Transmission of Heat

In general heat travels from one point to another whenever there is a difference of temperatures. Heat flows from a body at higher temperature to a lower temperature. Heat is transferred or propagated by three distinct processes - conduction, convection and radiation.


It is a process in which the heat energy is transferred from a particle to particle, without the particles leaving the mean positions but vibrating with amplitudes which depend on the temperature. For conduction the medium is actively involved.

The substances through which heat is easily conducted or for which the rate of conduction is large are good conductors of heat. All metals are good conductors of heat. In metals thermal conduction is due to vibration of the atoms and free electrons.

The substances which do not conduct heat easily are bad conductors. The substances like cork, wood, cotton, wool are bad conductors. Almost all gases and liquids (except mercury) are poor conductors of heat. The bad conductors do not have the free electrons, therefore, they cannot conduct heat. Whatever little heat they can conduct is by vibrations of the molecules.


The transmission of heat from one part to another by the actual transfer of particles of matter is known as convection.

Although conduction does occur in liquids and gases also, heat is transported in these media mostly by convection. Convection is the natural way of heat transmission in fluids. The region of a fluid when heated expands, becomes less dense and rises to the other parts of the fluids there by carrying heat. Convection is a quicker process than conduction. For convection molecules must be relatively free.

A wind is a convection current in the atmosphere caused by unequal heating. Trade winds and monsoons are convection currents on a global scale.


It is the process of transmission of heat from one place to another without any material medium. It is a quick process than conduction and convection. In this process medium is not heated.

Nature and properties of radiant energy

  1. It consists of long wavelength electromagnetic radiation.
  2. The wavelength of these waves is nearly 800 nm to 4,00,000 nm.
  3. It occupies the infrared region of the electromagnetic spectrum.
  4. It can be transmitted through vacuum.
  5. These waves propagate in vacuum with a velocity 3x108 ms-1 like light waves.
  6. It obeys laws of reflection, refraction, interference, polarization and diffraction.
  7. The intensity of radiant energy obeys inverse square law.