Second Law of Thermodynamics

Physics 202Professor Lee

CarknerLecture 18

PAL #17 Internal Energy II 3 moles of He at 300 K, raised to 400 K Fixed piston

Constant pressure

H2 gas, constant volume

H2 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

QH QC

THTC

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 QH-QC from the first law of thermodynamics

Eint=(QH-QC)-W =0

Efficiency We get work out of an engine,

what do we put into it?

QH is what you put in, W is what you get out so the efficiency is:

= W/QH

Efficiency and Heat

Since W=QH-QC we can rewrite efficiency as:

The efficiency depends on how

much of QH is transformed into W and how much is lost in QC:

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 QH 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: