Radiation

Radiation is the process of heat transfer by which heat energyEnergy is the ability to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Units of Energy: The SI unit... More is exchanged between two surfaces obeying the law of electromagnetics.

This method of heat transfer does not require any material medium. This heat transfer takes place in the form of electromagnetic waves mainly in the infrared region. It also occurs through a vacuumA vacuum is an area of space containing little or no matter (solids, liquids, and gases). It is a space with nothing in it, not even air. More or any transfer medium by means of photons.

Examples to demonstrate the transfer of heat by radiation

• We feel warmer when we sit near a fire or hot object.

• We feel how when we sit in the sun.

• Slices of bread get heat from the heating coil in a toaster.

Infrared Radiation Absorption:

It is observed that different bodies radiate heat at different rates when subjected to the same temperature depending on their nature. A black body or dull black surface radiates heat faster and better than a polished surface.

We can undertake an experiment to prove that this is the case. The setup for the experiment is shown below.

Two metal plates that are identical in width, height and thickness are placed as shown in the set-up above. A bunsen burner is lit and placed at an equal distance between the metal plates so that both of the metal plates receive the same quantity of infrared radiation.

The only difference between the two metal plates is the surfaces that are facing the Bunsen burner. One of the metal plates will have a dark matt side facing the Bunsen burner, and the other metal plate will have a light shiny side facing the Bunsen burner.

Both metal plates have a ball bearing attached with some wax to the side that is facing away from the Bunsen burner.

After some time, it will be discovered that the ball bearing attached to the dull black surface falls off while the one attached to the shining surface remains unaffected for a long time. It will eventually fall off at a later time.

This is because the dull black surface absorbs heat quickly from the heat source which melts the wax attaching the ball bearing to the metal plate, but the polished shiny surface reflects heat and is not a good absorber of heat.

Demonstration of Rate of Radiation by Different Surfaces:

Applications of Radiation:

1. A bright-coloured cloth reflects light/heat but a black cloth absorbs heat making the person wearing it uncomfortable

2. A coloured car, apart from a black coloured car, is preferable in the tropics because the body absorbs heat better than other surfaces.