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Old Sep 23rd 2019, 09:42 PM   #1
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Join Date: Sep 2019
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Constant Volume System

I keep seeing various variations of this problem in different thermodynamics problem sets:
A closed system of constant volume experiences a temperature rise of 25oC when acertain process occurs. The heat transferred in the process is 30kJ. The specific heat at constant volume for the pure substance comprising the system is 1.2kJ/kg oC,and this system contains 2.5 kg of this substance. Determine: 1) The change in internal energy 2)The work done.

The answer given in the above case is (75kJ) and (-45kJ).

However, as I know, constant volume systems should have W=0 and the transferred heat of 30kJ should be equal to the change in internal energy so the answer should be 30kJ and 0 and the temperature should only have increased by 10C. This however is inconsistent with the given conditions since a 1.2kj/kgC(2.5kg)(25)=75kJ.

If no work was done on the system and only 30kJ of heat was transferred to it, how could it have gained 75kJ of energy?

Is the problem formulation wrong or I am missing a thermodynamic principle?
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