A \"constant volume ideal gas thermometer\" is constructed as described (schemat

Question

A "constant volume ideal gas thermometer" is constructed as described (schematically). The closed bulb containing the gas is constructed of a material with a negligibly small coefficient of thermal expansion. The point ,4 is a reference point marked on the stem of the bulb. The bulb is connected by a flexible tube to a reservoir of liquid mercury, open to the atmosphere. The mercury reservoir is raised or lowered until the mercury miniscus coincides with the reference point A.The height h of the mercury column is then read.

a) Show that the pressure of the gas is the sum of the external (atmospheric) pressure plus the height h of the mercury column multiplied by the weight per unit volume of mercury (as measured at the temperature of interest).

b) Using the equation of state of the ideal gas, explain how the temperature of the gas is then evaluated

c) Describe a "constant pressure ideal gas thermometer" (in which a changing_ volume is directly measured at constant pressure).

Explanation / Answer

a) the pressure of the gas = P1 + P2

where, P1 = Po = atmospheric pressure

             P2 = rho * g * h

where, h = height of mercury column

=> the pressure of the gas = Po + rho * g * h

b)   As, PV = nRT

=> Temperature , T = PV/nR

c) The flask is immersed in an ice bath, and mercury reservoir is raised or lowered until the top of the mercury in column is at the zero point on the scale. The height h, the difference between the mercury levels in reservoir B and column A, indicate the pressure in the flask at 0°C.

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