The change in internal energy of a thermo-dynamical system which has absorbed $$2\,kcal$$ of heat and done $$400\,J$$ of work is
$$\left( {1\,cal = 4.2\,J} \right)$$
A.
$$2\,kJ$$
B.
$$8\,kJ$$
C.
$$3.5\,kJ$$
D.
$$5.5\,kJ$$
Answer :
$$8\,kJ$$
Solution :
According to first law of thermodynamics
$$\eqalign{
& Q = \Delta U + W \cr
& {\text{Given:}}\,Q = 2\,kcal = 2000 \times 4.2 = 8400\,J \cr
& W = 400\,J \cr
& \therefore \Delta U = Q - W = 8400 - 400 = 8000\,J \cr} $$
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