# Class 11 Physics Chapter 11 Thermal Properties of Matter

The universal law of conservation of energy states that,

“Energy can neither be created nor be destroyed, it changes from one form to another.”

Heat is such form of energy. Heat transfer takes place when there is temperature difference between system and its surrounding. In general, heat transfer takes place in three ways, conduction convection and radiation. In this article we will discuss these three types of heat transfer.

### Conduction

To understand conduction let’s perform an activity. Take a metal rod and keep it in wooden stand. Now attach some paper pins with wax at the bottom of rod at regular distances. Heat one end of the rod. If you observe carefully, the first paper pin closer to the heating end will fall down. Then the second pin falls , then third and so on. Pin falls due to melting of wax. Wax melts because heat is transferred from heating end towards other end. Such a transfer of heat from hot end towards cold end is called as conduction.
The quantitative relation for conduction is given by H = K A (TC – TD)/L
Where, H = Heat current,
K = Thermal conductivity
A = Area of cross section
L = Length of conductor

Greater the value of K faster will be the conduction of heat. The SI unit of thermal conductivity K is Js-1m-1K-1. Thermal conductivities of metals is greater than liquids and gases. Moreover, gases have poor thermal conductivity. Among metals copper has very high thermal conductivity. If you look at some daily examples, at homes roofs made of cement concrete gets very hot due to high thermal conductivity of concrete. Thus people prefer to have a layer of plastic foam which contains pockets of air, this acts as an insulator, thereby keeping homes cool.

### Convection

Heat Transfer: Convection

The mode of heat transfer by actual motion of medium is called convection. In case of forced heat transfer, convection is called forced convection. If convection takes place naturally then then convection is called natural or free convection. Wind is good example of natural convection. During day, land heats up fast as compared to seawater. The air in contact with warm ground get heated by conduction. Hence, its volume increases and becomes less dense. However, the air above the surface of water is cooler than air at the surface. This air is denser than hot air. Thus, hot air lifts up and cold air fills its space. Hence, during day wind flows from sea towards the land. However during night, surface of water is warmer than the ground. Thus, flow of wind reverses. Trade winds are the winds flowing steadily towards the equator from the northeast in northern hemisphere or south east in the southern hemisphere. Solar heating at equator is faster than polar region. So, air at the surface of equator heats up and rises up. Cold air from polar region get dragged to fill the empty space of this air. Thus convection sets up resulting into trade winds.

Radiation is third type of heat transfer. It is the emission of energy in the form of heat and light. In contrast to conduction and convection, heat transfer in radiation do not require medium. Best example of radiation is, heat of sun received at the earth. Sun and Earth are separated at huge distance, moreover, there is no medium between them. Thus, heat transfer takes place in form of radiation. All bodies either solid or liquid or gas emits energy in form of radiations. When these radiations fall on other body these are partly absorbed and remaining get reflected. It depends on the color of body. Black color absorbs most of the radiation while white color absorb least radiation. This is the only reason we wear white or light colored clothes in summer and dark colored clothes in winter.
It is interesting to know that, radiations takes place in the form of electromagnetic waves.

Keywords: Heat transfer, Conduction, Convection, Radiation, Electromagnetic waves, Thermal conductivity

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