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 | ||
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2. The increase in length is directly proportional to the change in temperature. | ||
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Combining (1) and (2) | ||
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OR | ||
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Where ![]() | ||
FINAL LENGTH OF BAR | ||
From figure: | ||
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Putting the value of ![]() | ||
L2 = L1 + ![]() ![]() L2 = L1 (1+ ![]() ![]() L2 = L1 {1+ ![]() | ||
Where [ ![]() | ||
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 “ ![]() ![]() | ||
UNIT OF ![]() | ||
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