Transfer of Heat

# Modes of Transmission of Heat

Heat gets transferred from one place to another by three modes. The modes of transmission of heat are conduction, convection and radiation.

## Conduction

Conduction is the process of transmission of heat from one point to another point through a substance by the transfer of energy from one molecule to another molecule without the actual movement of particles themselves. In this process, heat is transmitted in the direction of fall of temperature.

In solids, heat gets transmitted through conduction process. When one end of a metal rod is heated, another end becomes hot. This transmission of heat is conduction. When one end of the metal rod is heated, its molecules vibrate with greater amplitude and their average kinetic energy increases. As the molecules collide with their neighbouring molecules, the energy is shared between them. In the similar way, they also vibrate with greater amplitude and share their energy. Hence, energy of thermal motion is passed from one molecule to the other, while each individual molecule remains at its original position.

Different substances have different ability to conduct heat. In general, metals like copper, silver, etc. are good conductors of heat while other substances like woods, glass, etc. are bad conductors of heat.

## Convection

Convection is the process of transmission of heat from one point to another through a substance due to the actual movement of the heated particles themselves.
In liquids and gases, heat is transmitted through convection. When a liquid or gas gets heated, the heated fluid becomes lighter in density. Therefore, the heated fluid rises upwards while the cooler fluid moves down. So, the actual movement of the particles transfers heat. The currents set up in the heated fluid are known as convection currents.

In a room, ventilators are placed near its ceiling. It is because, hot air will go up because of its less density and it will escape from the ventilators.
We heat water in container from its below.

What if we heat water from its above? Well, when we heat water from below, the convection process takes place. The heated molecules become lighter in density and moves upwards. And, the top molecules comes down to take their place. And, then again the molecules get heated and become lighter in density and the same process takes place. In this way, all the portion of the liquid gets heated. But if we heat from above, the heated molecules cannot go anywhere. They evaporate and pass into atmosphere. Thus, water cannot be heated if we heat it from above.