Class 7 Science

# Transfer of heat by conduction and convection

Heat is transferred by conduction, convection and radiation.

## Conduction

Transfer of heat from one particle to the adjacent particle is known as conduction of heat. In solids, heat is transferred by the process of conduction. In this process, the transfer of heat takes place through adjacent molecules.

Example: When one end of an iron rod is put over flame then part which is nearer to the flame is heated first and heat is gradually transferred to the other end of the rod. This happens because particles of iron rod which are nearer to the flame receive the heat and transfer this to the adjacent particles. Subsequently, the adjacent particles transfer the heat to the next adjacent particles. This process continues and heat reaches to the other end of the rod. Thus, heat transfer in solid takes place through conduction.

### Conductor and Insulator

Materials which allow heat to pass through them are called conductor or good conductor of heat, such as iron, copper, aluminium, etc. All metals are good conductors of heat. Since, mercury is a metal and found in liquid state at room temperature, that's why it is used in thermometer.

Since metals are the good conductor of heat that's why kitchen utensils are made of metals or alloys of metals.

Materials which do not allow heat to pass through them are called bad conductor or poor conductor of heat. They are also called insulators. Example: rubber, wood, plastic, etc. This is the cause that handles of frying pan or other kitchen utensils are made of plastic.

## Convection

The transfer of heat because of movement of the molecules of the medium, via mass transfer, is called convection or convection of heat.

Water and air are bad conductors of heat. But they do become hot, in spite of being bad conductors. Heat transfer in fluids takes place through convection.

### Convection in water

When water is heated in a pan, the particles of water which are near the source of heat, get heated first. Because of heating, they become light, and rise in water. The gap which is created because of rise of hot particles is filled by cold particles of water from the surrounding area. Thus a cyclical movement of particles begins and ends up heating the whole water of the pan. The cyclical movement in fluids because of heating is called convection current.

### Convection in air

Air gets heated because of convection, the way water gets heated. Air near the source of heat gets heated and rises above. This leaves a gap, which is filled by the colder air from the surrounding. The convection current thus starts in air which results in heating up of air.

When you place your palm above a flame you will feel the hotness of the flame. But when you will place your palm below the flame the area will be colder. This shows how the colder air from below moves up, due to convection current.

## Land and Sea Breeze

### Sea Breeze

In coastal areas, the breeze that moves from sea surface to the land is called sea breeze. This happens because, during daytime, land gets heated more quickly than water. As a result, warm air from land rises up, leaving a gap. To fill that gap, colder air from the ocean surface rushes towards the land. This phenomenon continues and a continuous flow of cold air keeps coming towards the land. This gives rise to the phenomenon which is called the sea breeze. Because of this, people living in coastal areas prefer to live in a sea facing house.

### Land Breeze

In coastal areas, the breeze which moves from land towards the sea is called land breeze. In the night, the land cools down more quickly than the ocean surface. This makes the air over the water surface warmer than air over the land surface. Warmer air over the water surface rises in the air and air from the land rushes towards the water surface to fill the gap. This phenomenon continues which creates a flow of air from land to the sea. This phenomenon is called land breeze.