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 “L1” at temperature “T1” k . Let the bar is heated to “T2” k. | ||
 | ||
| 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 | ||
L2 = L1 + L1 T…………L2 = L1 (1+ T)……………L2 = 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 : | ||
 | ||