Question

What is the change in internal energy in joules for a system that releases 154 j of heat and does 125 j of work on its surroundings?

Answer 1

The change in internal energy of the system is given by the first law of thermodynamics:

`\Delta U = Q-W`
where
Q is the heat added to the system
W is the work done by the system

In this problem,
`Q=-154 J` (with a negative sign because the heat is released by the system (not absorbed)), and the work is
`W=+125 J`, with a positive sign since it is performed by the system. Therefore, the variation of internal energy of the system is

`\Delta U = Q-W=(-154 J)-(+125 J)=-279 J`

So, the internal energy of the system has decreased.