Question

Fifteen joules of heat are added to a cylinder with a piston. The system uses 7 joules of energy to raise the piston upward. By how much did the internal energy of the system increase? 2 joules 8 joules 15 joules 22 joules

Answer 1

Answer: 8j

Explanation:

Answer 2

The first law of thermodynamics states that:
$\Delta U = Q-L$
where 
$\Delta U$ is the variation of internal energy of the system
Q is the heat absorbed by the system
L is the work done by the system on the surrounding.

In this problem, the system absorbs 15 J of heat, so Q=+15 J (with positive sign, since it is heat absorbed by the system) while the work done by the system is L=+7 J (with positive sign, since it is work done by the system), so the variation of internal energy is
$\Delta U= Q-L=(15 J)-(7 J)=+8 J$