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Railways NTPC (Technical Ability) Thermodynamics Question Bank Thermodynamics

  • question_answer
    A closed system undergoes a process during which 150 kJ of heat is added to it. The system is then restored to its initial state. If the heat transfer and work transfer for the second process are \[-\,\,50\,\,kJ\] and \[+\,\,75\,\,kJ\] respectively, what is the work transfer for the first process?

    A) 25 kJ                

    B) 50 kJ

    C) 75 kJ                

    D) 100 kJ

    Correct Answer: D

    Solution :

    \[{{Q}_{1\,-\,2}}=+\,\,150\,\,kJ,\] \[{{Q}_{2\,-\,1}}=-\,50\,\,kJ,\] \[{{Q}_{2\,-\,1}}=+\,75\,\,kJ\] \[{{Q}_{1\,-\,2}}={{W}_{1\,-\,2}}+{{U}_{1\,-\,2}}\] \[150={{W}_{1\,-\,2}}+{{U}_{1\,-\,2}}\] \[{{U}_{1\,-\,2}}=150-{{W}_{1\,-\,2}}\] \[{{Q}_{2\,-\,1}}={{W}_{2\,-\,1}}+{{U}_{2\,-\,1}}-50=+\,75-\left( 150-{{W}_{1\,-\,2}} \right)\] \[{{W}_{1\,-\,2}}=25\,\,kJ\]


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