## Physics XI Content

# THERMAL EXPANSION

THERMAL EXPANSION | ||

Objects undergo changes in dimension when they are heated. This change in length or area or volume is called “Thermal Expansion”. | ||

WHY BODIES EXPAND ON HEATING | ||

At a given temperature, inter-molecular distances are definite. When a body is heated its molecules vibrate more energetically against the action of inter-molecular forces and the displacement of molecules is increased. Since the average distance between the molecules increases, the dimension of the body increases. Consequently, body expands. | ||

TYPES OF THERMAL EXPANSION | ||

There are three types of thermal expansion: (1) Linear Expansion (2) Superficial Expansion (3) Volumetric Expansion | ||

LINEAR EXPANSION | ||

Expansion in length of solid bodies on heating is called linear expansion. | ||

FACTORS ON WHICH LINEAR EXPANSION DEPENDS | ||

Consider a metallic bar of length “L” at temperature “_{1}T” k . Let the bar is heated to “_{1}T” k._{2} | ||

From experiments, it is observed that linear expansion depends on two factors: 1. The increase in length of a solid bar is directly proportional to its original length | ||

2. The increase in length is directly proportional to the change in temperature. | ||

Combining (1) and (2) | ||

…………….. | ||

OR | ||

………… | ||

Where = Coefficient of linear expansion of solid. | ||

FINAL LENGTH OF BAR | ||

From figure: | ||

……………….. | ||

Putting the value of L | ||

L……_{2} = L1 + L1 T…………L _{2} = L1 (1+ T)……………L _{2} = L1 {1+ (T2 – T1)} | ||

Where [T= T2 – T1] | ||

COEFFICIENT OF LINEAR EXPANSION | ||

It is a characteristic property of a material of solid and is defined as ” Increase in length per unit original length per Kelvin rise in temperature is known as coefficient of linear expansion”. It is denoted by ““(alpha). Value of is constant for a given material but different for different materials. It is independent of mass & dimensions of body . Coefficient of linear expansion depends on the nature of material. | ||

UNIT OF : | ||