Second Law of Thermodynamics Physics 202 Professor Lee Carkner Lecture 18

PAL #17 Internal Energy II  3 moles of He at 300 K, raised to 400 K  Fixed piston    Constant pressure    H 2 gas, constant volume    H 2 gas, constant pressure    Rank by heat: d > c = b > a

Irreversible Free Expansion

Irreversible and Reversible Processes

Second Law of Thermodynamics   No real process is truly reversible (due to friction, turbulence etc.), so we can say:   This is the second law of thermodynamics  Entropy always increases 

Hero’s Door Opener (1 AD)

Steam Engines (18th century)

Internal Combustion Engine (late 19th century)

Engines  An engine is a device for converting temperature differences into work by continuously repeating a set of processes  

Engine Elements

p-V and T-S Engine Diagrams

The Stirling Engine  As an example, we will examine the Stirling Engine   In between is an insulated chamber which can temporarily store energy 

Stirling Engine Diagram QHQH QCQC THTH TCTC Hot Piston Cold Piston

The First 2 Strokes  1) Isothermal Expansion   2) Isochoric process 

The Last 2 Strokes  3) Isothermal Compression   4) Isochoric process 

Sterling Engine Diagram

Heat and Work  Over the course of one cycle positive work is done and heat is transferred    Since the total heat is Q H -Q C from the first law of thermodynamics  E int =(Q H -Q C )-W =0

Efficiency  We get work out of an engine, what do we put into it?   Q H is what you put in, W is what you get out so the efficiency is:  = W/Q H 

Efficiency and Heat  Since W=Q H -Q C we can rewrite efficiency as:   The efficiency depends on how much of Q H is transformed into W and how much is lost in Q C :

Efficiency and Entropy  If we consider and engine as a closed system we must include the high and low temperature reservoir  If all the processes are reversible, the change in entropy between the two reservoirs must be zero so:  We can use this to rewrite the efficiency equation as:   

Ideal and Perfect Engines  The above equations hold only for ideal engines     It is also impossible to produce an engine where Q H is completely converted into work    Called a perfect engine (no energy lost to heat) 

Perfect Engine

Entropy and Real Engines  On a practical level, you always have heat losses in an engine    In other words the second law of thermodynamics can be stated: 