physics 1220/1320 thermodynamics & electromagnetism chapter 19 -20
TRANSCRIPT
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