Question

If the internal energy of a system decreases by $128\text{J}$ and at the same time it absorbs $56\text{J}$ of heat, then

• A. work is done by the system equal to $+71\text{J}$
• B. work is done by the system equal to $-184\text{J}$
• C. work is done by the system equal to $-71\text{J}$
• D. work is done by the system equal to $179\text{J}$

Answer 1

The correct option is B work is done by the system equal to $-184\text{J}$

$\Delta U=q+W$
Internal energy decreases by $128\text{J}$ thus, $\Delta U=-128\text{J}$
Heat is absorbed, $q=+56\text{J}$
Thus, $W=\Delta U-q=(-128-56)\text{J}=-184\text{J}$