In a thermodynamic process, pressure of a fixed mass of gas is changed in such a manner that the gas molecules gives out 30 joules of heat and $$10\,J$$ of work is done on the gas. If the initial internal energy of the gas was $$40\,J,$$ then final internal energy will be
A.
zero
B.
$$80\,J$$
C.
$$20\,J$$
D.
$$-20\,J$$
Answer :
$$20\,J$$
Solution :
Given that $$dQ = - 30\,J$$ and $$dW = - 10\,J,$$ $${E_i} = 40\,J$$ and let final internal energy $$ = {E_f}$$
So, $$dQ = {E_f} - {E_i} + dW$$
$$\eqalign{
& - 30 = {E_f} - 40 - 10 \cr
& {E_f} = 20\,J \cr} $$
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