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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Kinetic Theory of Gases Page 4

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Regnault's Method to Determine Specific Heat of a Gas at Constant Pressure

(Cp)

 

 

 

R - Gas cylinder kept in a thermostat, G - Pressure gauge, V - Regulator valve, M - Manometer, B - Oil bath, C - Water calorimeter, S - Screen, T, T1, T2 - Thermometers, E - Gas outlet

 

 

 

 

 

Formula:

  

Heat absorbed by calorimeter + water = Heat lost by the gas
Heat gained by calorimeter and water Q = (Mw + W) (
q2 - q1)
Where, Mw = mass of water
W = water equivalent of calorimeter
q1 = initial temperature
q2 = final temperature of calorimeter and water
Initial temperature of gas =
q

 

 

Final average temperature of gas =

q1 + q2

 

2

 

\ Fall in temperature of gas = q -

q1 + q2

 

2


If Mass of gas heated = m0
and specific heat of gas at constant pressure = Cp

\ Then, heat lost by the gas = m0Cp q -

q1 + q2

 

2

 

\ (Mw + W ) (q2 - q1) = m0Cp q -

q1 + q2

 

2

 


\
Cp =

(Mw + W)(q2 - q1)

 

m0 q -

q1 + q2

 

2

 

m1 =

P1VM

= Initial mass of the gas in reservoir

 

RT

 

m2 =

P2VM

= Final mass of the gas in reservoir

 

RT

 

\ m0 = m1 - m2 = (P1 - P2)

VM

= mass of the gas heated.

 

RT

P1 = Initial pressure
P2 = Final pressure
V = Volume of the gas
M = Molecular weight
T = Temperature of the reservoir
R = Universal gas constant

 

 

 

Latent Heat

 

The quantity of heat absorbed by unit mass of a substance during its change of state at constant temperature is called the latent heat.

It is called the Latent Heat of Fusion when the change of state is from solid to liquid and is termed as the Latent Heat of Vaporisation when the change of state is from liquid to gaseous state.

Internal and External Latent Heats
Internal Latent Heat (Li) is the part of the latent heat used by unit mass of substance, during its change of state, to perform work against the intermolecular forces of attraction. So that inter molecular distance increases.

External Latent Heat (Le) is the part of the latent heat used by unit mass of substance, during its change of state, for performing work against the external pressure.

  • Li helps to increase the intermolecular separation while Le helps to increase the volume of the substance.

If dV is the change in volume and P is the pressure, Le =

PdV

  (Cal or kcal)

 

J

\ L = Li + Le = Li +

PdV

(in neat units)

 

J

  •  
  • Intermolecular forces of attraction are very strong, hence a large amount of work is done against them to increase the separation between the molecules.
    \
    Li > Le.

 

 



 

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