What is Thermal or Heat Energy? Definition, Meaning, Equation, Formula, Types, Examples, Units

In this article, we will learn what is thermal energy or heat energy, its definition, meaning, equation or formula, types, examples, units, etc.

Let’s explore!

What is Thermal or Heat Energy? Definition, Meaning

Thermal or Heat Energy Definition

Thermal energy, more commonly known as heat energy, is a form of energy possessed by substances whose molecules or atoms vibrate faster due to increased temperature.

  • It may be defined as the form of energy that exists because of the temperature of the heated substance.
  • The energy that exists in a body by virtue of its temperature is referred to as Thermal energy.
  • The quantity is scalar since it’s a form of energy only having magnitude and not direction.

Thermal Energy or Heat Energy Meaning with Example

The matter is made up of molecules and atoms that are constantly moving. Their motion becomes faster if the substance is heated; hence they will collide with each other more. Thermal energy comes from heating a matter or substance.

It’s an internal energy present in a system in a state of thermodynamic equilibrium. That is why it can’t be converted into useful work more easily than systems, not in thermodynamic equilibrium.

An easy example can help in understanding the concept. Consider a flowing fluid possessing some energy that can be converted to work using a mechanical device. Suppose the same fluid is in thermodynamic equilibrium.

In that case, containing the same amount of energy (in the form of thermal energy), then useful work is only possible if combined with another substance having a different temperature, for instance, a heat engine.

Heat is a flow of thermal energy. The branch of Physics dealing with how heat transfers between certain types of systems, and work done during the whole process, is termed “Thermodynamics.”

Thermal Enegy History

In 1847 James Prescott Joule found several aspects and stated them which were closely related to heat and thermal energy. He proposed the terms such as “latent heat” and “sensible heat.”

He elaborated that these forms are responsible for affecting the potential and kinetic energies of bodies. Latent heat was called the energy of interaction, i-e, a form of potential energy, by James.

Moreover, he explained sensible energy as the form affecting the temperature of an entity due to thermal energy (measured using a thermometer). He called it a “living force.”

What are the Units of Thermal or Heat Energy?

Heat energy or thermal energy is measured in the following units:

  • BTU (British thermal unit)
  • Calorie, or
  • Joule.

The rate of heat transfer (heat flow) between systems has similar units as power which is energy per unit of time represented as Joule/s.

Let’s see the relationship between thermal energy units, BTU, Calorie, Joule

  • 1 BTU = 252.16 Calorie
  • 1 kcal = 4.2 kJ
  • 1 kcal = 4200 J
  • 1000 cal = 4200 J
  • 1 cal = 4.2 J

Thermal or Heat Energy Formula or Equation & Derivation

Thermal energy is present in a system and is responsible for the system’s temperature. Given below is the formula for thermal energy:

Specific Heat Capacity = thermal energy input / (mass) × (temperature change)

Rearranging the equation, we get:

Thermal energy input= specific heat capacity × mass × temperature change

Symbolically changing the formula. Hence the thermal energy equation is given as:

  • C= ΔEt / m. ΔT

Rearranging the equation, we get the thermal energy equation:

  • ΔEt = m.C. ΔT
  • ΔEt = m.C. (Tf – Ti)


  • ΔEt = change in thermal energy
  • m = mass
  • C= specific heat capacity
  • ΔT= change in time

Thermal energy equation can also be written as:

  • Q = m.C. ΔT (where “Q” is for thermal energy)

Thermal Energy and Heat Relation

We cannot deny the existence of thermal energy and the transfer of heat (heat flow). Although thermodynamic internal energy is thought of as thermal energy, it represents all types of energies in the form of kinetic or potential energies of the bodies, commonly termed as the thermal and mechanical elastic energies in terms of simple entropy.

Thermal and mechanical internal energies are referred to as the different energy forms of thermodynamic internal energy.

In thermodynamics, heat is the transfer of energy. Heat is a quantity transferred between systems. Hence, we conclude that it’s not a property of anyone’s system or is not contained in a specific system.

On the contrary, internal energy and enthalpy are properties of a single system. Heat and work depend on an energy transfer that occurred, while internal energy is a property of the state of a system.

If we consider an ideal gas, the molecules move freely between instantaneous collisions. So, internal energy is the sum of the kinetic energies of gas’s independent particles. In gas with no particle interactions except for instantaneous collisions, ‘ thermal energy’ is effectively synonymous with ‘internal energy.’

In physics, “thermal energy” refers to the product of Boltzmann’s constant and absolute temperature.  In other words, we can define the relationship between thermal energy and heat by saying that thermal energy is a phenomenon due to heat flow. Moreover, it is also called “heat or quantity of heat.”

Solved Problems of Thermal or Heat Energy

Some solved examples of thermal energy are stated below.

Thermal Energy or Heat Energy Problem#1


The specific heat of the copper is 386 J/kgºC. Calculate the amount of energy required to raise the temperature of 200 g of Cu by 20 ºC.


Using the formula for thermal energy:

  • Q = m.C. (Tf – Ti)
  • Q = 0.2 kg. 386/kg ºC. 20 ºC
  • Q = 1472 J

Thermal Energy or Heat Energy Problem#2


Calculate the temperature of 200 g of water rise if the energy of 2500 Joules is provided. The specific heat of the water is 4180 J/kgºC.


  •  m = 200g = 0.2 kg
  •  ΔEt = 2500 J
  •  C= 4180 J/kgºC.

According to the equation:

  • Q = m.C. (Tf – Ti)
  • (Tf-Ti) = ΔEt /m. C
  • (Tf-Ti) = 2500 J/ (0.2 kg × 4180 J/kgºC.)
  • (Tf-Ti) = 2.99 ºC

Thermal Energy or Heat Energy Problem#3


What is the thermal energy of a substance weighing 6 kg? The value for specific heat is 0.030 J/kg°c. The temperature difference of this system is given at 20°c.



  • m = 6 kg,
  • C = 0.030J/kg°c,
  • ΔT = 20°c

The thermal energy Equation is given by,

  • Q = mcΔT
  • Q = 6 x 0.030 x 20
  • Q = 3.6 J

Thermal Energy or Heat Energy Problem#4


An object weighing 5kg undergoes a temperature difference of 60°C whose specific heat is 0.07 J/kg°C, calculate the thermal energy.



  • m = 5kg,
  • c = 0.07J/kg°C,
  • ΔT = 60°C

The for thermal energy equation is given by,

  • Q = mcΔT
  • Q  = 5×0.07×60
  • Q = 21 J

Types of Thermal or Heat Energy Transfer

Thermal energy is proportional to its absolute temperature. Energy transfer, mainly in the form of work or heat due to thermodynamic processes, affects it (increases or decreases).

At the microscopic level, as per Kinetic Theory, it’s the total of mean kinetic energy existing due to random movement of atoms or molecules (thermal agitation) which vanishes in the process.

However, there are three forms in which thermal energy transfers. They are convection, conduction, and radiation. Each of these is elaborated in this section.

Thermal Convection

The heat transfer taking place due to the movement of a heated fluid (air or water) is known as convection. Naturally, convection occurs because of the property of fluids to expand after heating. It is because, after heating, the fluids become less dense and rise due to increased buoyancy.

When the molecules gain heat, they expand the space they are occupying because they collide more often with one another due to increased speed. They collide, rise, cool down, and come close together again with resultant sinking and increasing density. A typical example is heating water in a kettle or heating of air in a room.

When fluid transport occurs not because of a change in density due to temperature, it is called forced convection. Movement of water by pump or air by fan are typical examples of forced convection.

Local heating effects such as solar radiation can set up atmospheric convection currents. These currents move vertically and are responsible for natural phenomena such as thunderstorms and clouds.

Thermal Conduction

Temperature differences among adjacent parts of the body result in energy transfer in the form of heat. This phenomenon is known as thermal conduction. It is associated with the exchange of energy in a conducting medium between adjacent molecules.

Substances have different thermal conductivity “k” values. Those with higher values are good conductors of heat, while those with smaller values are bad heat conductors or good thermal insulators.

Mathematical Equation

The heat flow rate in a rod of material is proportional to the cross-sectional area of the rod and the temperature difference between the ends. Moreover, it is inversely proportional to the length. It means rate “H” equals the ratio of the cross-section “A” of the rod to its length l, multiplied by the temperature difference given as “T2 − T1” and thermal conductivity of the material, represented by constant “k.” Hence, the mathematical relation of thermal conduction is given below:

  •  H = −k (A / l) (T2 − T1)

The minus sign shows that heat flows from a region of higher to lower temperatures.

Thermal Radiation

Radiation is defined as the flow of atomic, subatomic particles or waves, especially those characterized by light rays, X-rays, or heat rays. Radiation of both types from cosmic and terrestrial sources constantly bombards all matter. The interaction between matter and radiation is an essential phenomenon in the universe. Radiation has several types. Some of them are discussed below.

The spectrum of Electromagnetic radiation is the first type.

It constitutes,

  • neutrino,
  • gamma rays,
  • x-rays,
  • UV rays,
  • visible light,
  • microwaves,
  • radio waves, and
  • IR rays.

All of them are characterized by zero mass when at rest (theoretically). These rays are named electromagnetic rays.

The second type includes electrons, protons, and neutrons. These particles have mass and are part of atoms or atomic nuclei. These constituents, when traveling at high velocities, are called radiation. They are particularly named as matter rays.

Neutrinos and antineutrinos are forms of radiation that travel with the speed of light and possess little or no rest mass and zero charges. They are similar to electromagnetic waves. The source for their production is ultra-high energy particle accelerators and certain types of radioactive decay.

Examples of Thermal Energy or Heat Energy

There are a large number of examples that we observe in everyday life related to thermal energy. They are either example of convection, radiation, or thermal conduction. 

Boiling water

When the water in a pot is placed on the stove, the particles start to heat up. The hot water goes up and replaces the cooler water. This cold water is then heated, and it is warmed too. The process goes on until all the water heats up.

Steam from a hot cup of coffee

Whenever you drink coffee, steam rises from the cup. It shows that heat is transferred to the air.

Melting ice

Whenever ice is taken out from a freezer, it starts to melt. It is because of heat from the surrounding move towards the ice. Hence, it transforms from a solid to a liquid state.

Frozen food thaws quickly under cold running water

When frozen food is placed under running cold water instead of simply putting it in water, it tends to melt quickly.

Convective clouds

The convection currents carry the moisture up the sky when the air contains more moisture, forming convective clouds. When a sufficient number of droplets build up in these clouds, the outcome is precipitation in the form of convective clouds.

Supercell and Squall Lines

A type of convective thunderstorm that produces a thunderstorm line along with heavy rain and blowing wind is a squall line. In contrast, a severe form of a convective thunderstorm is a supercell. It stays for an extended period (an hour or even more) and can form tornadoes.

Hot air balloon

There is a heater inside a hot air balloon that heats the air. This hot air rises inside the balloon and gets trapped, which causes the balloon to rise high. For descending, some hot air is released, and an amount of cold air replaces it. It causes the balloon to sink lower in the air.

Chimney effect

The Chimney effect is also known as the stack effect. It is the movement of air in and out of buildings or other areas due to buoyancy. Here buoyancy refers to the difference in outside and inside air densities. The greater structure height increases the buoyancy force, ultimately resulting in a more considerable difference between the heat level of inside and outside air.

Non-ionizing radiation

It is the radiation characterized by having enough energy to heat substances but not sufficient for ionizing molecules. The examples for non-ionizing radiations include the following:

  • Visible light
  • Infrared light
  • Microwaves
  • Shortwaves
  • Computer screens
  • Infrared lamps
  • Radio-frequency radiations
  • LEDs
  • Lasers
  • Light bulbs
  • Radiation from sun
  • Remote controls
  • Power lines
  • MRI
  • Extremely and very low-frequency waves
  • Strong magnets

Ionizing Radiations

Ionizing radiations can ionize molecules; therefore, one needs to be careful if exposed to such radiations. Ionizing radiation examples constitute the following:

  • Gamma rays
  • UV rays
  • X-rays
  • Cosmic rays
  • Alpha rays
  • Beta rays
  • Radioactive decay’s particles
  • Nuclear power generation
  • Medical tools sterilization
  • Medical imaging equipment
  • Coal mining
  • Production of power by utilizing coal
  • Radon
  • CT scans
  • Background natural radiation
  • Nuclear medical scans
  • Security scanners at airports and other required areas

Heating pad

The heating pad is often used to heat up the muscles or relieve pain. The heat pad transfers heat to the part where it is placed and produces a soothing effect.

Heat transfer from humans

If you are cold and your friend holds your hand to warm them up, heat transfers from their body to yours. It can be felt in the form of warmth.

Metal spoon

While cooking, if a metal spoon is left in the pot, it becomes hot. Heat transfers from the pot in which you are cooking to the metal spoon.

Melting chocolates

Whenever we buy chocolate, it melts after some time while we are holding it. It is because heat from our hands transfers to the chocolate, eventually melting it.

Light bulbs

Light bulbs, when turned on, give off heat. If they are touched, they will undoubtedly be hot.

Advantages of Thermal or Heat Energy

Some of the advantages of thermal energy are:

  • In terms of construction, the facilities generating thermal energy are pretty economical.
  • It’s a form of energy that is quickly convertible into electrical energy.
  • It is cost-friendly.
  • Thermal energy greatly favours rural areas not taken into account by power plants.
  • Energy plays a significant role in sustainable development.
  • The use of this energy form can help save water and electricity.
  • Thermal energy is obtainable from natural sources. Sun is the primary source.
  • It’s a renewable energy source. The heat can be generated in several ways.
  • The essential point is that this energy is prolonged in time if compared to energy from fossil fuels.
  • Power is generated in the motor of automobiles using thermal energy.
  • Recycled plastic material plants use it.
  • The energy has wide use in bakeries.
  • Thermal energy is utilized in incinerators to burn waste.
  • Electricity is generated using thermal energy, globally used in every sector from domestic to commercial to large industries.

Disadvantages of Thermal or Heat Energy

A few the disadvantages of thermal energy are as under:

  • The continuous emission of greenhouse gases is polluting the energy constantly.
  • Vapor and heat emissions occur in thermal plants, adversely affecting our ecosystem and planet earth. It, in turn, has adverse effects on plants, animals, and humans.
  • Water used during the process contaminates.
  • The use of nuclear energy leaves a lot of radioactive waste behind.
  • If the energy is produced from fossil fuels, it would be limited depending upon the number of fossil fuels available.
  • After being used for cooling down the plants, hot water is discharged into water streams that contaminate the water and adversely affect marine life.
  • Thermal power plants construction requires years.


Thermal energy is a form of energy contained in a body by virtue of its temperature. Since its energy, therefore, the unit is joules, and the quantity is scalar (only magnitude and no direction). The thermal energy equation is ΔEt = m.C. ΔT or ΔEt = m.C. (Tf – Ti). Another form of the equation is Q = m.C. ΔT. Thermal energy transfer exists in three primary forms that are convection, conduction, and radiation.

 The heat transfer taking place due to the movement of a heated fluid (air or water) is known as convection. Naturally, convection occurs because of the property of fluids to expand after heating. Temperature differences among adjacent parts of the body result in energy transfer in the form of heat. This phenomenon is termed thermal conduction. The equation for thermal conduction is H = −k (A / l) (T2 − T1). Radiation is defined as the flow of atomic, subatomic particles or waves, especially those characterized by light rays, X-rays, or heat rays. Hence, these were the three forms.

There are plenty of thermal energy examples from our daily life: boiling water, hot air balloons, microwaves, radio waves, visible light, MRIs, and many more, which are discussed above in the examples section. As far as thermal energy is concerned, it’s the flow of heat. Hence, there is a deep relationship between heat and thermal energy, which cannot be denied. The blog intends to introduce several aspects like the equation, derivation, types, advantages, and disadvantages of thermal energy.

Refer to our most interesting articles,

What is thermodynamics

Intensitve & Extensive Properties

What is Pressure

Bernoulli’s theorem

Venturi meter

Pump basics

Globe valve


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