The internal energy change in a system that has absorbed $$2\,kcal$$ of heat and done $$500\,J$$ of work is
A.
$$8900\,J$$
B.
$$6400\,J$$
C.
$$5400\,J$$
D.
$$7900\,J$$
Answer :
$$7900\,J$$
Solution :
According to first law of thermodynamics $$\Delta Q = U + W$$
or $$\Delta U = Q - W$$
$$\Delta U =$$ change in internal energy
$$Q =$$ heat given to system
$$W =$$ work done
$$\eqalign{
& \therefore \Delta U = 2 \times 4.2 \times 1000 - 500 \cr
& = 8400 - 500 = 7900\,J \cr} $$
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