# Heat Transfer

## What is Heat?

Heat is a form of energy. It makes a substance hotter. We cannot see heat. We can only feel it by the effect of hotness it produces. We can define heat as energy in transit.

### heat transfer definition :-

Since we know that heat is thermal energy in transit. Heat transfer is also referred to as heat. Here thermal energy moves from one place to another by virtue of the difference in temperature. Heat is the energy that is transferred from one body to another due to the temperature difference between the two bodies. With the definition of heat transfer or heat in mind, we now move on to know more about heat transfer.

## Modes of Heat Transfer

There are three modes of heat transfer. Heat transfer or transmission of heat from one place to another takes place by three different ways that are:
1. Conduction
2. Convection and

In solids, heat passes from one point to another through conduction. In Liquids and gases, heat transfer takes place by convection. Heat transfer takes place by the process of radiation when there are no particles of any kind which can move and transfer heat. So, in an empty space or vacuum heat is transferred by radiation. We shall now study heat transfer by conduction, convection, and radiation in detail.

## What is conduction

If we heat one end of a metal bar by keeping it over a gas burner, we find that its other end also gets hot after some time. So heat is transferred from hot end of the bar to its cold end. In this case of heat transfer, there is no movement of molecules (or particles) of the material of metal bar as shown below in the figure.

### Definition of Conduction

“Conduction is the transfer of heat from the hotter part of the material to its colder part without the actual movement of the particles. What I want to say here is that in the process of conduction there is no net movement of the particles of the body. Heat passes through solids by conduction only.

Thermal conduction plays an important role in our daily life. Some of the practical applications of thermal conduction include:-
(a) Cooking utensils are provided with wooden handles.
(b) Eskimos make double walled houses of the blocks of ice.

## Convection

Convection involves the bodily movement of the heated molecules. Convection is the process in which heat is transmitted from one place to other by the actual movement of heated particles. It is prominent in the case of liquids and gases. Land and sea breezes are formed due to convection. It also plays an important part in ventilation, electric lamps, and heating of buildings by hot water circulation. The figure below shows the transfer of heat by convection when water in a cooking pot is heated.

From this figure, you can clearly see convection current. So, when water is heated in a cooking pot then its heated molecules become lighter and moves upwards. As heated molecules of water move upwards, the denser and colder molecules of water begin to move downwards. So warm water rises and colder molecules of water descend. This sets up convection current as shown in the above figure.

Radiation is the process in which heat is transmitted one place to other directly without intervening medium. We get radiation of sun without an intervening medium. Heat radiations can pass through a vacuum. Their properties are similar to light radiations. Heat radiations also form a part of electromagnetic spectrum. Transfer of heat by radiation does not need any material medium. Energy from the sun comes to us by radiation.

1. Heat transfer through conduction is a slow process.
2. Heat transfer through convection is a rapid process
3. Out of these three processes radiation is the fastest one.
4. Radiation is the fastest because in this case, the transfer of heat takes place at the speed of light.
5. Both conduction and convection need a material medium for the transfer for heat.
6. Again consider the figure 2 that shows heating of water in a pot. Here In this process, all three modes of heat transfer can be seen.
1. Pot is kept over the burner. In this case, heat is transferred by conduction
2. Water in the pot transfers heat by convection
3. If you place your hand at a short distance from the burner, you can feel its heat on your hand. This means burner is transferring heat to the hand by radiation.
Question 1 Which of the following combination of properties would be most desirable for a cooking pot
(a) high specific heat and low conductivity
(b) low specific heat and high conductivity
(c) high specific heat and high conductivity
(d) low specific heat and low conductivity
Question 2 Why is it much hotter above the fire than by its sides?
Question 3 In which of the following process convection does not take place primarily?
(a) sea and land breeze
(b) boiling water
(c) warming of a glass of the bulb due to the filament
(d) heating air around a furnace
Question 4 Woollen clothes keep the body warm because of wool
(b) increases the temperature of the body
(c) decreases the temperature
(d) all of the above
Question 5 A liquid is generally heated from below. Why?