The Laws of Thermodynamics (01 of 38)

Physics Lecture Notes

The Laws of Thermodynamics

FOUNDRY: It requires about 289 Joules of heat to melt one gram of steel. In this chapter, we will define the quantity of heat to raise the temperature and to change the phase of a substance.

Photo © Vol. 05 Photodisk/Getty

• Sadi Carnot • 1796 – 1832• French Engineer• Founder of the science

of thermodynamics• First to recognize the

relationship between work and heat

Thermodynamics is the study of processes in which energy is transferred as heat and as work.

IThe internal energy is the sum of all the energy of all the molecules in an object:·    random translational kinetic energy·    rotational kinetic energy·    vibrational energy·    intermolecular energy associated with their bonding.

ZEROTH LAW The Zeroth law states that "If bodies A and B are each separately in thermal equilibrium with body C, then A and B are in thermal equilibrium with each other."

The common property between A and B is called temperature.

Topics

4) Thermodynamic Processes

5) The Second Law of Thermodynamics

6) Heat Engines

8) Entropy

1) The First Law of Thermodynamics

7) Carnot cycle

3) Pressure - Volume Graph

2) Work Done on a Gas

The Laws of Thermodynamics (02 of 38)

First Law of Thermodynamics

SystemU

Environment

DU = Q - W

Q W

The Laws of Thermodynamics (03 of 38)

PV = NkT

Pressure(Pa)

Volume(m3)

AbsoluteTemperature

(K)

Boltzmann’sConstant

(1.38 x 10-23 J/K)

Number ofMolecules

The Ideal Gas Law

Temperature and kinetic Theory13

Thermodynamic Processes

A. Isobaric Constant Pressure

B. Iso-volumetric Constant Volume

C. Isothermal Constant Temp.

D. Adiabatic No Heat Transfer between systems

The Laws of Thermodynamics (06 of 38)

Pressure - Volume Graph

P

V

Pressure

Volume

Isotherms(lines of constant

temperature)

Area under curve represents work

T1

T4

T2

T3Internal energyis proportionalto temperature

The Laws of Thermodynamics (05 of 38)

• Absorbs heat Qhot

• Performs work Wout

• Rejects heat Qcold

A heat engine is any device which through a cyclic process:

Cold Res. TC

Engine

Hot Res. TH

Qhot Wout

Qcold

HEAT ENGINES

THE SECOND LAW OF THERMODYNAMICS

It is impossible to construct an engine that, operating in a cycle, produces no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work.

Not only can you not win (1st law); you can’t even break even (2nd law)!

Wout

Cold Res. TC

Engine

Hot Res. TH

Qhot

Qcold

THE SECOND LAW OF THERMODYNAMICS

Cold Res. TC

Engine

Hot Res. TH

400 J

300 J

100 J

• A possible engine. • An IMPOSSIBLE engine.

Cold Res. TC

Engine

Hot Res. TH

400 J 400 J

EFFICIENCY OF AN ENGINE

Cold Res. TC

Engine

Hot Res. TH

QH W

QC

The efficiency of a heat engine is the ratio of the net work done W to the heat input QH.

e = 1 - QC

QH

e = = W

QH

QH - QC

QH

EFFICIENCY EXAMPLE

Cold Res. TC

Engine

Hot Res. TH

800 J W

600 J

An engine absorbs 800 J and wastes 600 J every cycle. What is the efficiency?

e = 1 - 600 J

800 J

e = 1 -

QC

-----

QH

e = 25%

Question: How many joules of work is done?

EFFICIENCY OF AN IDEAL ENGINE (Carnot Engine)

maember perfect engine, the quantities Q of heat gained and lost are proportional to the absolute temperatures T.

e = 1 - TC

TH

e = TH - TC

TH

Cold Res. TC

Engine

Hot Res. TH

QH W

QC

ch QQW

Work done by engine each cycle

hQW

e

J 470J 890W J 420

The efficiency of the engine

J 908J 420 % 2.47 472.0

Th= 550 K

Tc

Engine

Qh

Qc

W

= 470 J

= 890 JFor the engine

The Carnot Cycle

The Laws of Thermodynamics (26 of 38)

Th= 550 K

Tc

Engine

Qh

Qc

W = 420 J

h

c

h

cT

T

Q

Q

= 470 J

= 890 J

Temperature of the cool reservoir

h

chc Q

QTT

J 505J 470

K 550Tc K 290

The engine undergoes 22 cycles per second, its mechanical power output

tW

P Wf

s

cycles 22

cycleJ

420 kW 24.9

The Carnot Cycle

The Laws of Thermodynamics (27 of 38)

Th

Tc

Engine

Qh

Qc

W = 350 J

= 900 JA carnot engine absorbs 900 J of heat each cycle and provides 350 J of work

The heat ejected each cycle

ch QQW

hQW

e

The efficiency of the engine

J 009J 503 % 9.38 389.0

WQQ hc J 350J 900 J 550

The Carnot Cycle

The Laws of Thermodynamics (28 of 38)

Th

Tc = 283 K

Engine

Qh

Qc

W = 350 J

= 900 JA carnot engine absorbs 900 J of heat each cycle and provides 350 J of work

The engine ejects heat at 10 oCThe temperature of the hot reservoir

=550 J

h

c

h

cT

T

Q

Q

c

hch Q

QTT

J 505J 900

K 283Tc K 463 C 190 o

The Carnot Cycle

The Laws of Thermodynamics (29 of 38)

A carnot engine operates between a hot reservoir at 650 K and a cold reservoir at 300 K. If it absorbs 400 J of heat at the hot reservoir, how much work does it deliver?

Th= 650 K

Tc= 300 K

Engine

Qh

Qc

W = ?

= 400 J

hQW

e h

chT

TT

h

chh T

TTQW

K 650K 300K 650

J 400W J 215

The Carnot Cycle

The Laws of Thermodynamics (30 of 38)

Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law:

Natural processes tend to move toward a state of greater disorder.

Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet.

No amount of stirring will get the milk and sugar to come back out of solution.

Entropy

The Laws of Thermodynamics (33 of 38)

Another example: when a tornado hits a building, there is major damage.

You never see a tornado approach a pile of rubble and leave a building behind when it passes.

Thermal equilibrium is a similar process –

the uniform final state has more disorder than the separate temperatures in the initial state.

Entropy

The Laws of Thermodynamics (34 of 38)

Another consequence of the second law:

In any natural process, some energy becomes unavailable to do useful work.

If we look at the universe as a whole, it seems inevitable that, as more and more energy is converted to unavailable forms, the ability to do work anywhere will gradually vanish. This is called the heat death of the universe.

Entropy

The Laws of Thermodynamics (35 of 38)

WQUΔ First law of thermodynamics:

Isothermal process: temperature is constant.

Adiabatic process: no heat is exchanged.

UΔPW Work done by gas at constant pressure:

Heat engine changes heat into useful work (requires temperature difference).

HQW

e H

LQ

Q1 Efficiency of a heat engine:

Carnot efficiency:H

Lc T

T1e

Summary

The Laws of Thermodynamics (36 of 38)