1. ATOC 4720: class 15 The second law of thermodynamics
The Second Law of Thermodynamics: the maximum fraction of heat that can be converted into useful work.

2. Carnot cycle: cyclic and reversible processes
A cyclic process: a series of changes in the state
of a working substance in which its volume
changes and it does external work, subject to
the conditions that the working substance
returns to its initial conditions.
Since:

3. Net heat absorbed is equal to external work
Reversible: If each state of the system is in
equilibrium during the transformation so that a
reversal in the direction of an infinitesimal change
returns the working substance and the environment
to their original states.

4. Absorb:
Reject:
External work:
Efficiency:

5. Carnot cycle: 4 operationsQ1
Begins at A Q2
Heat is transferred from warmer (H) to colder (c) body

6. It is only by transfering heat from a
warmer to a colder body that heat can
be converted into work in a cyclic
process.

8. No engine can be more efficient than a reversible engine working between the same limits of temperature and all reversible engines working between the same temperature limits have the same efficiency

9. Example: heat engine H: boiler C: condenser Water: liquid and vapor
Reversible Carnot cycle: refrigerating
Machine. An outside agency is required
(motor) to drive the motor.

10. Entropy

11. The ratio
Stays the same no matter
Which isothermo is chosen in passing from one
adiabat to anotoher. Therefore,
Is a measureof the difference
between the two adiabats;
It is called the difference in
Entropy.

12. We define:
As the increase in entropy S.
Unit mass:
The first law of thermodynamics:

13. We can derive that:
Transformations in which the entropy
remains constant (and therefore potential
Temperature remains constant) are called
Isentropic.