# After a system is added 40-J heat, the system does 30-J work. How do you find the change of the system’s internal energy?

Mar 20, 2018

$10 J$

#### Explanation:

1st Law of Thermodynamics:
$\Delta U = Q - W$
$\Delta U =$ change in internal energy.
$Q =$ Heat energy supplied .
$W =$ work done by the system.

$\Delta U = 40 J - 30 J = 10 J$

Some physicist and engineers use different signs for $W$.

I believe this is the engineer's definition:
$\Delta U = Q + W$ here, $W$ is the work done on the system.
The system does work of $30 J$ hence the work done on the system is $- 30 J$.