It is a matter of common experience that when we sit near a fire we feel warm or if we hold our hand below a lighted bulb our hand feels warm. Similarly, when we sit in the sun, we feel warm. How do we receive heat in the above cases? The process responsible for transmission of heat cannot be conduction, because the intervening medium (air) is very poor conductivity of heat and heat from the sun travels almost the entire distance in vacuum. The process cannot be convention because the hot air will always rise upwards and not sideways or downwards. Moreover, the intervening medium is not strongly heated as can be seen by putting a screen between yourself and the source of heat. On that case we immediately feel comfortable which would not be case if the surroundings were also heated. Thus, there must be a third process by which heat can be maintained transmitted without the intervening medium(which is not heated if it is present). This process of transmission of heat is known as radiation. Transmission of energy by radiation takes place in the form of electromagnetic waves. These waves can travel in vacuum and travel with the speed of light. This heat is known as infra red radiation because ‘heat waves’ occupy a place in the electromagnetic spectrum just above the red color of the visible spectrum.

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## Resistance

Obstruction or opposition to the flow of electrons. This opposition flow of electrons in a conductor is known as the resistance of the conductor.

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The resistance of a conductor depends on the following factors:

1. It is directly proportional to the length of the conductor.
2. It is inversely proportional to the area of cross section of the conductor, combining these we get

R = `rho*l/A`

Where `rho` depends on the material of the conductor and is known as the specific resistance.

Its unit is ohm-meter.