Changes in Internal Energy
We cannot measure the internal energy in a system, we can only determine
the change in internal energy, 
E, that accompanies a change in the system.
The change in internal energy that accompanies the transfer of heat, q, or work, w, into or out of a system can be calculated using the following equation:
Note the value of heat and work as they are transferred into or out of a system.
| Reduces E | Increases E |
| q < 0 (q negative), heat lost | q > 0 (q positive), heat gained |
| w < 0 (w negative), work done by system | w > 0 (w positive), work done on system |
