# Thermal Energy

Thermal energy is a kind of energy and it is generated when the temperature rises. Thermal energy is directly proportional to the change in temperature of the object. Heat is the form of thermal energy. The hotter the substance, the more will be its thermal energy. The rise of thermal energy by increasing the temperature is due to the faster movements of atom and molecules of the substance.

Sometimes the temperature is so high that the molecules of the substance break the bonds from it and leave it. Surprisingly, the states of matter are also dependent on thermal energy. Change in the thermal energy changes the states of matter. Increase in the thermal energy allows the material to change from solid to liquid and then liquid to gas. This is due to the increase in the internal energy of the molecules and atoms.

Thermal Energy

## Types of Thermal Energy

All matters are made from tiny particles called molecules and atoms. They are always in motion and move here and there or vibrating back and forth even at constant temperature and the total sum of all their kinetic energies in all directions is zero. An increase in the temperature raises the kinetic energy of the molecules which tend to change the state of matter. The thermal energy of the matter is increased by three methods, namely, conduction, convection and radiation. These transfer methods of thermal energy are discussed below in details:

### Conduction:

Thermal energy transfer is made easy in case of conduction as in this case, energy moves from one molecule to another by vibration. Molecules do not move from their places but their speedy back and forth vibrations transfer the energy effectively. Heat, a form of thermal energy, transfers the energy between the bodies if there are no external hindrances present. Even at the thermal equilibrium the molecules and atoms possess kinetic and potential energies in all directions but their net effect is zero. When heat is transferred to them, they vibrate at their places and transfer the extra energy to the neighbouring particles. This way, heat transfers from one place to another. This type of flow of thermal energy generally happens in the solid phase of the matter.

### Convection:

When the heat or thermal energy transfer takes place in the liquid state of the matter, it transfers in layers. In the liquids, there is no restriction for the movements of molecules. When the liquid is heated up, the liquid molecules near to the heat source do move to the place where the temperature is low. This way a current is developed within the liquid and when the hot current moves upwards the blank space is filled up by the cold current. This process continues until all the liquid attains the same temperature throughout.

In the gaseous state of matter, the movement of molecules is so random that they can move to any direction. In radiation, energy transfer takes place in the waveform. These waves are electromagnetic waves which transfer the energy from one molecule to another. Thermal transfer of heat through radiation does not require any medium wherein a medium is required in case of conduction and convection methods to transfer heat or energy. They can travel in a vacuum also. Therefore energy transfers easily in case of radiation as compared to conduction and convection.

### Mathematical Representation for Transfer of Thermal Energy

#### Conduction:

Thermal energy transfer in case of conduction method, the rate of heat transfer is directly proportional to the temperature difference between the hot and cold surface and area of thermal transfer. It is inversely proportional to the thickness of the body. Mathematically,

Q= $$\frac{KA(T_{hot}-T_{cold})}{d}$$

Where Q is the rate of heat transfer

A is the area of heat transfer

$$T_{hot}$$ is the temperature at the hot surface

$$T_{cold}$$ is the temperature at the cold surface

d is the thickness of the body and

K is the constant of proportionality, known as the thermal conductivity of the body

#### Convection:

In this method of heat transfer, layer by layer transfer of heat takes place. In this case, the rate of heat transfer is directly proportional to the temperature difference between the hot and cold area and area of heat transfer. Mathematically,

Q=h $$A(T_{s}-T_{l})$$

Where Q is the rate of heat transfer

A is the area of heat transfer

$$T_{s}$$ is the liquid temperature near to the hot plate

$$T_{l}$$ is the liquid temperature at large

h is the constant of proportionality, known as the coefficient of convective heat transfer

In radiation heat or thermal transfer, the rate of heat transfer is governed by the Stefan-Boltzmann law of radiation. According to this law, the rate of heat transfer is directly proportional to the fourth power of difference in temperature between the source and surroundings and surface area of the object. This shows that the thermal transfer through radiation is strongly dependent on the temperature difference between the surface and source. Mathematically,

Q= $$\sigma \epsilon A\left ( T_{s}-T_{0} \right )^{4}$$

Where Q is the rate of heat transfer

A is the surface area of the object

$$\epsilon$$ is the emissivity of the object

$$T_{s}$$ is the temperature at the source

$$T_{0}$$ is the temperature of the object or surroundings

$$\sigma$$ is the constant of proportionality, known as Stefan-Boltzmann constant and the value of it is equal to $$5.6703\times 10^{-8}Wm^{-2}K^{-4}$$

In this formula, a term emissivity has come. The emissivity of an object is the value which measures how effectively it radiates the heat. A perfect black body has the emissivity equal to 1 whereas a perfect mirror has the emissivity equal to 0. The emissivity of real objects falls between these two values.

## FAQs on Thermal Energy

Q.1: What is the use of thermal energy?

Answer: There are numerous uses of thermal energy. Almost all the heat transfers are by thermal energy transfers one way or the other. Heating an iron rod, melting of chocolate in the hand, baking, ironing the clothes and heating the sand are some examples of heat transfer through conduction.

Heating of water or other liquids, cooking food, heating of air in air-balloons, blood circulation in warm-blooded animals are some examples of heat transfer through convection.

Cooking food in the microwaves, transfer of heat from the sun and other stars, heating of seawater are some examples of heat transfer through radiation.

Q.2: What are the ways through which thermal energy transfer?

Answer: There are three ways to transfer thermal energy from one place to another. These ways are:

• Conduction,
• Convection and

Q.3: What is Stefan-Boltzmann law for thermal energy transfer?

Answer: According to the Stefan-Boltzmann Law, the rate of heat transfer is directly proportional to the fourth power of temperature difference between the surface of the source and surface of the object. It is also directly proportional to the surface area of the object. Mathematically, it is represented as

Q=$$\sigma \epsilon A\left ( T_{s}-T_{0} \right )^{4}$$

Where Q is the rate of heat transfer

A is the surface area of the object

$$\epsilon$$ is the emissivity of the object

$$T_{s}$$ is the temperature at the source

$$T_{0}$$ is the temperature of the object or surroundings

$$\sigma$$ is the constant of proportionality or Stefan-Boltzmann constant and the value of it is equal to $$5.6703\times 10^{-8}Wm^{-2}K^{-4}$$.

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