Q26 of 27 Page 88

Consider that an ideal gas (n moles) is expanding in a process given by P = f (V), which passes through a point (V0, P0). Show that the gas is absorbing heat at (P0, V0) if the slope of the curve P = f (V) is larger than the slope of the adiabatic passing through (P0, V0).

If an adiabatic process is passing through the point P0, V0 we can write its equation as






As


Slope at P0, V0=



Now, coming to the process given,


P = f (V)


We can write the equation,



Since,




Now, as we know PV=nRT by rearranging it we get,


T= and P = f (V)


T=



Now,


dQ=ncv


we know,=


and




And P=f(V)





Now, the volume is increasing. dV=+ve


dQ should also be +ve if heat is given to the system.


at point (P0, V0) for heat to be absorbed.




Since,



Hence proved.


More from this chapter

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23

Consider a P-V diagram in which the path followed by one mole of perfect gas in a cylindrical container is shown in Fig. 12.9.

(a) Find the work done when the gas is taken from state 1 to state 2.


(b) What is the ratio of temperature T1/T2, if V2 = 2V1?


(c) Given the internal energy for one mole of gas at temperature T is (3/2) RT, find the heat supplied to the gas when it is taken from state 1 to 2, with V2 = 2V1.



24

A cycle followed by an engine (made of one mole of perfect gas in a cylinder with a piston) is shown in Fig. 12.10.


A to B: volume constant


B to C: adiabatic


C to D: volume constant


D to A: adiabatic


Vc = VD = 2VA = 2VB


(a) In which part of the cycle heat is supplied to the engine from outside?


(b) In which part of the cycle heat is being given to the surrounding by the engine?


(c) What is the work done by the engine in one cycle? Write your answer in term of PA, PB, VA.


(d) What is the efficiency of the engine?


[ γ = for the gas], ( Cv = R for one mole)


25

A cycle followed by an engine (made of one mole of an ideal gas in a cylinder with a piston) is shown in Fig. 12.11. Find heat exchanged by the engine, with the surroundings for each section of the cycle. (Cv = (3/2) R)

AB: constant volume


BC: constant pressure


CD: adiabatic


DA: constant pressure



27

Consider one mole of perfect gas in a cylinder of unit cross section with a piston attached (Fig. 12.12). A spring (spring constant k) is attached (unstretched length L) to the piston and to the bottom of the cylinder. Initially the spring is unstretched and the gas is in equilibrium. A certain amount of heat Q is supplied to the gas causing an increase of volume from Vo to V1.

(a) What is the initial pressure of the system?


(b) What is the final pressure of the system?


(c) Using the first law of thermodynamics, write down a relation between Q, Pa, V, Vo and k.