Question

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

• A. work done by the system is equal to $+72J$
• B. work done on the system is equal to $-184J$
• C. work done by the system is equal to $-72J$
• D. work done on the system is equal to $179J$

Answer 1

The correct option is B work done on the system is equal to $-184J$

We know from first law of thermodynamics,
$△U=q+W$
Internal energy decreases by $128J$
Thus, $△U=-128J$
Heat is absorbed, $q=+56J$
Putting values in the above equation we get,
$W=△U-q=(-128-56)J=-184J$