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.

What is Heat Transfer?

According to 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:

  • Conduction
  • Convection
  • Radiation

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

Convention,Conduction, Radiation

What is Conduction?

Conduction is defined as

The process in which heat flows from objects with higher temperature to objects with lower temperature.

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. Conduction is also known as thermal conduction or heat conduction.

Conduction Equation

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 = \( \frac{[K.A.(T_{hot}-T_{cold})]}{d} \)

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

Conduction Examples

Following are the examples of conduction:

  • Ironing of clothes is an example of conduction where the heat is conducted from the iron to clothes.
  • Heat is transferred from hands to ice cube resulting in melting of ice cube when held in hands.
  • Heat conduction through sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.

What is Convection?

Convection is defined as

The movement of fluid molecules from higher temperature regions to lower temperature regions.

Convection Equation

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

Convection Examples

Examples of convection include:

  • Boiling of water, that is molecules that are denser move at the bottom while the molecules which are less denser move upwards resulting in circular motion of the molecules so that water gets heated.
  • Warm water around the equator moves towards the poles while cooler water at the poles moves towards the equator.
  • Blood circulation in warm blooded animals takes place with the help of convection, thereby regulating the body temperature.

What is Radiation?

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.

Radiation Equation

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

Radiation Example

Following are the examples of radiation:

  • Microwave radiation emitted in oven is an example of radiation.
  • UV rays coming from the sun is an example of radiation.
  • Release of alpha particles during the decaying of Uranium-238 into Thorium-234 is an example of radiation.

Unit of Heat Transfer

SI system Joule
MKS system cal
Rate of transfer of heat KW

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