Mechanical Engineering - Thermodynamics - Discussion


The value of specific heat at constant pressure (cp) is __________ that of at constant volume (cv).

[A]. less than
[B]. equal to
[C]. more than

Answer: Option C


No answer description available for this question.

Sameer Satpathy said: (Dec 21, 2013)  
Cp>Cv since,

Cp-Cv = R,
Cp = R+Cv.

Where R is a specific gas constant which is positive so Cp is greater than Cv.

Sudharshan Reddy K said: (Feb 7, 2014)  
Because Cp=gama(Cv) which implies Cv should be multiplied with "Gama" to get Cp.

Pavan said: (Apr 2, 2014)  
Cp/Cv = Y(gamma),

Gamma should be greater than 1.

Dilip Kumar Kanoujia said: (Jan 11, 2015)  
In const vol process w = pdv become zero so that's why cp>cv?

Swajan Manna said: (Feb 8, 2015)  
Cp-Cv=R so Cp is more.

Arjun said: (Apr 17, 2015)  
When gas is heated at Constant Pressure (Cp) it requires more heat energy as there is change is in internal energy as well as external work done.

But in case of Constant Volume (Cv) there is no external work done, so that the given heat has to increase only external energy. Therefore Cp is always greater than Cv.

Y.Ganesh said: (Jun 4, 2015)  
At temperature 300 k, the values of air is:

Cp = 1.005 kj/kg.
Cv = 0.718 kj/kg.

Maqusud Alam said: (Jul 29, 2015)  
At constant pressure there is temperature increase as well as external work done by the system against atmosphere but in constant volume external work done is zero so Cp is greater than Cv.

Manzoor Ahmed Mir said: (Sep 12, 2015)  
Kindly explain in simple method.

Punit Kongi said: (Oct 6, 2015)  
Just imagine an IC engine.

At Cv-volume is constant, so if you give the heat (Q) to this, the piston does not move until total heat is being supplied.

At Cp-pressure will be constant, so the same heat (Q) what you are giving to will do addition work i.e. the piston will move down in this case to keep the pressure constant in this case.

Therefore Cp>Cv.

Sourabh said: (Nov 22, 2015)  
Consider the piston cylinder arrangement and supply heat to it.

In constant pressure process piston is free to move as p is constant so drag force is neglected it means there will be increase in internal energy and boundary work.

In constant volume process: All the energy will be used to raise the internal energy as the boundary work is zero.

So cp is greater then cv always.

Engr Arshad Jamali said: (Dec 9, 2016)  
The value of cp always greater than cv because the relationship is cp - cv = R.

Sambasiva said: (Apr 12, 2018)  
Cp/Cv is greater than 1 so, Cp gerater than Cv.

Nitish said: (Jan 21, 2019)  
Cp = Cv for liquids.
And Cp>Cv for gases.

Yash said: (Jul 2, 2019)  
Cp = R+Cv.
CP = 1.00 kJ/kg.K.
Cv = 0.718 kJ/kg.K.

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