WORK W:
Useful Energy Transfered across the System's Boundaries, capable of producing Macroscopic-Mechanical Motion of a the system's Center-of-Mass.

Work done by a Gas :

		Constant Volume Process
		Constant Pressure Process
		Constant Temperature Process
		Adiabatic Process Q = 0

HEAT Q:
Energy Transfer across the System's Boundaries that cannot produce Macroscopic-Mechanical Motion of the system's Center-of-Mass. Energy Transfer at the Molecular Level .

Some common types of Heat Lost:

		Solids or Liquids
		Gas- Constant Pressure Process
		Gas - Constant Volume Process
		Gas - Constant Temperature Process
		Gas - Adiabatic Process

INTERNAL ENERGY U
Energy Stored in a System at the Molecular Level.
The System's Thermal Energy -the Kinetic Energy of the atoms due to their random motion relative to the Center of Mass plus the binding energy (Potential Energy) that holds the atoms together.

It is important to observe that the D in DU is absolutely necessary because both work W and heat Q represent a transfer of energy where as the internal energy U is a quantity of energy that a system contains.
Another way to state his difference is that U is a state variable were as Q and W are not state variables. What this means is that if you take the system from one state to another state by two different processes, DU will be the same independent of the path taken but not Q or W.


The quantities Q_in, Q_out, W_in, and W_out are all taken to be positive quantities where as Q and W can be either positive or negative.