What is Thermal Energy

Read on to know more.
Omkar Phatak Jul 7, 2019
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Studying physics is all about studying the interaction between matter and energy. As you go deeper in studying these two entities, you realize that matter and energy are in fact just two sides of the same coin. As modern quantum field theory and nuclear physics reveals, energy and matter are inter-convertible.
Energy manifests itself in various forms and every change that happens in nature is an energy change of some kind. Of the many forms of energy, the most closely experienced of its manifestations is thermal energy.

What is Thermal Energy?

Every piece of matter is made up of atoms and every atom is in an incessant vibrational motion. This constant oscillatory motion of atoms, which never ceases, constitutes the thermal energy that pervades all matter.
The sum total of the kinetic energy of atoms and molecules, due to their incessant motion, is known as thermal energy. According to thermodynamics, the thermal energy of any system is just a part of the total internal energy of the system.
Temperature is a measure of the thermal energy of matter, which in turn is a measure of the kinetic energy of atoms and molecules. When substances receive heat energy, they vibrate with greater amplitudes and frequencies, giving rise to an increase in the thermal energy of the system.
Heat is not same as thermal energy. Rather, thermal energy acquired by matter from surroundings is an effect of absorption of heat. When two systems with unequal temperatures come together, in contact with each other, there is a transfer of heat which continues until both acquire the same levels of thermal energy.

Thermal Energy Equation for an Ideal Gas

Thermodynamics is the study of work and energy acquired from any system. Physicists like to study the simplest case of a problem, before studying more complex ones. Real gases are complex systems to study and hence they study what are known as ideal gases first, which are monatomic gases with negligible inter-atomic forces.
Here is the thermal energy equation for an ideal gas.

Thermal Energy (EThermal) = ½ N x M x v2 = 3/2 NkT

Where N is the number atoms, M is the mass of every atom and v2 is the average value of square of velocity, for that system of gaseous atoms, k is the Boltzmann's constant and T is the overall temperature of the gas.
The first term tells us how the thermal energy is a function of the velocity of the atoms, while the second term identifies its dependency on the temperature of the system, which is again a measure of its kinetic energy (that is again function of velocity).


There are several sources of thermal energy on Earth. However, the prime source of thermal energy which drives all the natural systems on Earth is the Sun. The thermal energy of the Sun, generated through nuclear fusion is radiated to Earth in the form of electromagnetic waves.
A second source of thermal energy is the hot core of the Earth, providing what is called geothermal energy. Other than these two sources, any combustion reaction including any exothermic reaction which generates heat, is a source of thermal energy.
Solar energy, as a thermal energy source, is being increasingly tapped into to take advantage of the almost limitless energy it provides.
To put it in the simplest of words, thermal energy is the macroscopic manifestation of the collective kinetic energy of atoms and molecules that constitute all of matter.