Physics XI Content
THERMAL EXPANSION
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THERMAL EXPANSION
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| Objects undergo changes in dimension when they are heated. This change in length or area or volume is called “Thermal Expansion”. | ||
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WHY BODIES EXPAND ON HEATING
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| 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. | ||
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TYPES OF THERMAL EXPANSION
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| There are three types of thermal expansion: (1) Linear Expansion (2) Superficial Expansion (3) Volumetric Expansion |
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LINEAR EXPANSION
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| Expansion in length of solid bodies on heating is called linear expansion. | ||
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FACTORS ON WHICH LINEAR EXPANSION DEPENDS
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| Consider a metallic bar of length “L1” at temperature “T1” k . Let the bar is heated to “T2” k. | ||
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| 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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 ………… |
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 ….. |
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Where  = Coefficient of linear expansion of solid. |
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FINAL LENGTH OF BAR
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| From figure: | ||
 ……………….. |
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Putting the value of  L |
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L2 = L1 +
L1 T…………L2 = L1 (1+ T)……………L2 = L1 {1+ (T2 – T1)}…… |
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Where [
T= T2 – T1] |
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COEFFICIENT OF LINEAR EXPANSION
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| 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. |
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