Transfer of heat

Modes of transfer of heat

How does heat transfer ...?

Will heat flow from a colder object to a hotter object ...?

Is there heat transfer between two objects of same temperature ...?

Consider a situation ...

• What will happen if we keep a frying pan on a flame ? The pan become hot .
• What will happen if we remove the pan from the fire ? The pan slowly cools down .
• Why does it cools down ?

The frying pan becomes hotter if we keep that in flame . It is because of the heat transfer from flame to the utensil . When we remove the pan from fire , the pan slowly cools . It is due to the heat transfer from the pan to the surrounding . So we can say that heat is transferred from the hotter object to the colder object . When there are objects which are at different temperatures or there is an object at different temperature from the surroundings, then the transfer of heat takes place so that the object and the surrounding, both reach an equilibrium temperature.

There are three modes of heat transfer between the two bodies:

1. Conduction
2. Convection
3. Radiation

Any energy exchange between bodies occurs through one of these modes or a combination of them.

Conduction of heat:

Heat conduction is a process in which heat is transferred from the hotter part to the colder part in a body without involving any actual movement of the molecules of the body. Heat transfer takes place from one molecule to another molecule as a result of the vibratory motion of the molecules. Heat transfer through the process of conduction occurs in substances which are in direct contact with each other. It generally takes place in solids.

Eg : Frying vegetables in a pan .

Convection of heat:

In this process, heat is transferred in the liquid and gases from a region of higher temperature to a region of lower temperature. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer.

Eg : Heating of milk in a pan.

Radiation of heat:

It is the process in which heat is transferred from one body to another body without involving the molecules or the medium. Radiation heat transfer does not depend on the medium.

Eg : In a microwave the substances are heated directly without any heating medium.

If we spend time in coastal areas we can experience an interesting phenomenon : Sea breeze and Land breeze

During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

At night it is exactly the reverse. The water cools down more rapidly than the land . So the cool air from the land moves towards the sea . This is called land breeze.

This is an example for transfer of heat by convection in heat .