Energy Equation 2nd Law of Thermodynamics

1. Energy Equation2nd Law of Thermodynamics • Energy cannot be created or destroyed. (see Conservation of Mass for exception) • It can, however, change from one form to another; transfer from one system to another. • Energy conservation is a Postulate

2. Closed System • In a closed system no mass enters or leaves the system. • However, energy can leave as • Work [W] (organized motion) • Heat [Q] (random motion)

3. 1st Law for Closed System dECM = Q + W • Work done by system (out) +ve • Heat supplied to system (in) +ve • d - exact differential (state variable) •  - not e. d. (path dependant) • Same per mass, per mole.

4. Other Forms of 1st Law • In rate form: • In integral form:

5. Applications of 1st Law • Fixed volume: pdV = 0 only internal energy, E = U

6. Applications of 1st Law • Constant pressure, slow moving piston • No kinetic energy, no potential energy

7. Application of 1st Law • Piston with friction: Without (a): With (b): 1st Law still:

8. Application of 1st Law Assume: ideal gas, same initial/final volume For either a) or b): Then: But:

9. Application of 1st Law Less energy (heat transfer) required for same expansion process without friction. Show for yourselves: if pa same as pb the gas would expand less since some of the work used to overcome friction

10. Application of 1st Law • Heat from saturated liquid to saturated vapor at constant p: Q12 = H12 = m(hg - hf) = mhfg hfg - latent heat of vaporization Similarly: latent heat of fusion latent heat of sublimation

11. 1st Law with Shaft Work

12. Cycles • A repeated thermodynamic cycle where the initial state equals the final state. • denotes integration over a cycle • By definition:

13. Cycles First law: Net work out = net heat in: Cycle efficiency: