Physics 101: Lecture 31 Thermodynamics, part 2 z z z z
Review of 1st law of thermodynamics 2nd Law of Thermodynamics Engines and Refrigerators The Ca...
Physics 101: Lecture 31 Thermodynamics, part 2 z z z z
Review of 1st law of thermodynamics 2nd Law of Thermodynamics Engines and Refrigerators The Carnot Cycle
Physics 101: Lecture 31, Pg 1
Quick Review Î1st Law of Thermodynamics: Îenergy conservation
Q = ∆U + W Work done by (or on) system Increase (or decrease) in internal energy of system P Heat flow into (or out of) system z z
z z
U depends only on T (U = 3nRT/2 = 3PV/2) Point on P-V plot completely specifies state of system (PV = nRT) work done is area under curve for complete cycle
V
∆U=0 ⇒ Q=W Physics 101: Lecture 31, Pg 2
Second Law of Thermodynamics Not all processes that are allowed by energy conservation occur in nature. Why ? Example: Stone falls from height h: mgh -> ½ m v2 (just before impact) -> heat (contact with floor) This process is consistent with energy conservation. The reversed process: Stone lying on floor cools down and moves upward to height h, has never been observed in nature, although it is also allowed by energy conservation: Q->1/2 mv2->mgh z
Or: Ice melts but water does not spontaneously freeze, heat flows from hot to cold but never from cold to hot. ⇒
We need a new concept which makes these (reversed) processes highly unlikely. Physics 101: Lecture 31, Pg 3
New concept: Entropy (S) z
A measure of “disorder” or probability of state of a system.
z
A property of a system (=state function, just like P, V, T, U) Îrelated to number of different “states” of system
z
Examples of increasing entropy: Îice cube melts Îgases expand into vacuum
