells. A fuel cell generates energy from a chemical reaction, which takes place in its electrodes. The reaction between two electrodes produces ions or charged particles, which are carried by an electrolyte. The heat generated during this process is harnessed and used to enhance energy efficiency.

5. A Glass of Cold Chocolate and a Cup of Hot Chocolate Milk

A cup of hot chocolate tends to be warmer than a glass of the same chocolate when cold. In this case, it can be deduced that the hot chocolate milk exhibits higher thermal energy as compared to the cold chocolate milk. By putting the chocolate milk on a hot stove, there will be a rise in the temperature of the milk. It is so because; the milk will be able to absorb the thermal energy coming from the hot stove.

On the other hand, when the hot chocolate milk cools down, it will be losing thermal energy since the movement of the particles is lowered. It does not absorb any heat from the burning stove. As such, it loses the heat to the surrounding cold areas. Therefore, the kinetic energy of its particles reduces hence reducing the thermal energy of the chocolate milk.

6. Melting Ice

When ice is added to a glass of water, the temperature of the water decreases as the temperature of the ice increases. This occurs because the thermal energy of the warmer water than the ice will make the ice to melt.

Other common examples of thermal energy include the following:

7. Adding ice to a glass of water results in a decrease of the water temperature since the thermal energy in the water is utilized in melting the ice.

8. An eight-ounce glass of water at 70 degrees possess a higher amount of thermal energy compared to an eight-ounce glass of water at 60 degrees.

9. A grill generates thermal energy by burning propane

10. When a computer is turned on, its internal components generate heat energy. This energy has to be cooled with a little fan installed within the computer.

11. A hot stove has thermal energy, which is transmitted to a metal pot, increasing the speed of water molecules and hence an increased water temperature.

12. A cat has heat energy within it, which can be transmitted to a person when the warm animal sits on his lap.

13. A bathtub full of hot water contains enough heat energy to warm your cold body and make you feel comfortable once again on a chilly day.

Similarly, the objects can be separated by heating both of them to ignite the motion of the molecules making them part ways. This increases their thermal energy which comes as a result of the increase in the temperatures of the matter. Again, if this happens in liquids in the process of evaporation, it is known as the latent heat of vaporization, which is a form of thermal energy.

Interesting Facts About Thermal Energy

Fact 1: Thermal energy is measured in joules.

Fact 2: There’s a relationship between thermal energy and the temperature of an object.

Fact 3: Thermal energy is a component of the total energy within an object.

Fact 4: Thermal energy is more difficult to transform into other energy forms compared to other forms of energy.

Fact 5: Objects can’t contain heat. They contain thermal energy. This is because heat is considered a process.

Fact 6: When thermal energy is transmitted to or from an object, it is referred to as heat.

Fact 7: You need a machine, such as an engine, to convert thermal energy into other energy forms.

Fact 8: Heat from the sun replaces the heat from Earth that gets lost to space.

Fact 9: The amount of thermal energy does not depend on the amount of work something does. It is unlike other energy forms.

Fact 10: James Joule is recognized as the first person to talk about the gain and loss of heat, but he was not the first to use the term “thermal energy”.

Fact 11: Temperature and heat are two different things. Namely, temperature refers to how cold or hot an object is.

Fact 12: Heat can be transferred in three ways – radiation, convection, and conduction.

Fact 13: Objects that allow thermal energy to be transmitted easily through them are known as conductors. Metals are a good example of conductors.

Fact 14: Objects that don’t allow thermal energy to be transmitted easily through them are known as insulators. Plastic is a good example of insulators.

Fact 15: Thermal energy is often transferred in three primary means namely; conduction, convection, and radiation.

Fact 16: Thermal energy and temperature are not the same. This is because temperature simply defines how hot or cold matter is while the thermal energy deals with the kinetic or potential state of the object’s molecules and atoms.

Fact 17: Even though James Joule was not the first person to utilize thermal energy, he is credited for being the first individual to discuss the gain and loss of heat.

Fact 18: There is a close relationship between thermal energy and the temperature of any matter.

Fact 19: The amount of thermal energy in an object is in no way related to the amount of work performed by the object.

Fact 20: Objects which are used as insulators such as wood and plastics do not allow thermal energy to move through them quickly.

Fact 21: Thermal energy forms a significant part of the total energy possessed by an object.


ScienceDirect, Wikipedia

Sonia Madaan

Sonia is a High School Graduate and Runs the Writing and Editing Team for EarthEclipse.com. She is Extremely Passionate about Environment, Technology and Computing.
Sonia Madaan