Physics 1220/1320 Thermodynamics & Electromagnetism Chapter 19 -20

Physics 1220/1320

Thermodynamics&

ElectromagnetismChapter 19 -20

Ideal Gas: Phase Diagram:Substance expanding on melting

Ideal Gas to real gas:Phase transitionsHere: liquid/gas

Phase diagrams – 3-dim pVT system

Examples of quasi-static processes:isothermal constant Tisobaric constant pisochoric constant Vadiabatic no heat flow

1st Law of Thermodynamics

Conservation of energy:

State Functions

Graphical representation of quasi-static processes

State Functions Ideal gas- Isothermal processes

g = Cp/Cv= (5/2R)/(3/2R)=5/3

TVg-1 = const.

Second Law of Thermodynamics:gives direction to processes

No system can undergo a process where heatis absorbed and convert the heat into workwith the system ending in the state where itbegan: No perpetuum mobile.

b/c heat cannotflow from a colder toa hotter body w/oa cost (work).

iow e=100% is notpossible!Reversible vs irreversibleprocesses.

Heat engines: heaters and refrigerators

Early heat engines:

4 stroke or Otto engine intake stroke compression stroke ignition power stroke exhaust stroke

Bottom right Ta, bottom left Tb

Use T*Vg-1 = const. lawfor the two adiabatic processes

For r=8, g = 1.4 e=56%

Diesel Cycle

Typical rexp ~ 15, rcomp ~ 5

Carnot CycleThe best-efficiency cycle

2 reversible isothermaland 2 reversible adiabaticprocesses.

There are many other useful state functions:the thermodynamic potentials

Enthalpy H = U + PV

Free energy at const P, T G = U + PV - TS

Free energy at const. TF = U – TS

Entropy Setc.

The first law revised (for a p-V-T system):DU= TdS - pdV

Entropy: Cost of Order – macroscopic interpretation:

reversible and irreversible processes

Total entropy change zero:reversible

Use dQ = S dT in first law!

w is no. of possible states

Entropy ~ microscopic interpretation

The 3rd Law of Thermodynamics

3rd Law: It is impossible to reach absolute zero.

‘Fun’ mnemonic about thermodynamic laws:

1st Law ‘You can’t win, you can only break even’

2nd Law ‘You can break even only at absolute zero’

3rd Law ‘You cannot reach absolute zero’

Moral: one can neither win nor break evenThe American Scientist , March 1964, page 40A

The laws of thermodynamics give processes a direction