Q. 21

A barometer is constructed with its tube having radius 1.0 mm Assume that the surface of mercury in the tube is spherical in shape. if the atmospheric pressure is equal to 76 cm of mercury, what will be the height raised in the barometer tube? The contact angle of mercury with glass = 135° and surface tension of mercury = 0.465 N m-1. Density of mercury 13600 kg m-3.

Answer :

75.5 cm

Given, radius of capillary,

surface tension of mercury,

Contact angle,

Density of mercury,

Length in mercury column of atmospheric pressure,

Due to surface tension, there is a capillary rise seen in the tube.

The rise of mercury column,

The negative sign is used as mercury goes down in a capillary tube.

The height of mercury in the tube thus becomes,

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For a surface molecule

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