Question

A system absorbs $20KJ$ of heat and also does $10KJ$ of work. The net internal energy of the system:

• A. increases by $10KJ$
• B. decreases by $10KJ$
• C. increases by $30KJ$
• D. decreases by $30KJ$

Answer 1

Answer: (A)

increases by $10KJ$

As per the sign convention, the heat absorbed by the system has positive sign. Hence, $q=+20kJ$
When work is done by the system on the surroundings, it has a negative sign.
Thus, $w=-10kJ$
The expression for the change in the internal energy is $\Delta E=q+w=20kJ-10kJ=10kJ$
Thus, the net internal energy of the system increases by $10KJ$.