Question

How much heat is released by a system when 45 J of work is done on it to decrease its energy to 23 J

Answer 1

To solve the problem, the use of the first law of thermodynamic is required:

$\Delta U = \Delta Q - \Delta W$

where $\Delta U$ is a change in internal energy of a system, $\Delta Q$ is a heat put into the system and $\Delta W$ is a work done by or on the system on its surroundings.

To find the amount of heat released by the system the equation have to be re-written in a following form:

$\Delta Q = \Delta U + \Delta W$

Since the work was done by the system, the $\Delta W$ has a negative sign and equals to -45J.

The heat released then equals to:

$\Delta Q = 23J - 45J = -22J$