review questions from chapter 17 if the temperature of an object is increased from 0°c to 273°c,...

Review Questions from Chapter 17

• If the temperature of an object is increased from 0°C to 273°C, what happens to the power of the radiation emitted?

• If you mix 1 g of ice at 0°C with 1 g of water vapor at 100°C in an insulated container, what will be the equilibrium temperature of the mixture?

• How much ice at 0°C would you have to mix with 1 g of water vapor at 100°C in an insulated container to ensure that there is no water vapor left at equilibrium?

Thermodynamics

Thermodynamics started out as a macroscopic theory.

Max Plank pushed the idea that you could describe things both macroscopically and microscopically.

What is heat? It was originally thought that there is a thing called caloric

fluid that flows from hot to cold.

This was disproved experimentally. Ex: cannon makingCannons got hotter without adding heat

For our purposes, unless otherwise specified, we will think that changes in internal energy come only from changes in thermal energy. This will be true of there are no chemical reactions, no nuclear reactions, and no changes in the magnetic or electric properties.

Recall that work is force times displacement (a special sort of “times” called the dot product).

xFW

Adding heat to the system increases internal energy

The system doing work on its surroundings decreases internal energy

Which has more internal energy, solid water at 0C or liquid water at 0C?

Liquid water, you add Q to melt ice.

Example - blow on hand, then purse lips, compressed air.

Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool.

Which has more internal energy, solid water at 0C or liquid water at 0C?

Liquid water, you add Q to melt ice.

Example - blow on hand, then purse lips, compressed air.

Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool.

Why does air cool as it rises?

What is an adiabatic process?

What happens to the temperature of compressed air when it is allowed to escape through an expansion nozzle into the atmosphere?

What happens to the air temperature when an air mass carried by prevailing wind rises from a valley floor up a mountain?

First law of thermodynamics

• Relation of heat, work and internal energy• As a specific case of conservation of energy• Historical significance

• End of “caloric” theory of heat• Examples of phenomena explained by the 1st law

• Heating during rapid compression

Second law of thermodynamics

• The most basic way of saying the second law is that heat tends to flow from hot objects to cold objects

• We will have three other ways of saying this – you need to know all of them

• Concerns of the 2nd law• Irreversibility of processes• The quality of energy (usefulness of energy)

There are several ways to express the 2nd Law:

Macroscopic:1. Heat flows from hot to cold2. You can’t have a perfect heat engine3. You can’t have a perfect refrigerator.

Microscopic:4. In a closed system, entropy increases as time

goes by (or at least does not decrease). Entropy is a measure of disorder.

2nd Law of Thermodynamics

• Experimental evidence suggests strongly that it is impossible to build a heat engine that converts heat completely to work

hot

W

Qh device Cannot happen


• Heat engine statement of 2nd law - it is impossible to build a heat engine that converts heat completely to work, some heat must be dumped to a colder place

hot

W

Qh device coldQc

Some heat must be dumped to a colder place

W<Qh

Thermodynamic Efficiency of a heat engine

• Efficiency of a heat engine is the ratio of output work to the heat from the hot reservoir (often the heat from burning fossil fuel)

The heat dumped to a colder place

causes W to be smaller than Qh, and

therefore the efficiency to be less

than 100%

hot

W

Qh device coldQc

hQ

Wefficiency

Thermodynamic Efficiency of a heat engine

• The best possible efficiency of a heat engine (the ideal efficiency or Carnot efficiency) depends on the ratio of the cold temperature to the hot temperature, or more directly, the ratio of the temperature difference to the hot temperature. (the temperatures are in kelvin)

h

Ch

h

C

T

TT

T

TefficiencyCarnot

1

Most power plants boil water with the heat from burning fossil fuel or from nuclear reactions, and this steam does work on a turbine. Why is high pressure useful for the steam that is used in a power plant?


• Refrigerator – one would like to be able to remove heat from a cold place (the inside of a refrigerator) and put it in a warm place (a room inside a house)

hot Qh device coldQc

Cannot happen, work has to be done


• Refrigerator statement of 2nd law – to move heat from a cold place to a warmer place, work must be done

hot

W

Qh device coldQc

Need to do work to move heat from a cold place to a warmer place

Refrigerator

• Recall that another way to state the second law is that heat flows spontaneously from a hot place to a cold place, so the picture on the right is in violation of this

Inside refrigerator

Room around

refrigerator

Try refrigerator in class?

(hotter)

Couldn’t I just use my refrigerator to cool my home?

Couldn’t I just use my refrigerator to cool my home?

What if the heat was vented to the outside

(the compressor and condenser were outside)?

Heat pumpThis is the same picture as the refrigerator. But if the device is a heat pump, the cold place is outside, and the hot place is inside the house or the building. Note that more energy goes into the house than the work that is done (in some sense, more than 100% efficient – but not in a thermodynamic sense).

hot

W

Qh device coldQc

Need to do work to move heat from outside to the warmer inside

Second law of thermodynamics

• The 2nd law in terms of organization (microscopic)

• Entropy – labeled S, a measure of disorder (microscopic disorder – may not be apparent macroscopically)

• In a closed system, the entropy tends to increase – it cannot decrease.

• For reversible processes, the entropy does not change

• Caution: 2nd law applies to

• Systems, not processes

• Closed, not open systems

0S


Macroscopic:1. Heat flows from hot to cold.2. You can’t have a perfect heat engine3. You can’t have a perfect refrigerator.

Microscopic:4. In a closed system, entropy never decreases

Suppose you drop an ice cube into a hot cup of tea. The ice melts and the water cools. Why doesn’t the reverse happen – the water warming up and the ice cube formed?

Review question: pick up water and let it fall. How high must it be to change the temperature by 1 K when it hits the ground?

use mgh = mCΔT

Rayleigh Benard convection – uniform temperature on bottom plate and on top plate

Rayleigh Benard convection – with enough temperature difference, convection begins and forms patterns that were

not there in the system to begin with. Order increase

Rayleigh Benard convection videos

• 2-dimensional simulation – note that the small fluctuations are enhanced (positive feedback), but are opposed by viscosity and dissipation

http://www.youtube.com/watch?v=5ApSJe4FaLI

• 3-dimensional simulation – note the stable pattern that develops

http://www.youtube.com/watch?v=eb8PzSmZqRM

• http://hmf.enseeiht.fr/travaux/CD9598/travaux/optmfn/BES_PHOENICS/96-97/Masson-Nicole/rb.html (development of instability)

http://www.youtube.com/watch?v=5ApSJe4FaLI

http://www.youtube.com/watch?v=eb8PzSmZqRM

http://hmf.enseeiht.fr/travaux/CD9598/travaux/optmfn/BES_PHOENICS/96-97/Masson-Nicole/rb.html

http://hmf.enseeiht.fr/travaux/CD9598/travaux/optmfn/BES_PHOENICS/96-97/Masson-Nicole/rb.html

Conditions Needed for order to develop (within some portion of

the system)Must be open (energy must flow through)

Must be far from equilibrium

Must have a feedback system

If the Rayleigh Benard system is placed in an insulated box, so that it becomes a whole system, convection can start and order can develop. Is the entropy of the system increasing? That is, is the total order decreasing? Why or why not?

Spontaneous organization

• In which portion does the entropy change in which ways?

• Necessary conditions for occurrence

• Open system

• Far from equilibrium

• Description of process

• Order through fluctuations

• Dissipative structures

• Common misunderstandings of spontaneous organization

– As a violation of the 2nd law– Impossibly rare according to a naïve view of

development by chance

•http://www.bitstorm.org/gameoflife/

http://www.bitstorm.org/gameoflife/



ŅMaxwellÕs demonÓ

Thought experiment: A little "demon" guards a trapdoor between thetwo containers. When a faster-than-average molecule from A fliestowards the trapdoor, the demon opens it, and the molecule willfly from A to B. The average speed of the molecules in B will haveincreased while in A they will have slowed down on average.

Violation of 2nd Law?

AB

Maxwell’s Demon

The drinking bird

1st law of thermodynamics, 2nd law of thermodynamics, evaporation as a cooling process, saturated vapor pressure changes rapidly with temperature (compared with single phase gas pressure)

http://www.wimp.com/drinkingbird/

http://www.wimp.com/drinkingbird/

• If the cold place to which you are dumping heat is outside on a coolish day (10°C), how hot must the steam be in an ideal power plant be to get 50% efficiency? (ideal means that there is no waste whatsoever, and there is no friction in any of the moving parts)

• The vapor pressure of water is given in the table on the right (for interest)

T (°C) P (torr) P (atm)

0 4.58 0.0060

25 23.8 0.031

50 92.6 0.122

75 289 0.381

100 760 1

125 1,741 2.29

150 3,570 4.70

175 6,694 8.81

200 11,659 15.34

225 19,123 25.2

250 29,818 39.2

275 44,581 58.7

300 64,433 84.8

325 90,448 119.0

350 124,002 163.2

360 139,893 184.1

365 148,519 195.4

373.946 165,452 217.7