Change in Internal Energy

[105743571]

How do I know if the change in internal energy is a negative or positive value if the loss/gain of energy is heat?

[Caitlin Gee 2K]

If the system is giving off heat, the system is losing heat and thus it is a negative value. If the system requires heat, it absorbs heat and has a positive value. When systems give off heat, they lose it to their surroundings. An example is a hot cup of coffee, where the coffee is giving off heat to it's surroundings, the cup. An ice pack is an example of a system taking heat from the surroundings, thus having an increase of heat inside the system.

[Anne Mariano 3F]

The way I think of it is that heat and internal energy are proportional. The molecules of something hot vibrate fast which then describes a high energy level. As a result, things gaining heat have a positive value of energy. The same thing goes for something cold but in the opposite way. If it is cold, the molecules slow down and do not have as much energy, thus the negative value for internal change.