# Thermal Expansion

## Thermal Expansion

When a body is heated its length, surface area and volume increase. The length, area and volume with the increase in terms of coefficient of linear expansion ( α ), coefficient of superficial expansion or superficial expansion ( β ) and coefficient of cubical expansion or cubic expansivity (γ ).

Relation between α , β and γ
α : β : γ = 1 : 2 : 3

## Anomalous expansion of water

Almost every liquid expands with the increase in temperature. But when temperature of water is increases from 0°C to 4°C its Volume decreased. If the temperature is increased above 4°C, its volume decreased. If the temperature is increased above 4°C, its volume starts increasing. Clearly, density of water is maximum ar 4°C.

## Transmission of heat

The transfer of heat from one place from one place to other place is called transmission of heat.
There are three modes of heat transfer :
1. Conduction
2. Convection

Conduction
In this process, heat is transferred from one place to other place by the successive vibrations of the particles of the medium without movement of the particles of the medium. In solids, heat transfer takes place by conduction.

Convection
In this process, heat is transfer by the actual movement of particles of the movement from on place to other place. Due to movement of particles, a current of particles set up which is called convection current.
Earth’s atmosphere is heated by convection.

In this methods transfer of heat takes place with the speed of light without affecting the intervening medium.

## Newtons’s law of cooling

The rate of loss of heat by a body is directly proportional to the difference in temperature between the body and the surrounding.

## Kirchhof’s Law

According to Kirchhof’s law the ratio of emissive power to absorptive power is same for all surface at the same temperature and is equal to emissive power of black body at that temperature.
Kirchhoff's law signifies that good absorbers are good emitter.
If a shining metal ball with some black spot on its surface is heated to a high temperature and seen in dark, the shining ball becomes dull but the black spots shines brilliantly, because black spot absorbs radiation during heating and emit in dark.

## Stefan's law

The radiant energy emitted by a black body per unit area per unit time (i.e. emissive power) is directly proportional to the fourth power of its absolute temperature.
E = σ T4
where σ is a constant called Stefan’s constant.