1. Brief Further Discussion of the 1st Law: The Signs of Work & Heat

2. The Signs of Work & Heat A' A By convention, Heat Q & Work W are
W > 0, Q > 0 A' A
W < 0, Q < 0
By convention, Heat Q & Work W are
both taken as positive when their
direction is into the system of interest.
Q & W are negative when their
direction is out of the system of interest.

3. The Signs of Work & Heat

4. Common Properties of Heat & Work
1. Both are only recognized at the boundaries of a system as they cross the boundaries.
2. Systems possess energy, but not heat or work. We can say a process has done some work, but we can’t say that a state has done some work.

5. Common Properties of Heat & Work
3. Neither heat nor work are state functions. They can’t be expressed in total differential form. Infinitesimal amounts are denoted by đQ & đW instead of dQ and dW, respectively. Their magnitudes depend on the path.

7. Reversible Pressure-Volume Work
Reversible (Quasi-Static) Processes
 Processes for which the timescale is assumed to
be so slow that every intermediate state deviates
only infinitesimally from equilibrium.
After a reversible process has happened, the
process can be reversed & causes no change in
either the system or its surroundings. That is, all
properties of both system & surroundings must be
recovered to their original states. There must be
no net work or heat. These are only ideal
processes, & do not exist in real world.

8. Example of a Reversible Process
A Reversible Process could be realized in the situation in the figure by removing the sand one grain at a time!

9. Irreversible Processes
 Processes which can’t return both the system & the surroundings to their original conditions without extra work from the outside being done. That is, the system & the surroundings would not return to their original conditions if the process was reversed,
unless work is done from the outside.
In irreversible processes, the intermediate states can’t be specified by any set of macroscopic variables.
All real processes are irreversible!!

10. An Irreversible Process
                                                   
An Irreversible Process

11. Reversible Pressure-Volume Work
For the Ideal Gas ONLY

12. 3-step expansion of an ideal gas at constant temp
Expansion Work
3-step expansion of an ideal gas at constant temp

13. 3-step compression of an ideal gas at constant temp