# Heat Transfer - Radiation, Convection And Conduction

Any matter which is made up of atoms and molecules has the ability to transfer heat. The atoms are in different types of motion at any time. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy. More the motion of molecules more will be the heat energy.

However, talking about Heat transfer, it is nothing but the process of transfer of heat from the high-temperature body to a low temperature one. Considering thermodynamic systems, heat transfer is defined as the movement of heat across the border of the system due to a difference in temperature between the system and its surroundings. Interestingly, the difference in temperature is said to be a ‘potential’ that causes the transfer of heat from one point to another. Besides, heat is also known as flux.

## How is Heat Transferred?

Heat can travel shift from one place to another in several ways. The different modes of heat transfer include radiation, convection, and conduction. Conduction and convection need matter to transfer heat.

Meanwhile, if the temperature difference exists between two systems, heat will find a way to transfer from the higher to the lower system.

### Conduction:

We can define conduction as the heat by the direct collision of molecules. An area of higher kinetic energy transfers thermal energy towards the lower kinetic energy area. High-speed particles clash with particles moving at a slow speed, as a result, slow speed particles increase their kinetic energy. This is a typical form of heat transfer and takes place through physical contact.

For example, place a metal over an open flame.

Which material transfers heat better can be determined by physical property? The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction. The rate of conduction can be calculated by the following equation.

Q = $\frac{[K.A.(Thot-Tcold)]}{d}$

Where,

• Q is the transfer of heat per unit time.
• K is the thermal conductivity of the body.
• A is the area of heat transfer
• Thot is the temperature of the hot region
• Tcold is the temperature of the cold region.
• d is the thickness of the body.

### Convection:

When fluid is heated, it carries thermal energy along with it and moves away from the source. This kind of heat transfer is known as convection. The liquid over the hot surface expands and rises up.

A natural example is in the atmosphere. The earth’s surface gets warmed by the sun and the warm air moves up and cool air comes in.

As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows.

Q = hc ∙ A ∙ (Ts – Tf)

Where,

• Q is the heat transferred per unit time.
• Hc is the coefficient of convective heat transfer
• A is the area of heat transfer
• Ts is the surface temperature.
• Tf is the fluid temperature.

Thermal radiation is generated by the emission of electromagnetic waves. These waves carry away the energy from the emitting body. Radiation takes place through a vacuum or transparent medium which can be either solid or liquid. Thermal radiation is the result of the random motion of molecules in the matter. Movement of charged electrons and protons is responsible for the emission of electromagnetic radiation.

As temperature rises, the wavelengths in the spectra of the radiation emitted decreases and shorter wavelengths radiations are emitted. Thermal radiation can be calculated by Stefan-Boltzmann law:

P = e ∙ σ ∙ A· (Tr – Tc)4

Where,

• P is the net power of radiation
• A is the area of radiation
• Tr is the radiator temperature
• Tc is the surrounding temperature
• e is emissivity and σ is Stefan’s constant.

## Unit of Heat Transfer

As for the measurement, going by MKS system the unit of heat transfer is cal and in SI system it is Joule. The rate of transfer of heat is given in KW.