© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 18: THERMODYNAMICS

© 2010 Pearson Education, Inc.

Conceptual Physics11th Edition

Chapter 18:

THERMODYNAMICS


This lecture will help you understand:• Thermodynamics• Absolute Zero• Internal Energy• First Law of Thermodynamics• Adiabatic Processes• Meteorology and the First Law• Second Law of Thermodynamics• Order Tends to Disorder• Entropy


Thermodynamics• The science of thermodynamics was developed

in the early 19th century. • Atomic and molecular theory of matter was not

understood then, so early thermodynamics invoked macroscopic ideas such as mechanical work, pressure, and temperature.

• The foundation stones of thermodynamics are:– The conservation of energy and – The fact that heat flows spontaneously from hot to

cold and not the other way around


Absolute ZeroAs the temperature of a gas changes, the volume of a gas changes.• At zero degrees with pressure constant, volume

changes by 1/273 for each degree Celsius.

Absolute zero• Lowest limit of temperature.• Molecules have lost all available kinetic energy.


Internal Energy

• Energy at the particle level within a substance in several forms, which, when taken together, are called internal energy.

• The internal energy of a substance is quite complicated; the simplest form are the kinetic and potential energies of the molecules.

• Our study focuses not on the internal energy, per se, but rather on the changes in internal energy of a substance.


The First Law of Thermodynamics

States that the heat added to a system transforms to an equal amount of some other form of energy.

Heat added to system =

increase in internal energy + work done by system

The first law of thermodynamics is a restatement of the law of conservation of energy:

Energy can neither be created nor destroyed.


The First Law of ThermodynamicsAnother implication: Instead of adding heat, if we do mechanical work on a system, we can expect an increase in internal energy, i.e., temperature rise.

Examples:• Rubbing your hands, makes them

warmer.• Joule’s apparatus: As the weights fall,

they lose potential energy (mechanical), which is converted to heat that warms the water.


When work is done on a system—for example, compressing air in a tire pump—the temperature of the system

A. increases.

B. decreases.

C. remains unchanged.

D. is no longer evident.

First Law of ThermodynamicsCHECK YOUR NEIGHBOR


When work is done on a system—for example, compressing air in a tire pump—the temperature of the system

A. increases.

B. decreases.

C. remains unchanged.

D. is no longer evident.

Explanation:

In accord with the first law of thermodynamics, work input increases the energy of the system.

First Law of ThermodynamicsCHECK YOUR ANSWER


Adiabatic Processes

• Compressing or expanding a gas while no heat enters or leaves the system is an adiabatic process.

• Adiabatic conditions are achieved by– thermally insulating a system from its

surroundings, or – performing the process so rapidly that heat

has no time to enter or leave.


Adiabatic Processes

Example:

When we compress air using a bicycle pump, i.e., when we do work on the system, we heat the air up, i.e., increase its internal energy.

Heat added to system = 0

So:

Increase/Decrease in internal energy

= work done on/by system


Blow air on your hand first with your mouth wide open, then with puckered lips. In which case is the air coming out of your mouth cooler?

A. When your mouth is wide openB. When your lips are puckered C. It is equally cool in both cases.D. It depends upon who does the experiment.

Adiabatic ProcessesCHECK YOUR NEIGHBOR


Blow air on your hand first with your mouth wide open, then with puckered lips. In which case is the air coming out of your mouth cooler?

A. When your mouth is wide openB. When your lips are puckered C. It is equally cool in both cases.D. It depends upon who does the experiment.

Adiabatic ProcessesCHECK YOUR ANSWER

Explanation:

When you pucker your lips the air expands as it comes out.

Air expanding adiabatically does work on the surroundings.

So, it loses internal energy which makes it cooler.


Meteorology and the First Law• Thermodynamics is useful to meteorologists when

analyzing weather. • The first law of thermodynamics is expressed as:

Air temperature rises as heat is added or as pressure is increased.

• Heat may be added as– incoming solar radiation.– radiation back from Earth.– moisture condensation.– contact with ground.


Meteorology and the First Law• In the adiabatic form (i.e., when no heat is added),

the first law of thermodynamics becomes:

Air temperature rises (or falls) as pressure increases (or decreases).

• Adiabatic processes in the atmosphere occur in large parts of the air, called parcels.– Parcels are large enough that outside air doesn’t

appreciably mix with the air inside them. – They behave as if they are enclosed in giant, tissue-light

garment bags.


Meteorology and the First Law• As parcels of air rise, they

experience lower pressure and so they expand.

• The expanding air cools down— 10°C for every 1-km rise in altitude.

• Air continues to rise and expand as long as it has a higher temperature than its surroundings.

• When it gets cooler than the surroundings, it sinks


Meteorology and the First Law• As parcels of air drop, they experience higher

pressure and heat up.• Example: Chinook wind that descends from the

Rockies into the Great Plains warms up.


Meteorology and the First Law• Air continues to rise and expand as long as it has

a higher temperature than its surroundings.

• When it gets cooler than the surroundings, it sinks.

• So, sometimes cooler air occurs at an altitude lower than warmer air: temperature inversion.


Meteorology and the First LawDuring a temperature inversion, if rising warm air is denser than upper layers of warm air, it will no longer rise.

Example: Smoke from a campfire sometimes may not rise.

Example: Smog in LA is trapped by hot air from the desert coming over the mountains.


If a parcel of dry air initially at 10°C at ground level expands adiabatically while flowing upward alongside a mountain a vertical distance of 5 km, what will its temperature be?

A. 10°C

B. 20°C

C. 40°C

D. 50°C

Metrology and the First LawCHECK YOUR NEIGHBOR


If a parcel of dry air initially at 10°C at ground level expands adiabatically while flowing upward alongside a mountain a vertical distance of 5 km, what will its temperature be?

A. 10°C

B. 20°C

C. 40°C

D. 50°C

Metrology and the First LawCHECK YOUR ANSWER

Explanation:Air cools at the rate of 10°C for every 1 km. So, if it rises 5 km, it will cool: 10°C 5 = 50°C.So, it will be 50°C cooler than on the ground.Its temp. on the ground was 10°C.So, its temp. at the top will be 10°C – 50°C = 40°C.


Second Law of ThermodynamicsHeat itself never spontaneously flows from a cold

object to a hot substance.• Example:

– In summer, heat flows from the hot air outside into the cooler interior.

– In winter, heat flows from the warm inside to the cold exterior.

• Heat can flow from cold to hot only when work is done on the system or by adding energy from another source.– Example: heat pumps, air conditioners


Second Law of Thermodynamics• A heat engine is any device that converts

internal energy into mechanical work.

• The basic idea behind a heat engine is that mechanical work can be obtained only when heat flows from a high temperature to a low temperature.

• In every heat engine, only some of the heat can be transformed into work.


Second Law of Thermodynamics• Every heat engine has

– a reservoir of heat at a high temperature.– a sink at lower temperature.

• Every heat engine1. gathers heat from the

reservoir at high temperature.

2. converts some of this heat into mechanical work.

3. expels the rest of the heat to the sink at lower temperature.


Second Law of ThermodynamicsApplied to heat engines, the second law of

thermodynamics is stated:

When work is done by a heat engine operating between two temperatures, Thot and Tcold, only

some of the input heat at Thot can be converted to work, and the rest is expelled at Tcold.

Every heat engine expels some heat, for example:• The hood of a car becomes hot.• Hot air is expelled from a laundry or baking oven.


Second Law of Thermodynamics

• The ideal (i.e., maximum possible) efficiency of a heat engine was determined by Carnot.

• It depends upon the temperature of the hot reservoir (Thot) and the cold sink (Tcold).

where, Tbot and Tcold are in kelvin.

• In real heat engines, the efficiency is actually less than ideal, due to friction.

hot cold

hot

- Ideal efficiency

T T

T


What is the ideal efficiency of a heat engine that is operating between a hot reservoir at a temperature of 400 K and a cold sink at a temperature of 300 K?

A. 1/4

B. 1/2

C. 1/3

D. 3/4

Second Law of ThermodynamicsCHECK YOUR NEIGHBOR


What is the ideal efficiency of a heat engine that is operating between a hot reservoir at a temperature of 400 K and a cold sink at a temperature of 300 K?

A. 1/4

B. 1/2

C. 1/3

D. 3/4

Second Law of ThermodynamicsCHECK YOUR ANSWER

Explanation:Use the formula by Carnot:

In this case: Thot = 400 K, Tcold

= 300 KSo:

4/1400

3001 efficiency Ideal

h t co d

ho

Ideal eff e o l

t

-ici ncy

T TT

Ideal ef


Order Tends to Disorder

Restatement of the second law of thermodynamics

In natural processes, high-quality energy tends to transform into lower-quality energy—order

tends to disorder.

• Processes in nature moving from disorder to order do not occur without external assistance.


Entropy

• The measure of the amount of disorder in a system is called entropy.

• If disorder increases, then entropy increases.

• Entropy can decrease if work is put into the system.

Example: Living organisms take in food or extract energy from their surroundings and become more organized.


Your locker gets messier each week. In this case, the entropy of your locker is

A. increasing.

B. decreasing.

C. hanging steady.

D. nonexistent.

EntropyCHECK YOUR NEIGHBOR


Your locker gets messier each week. In this case, the entropy of your locker is

A. increasing.

B. decreasing.

C. hanging steady.

D. nonexistent.

Comment:

If your locker became more organized each week, then entropy would decrease in proportion to the effort expended.

EntropyCHECK YOUR ANSWER


Entropy

The net entropy in the universe is continually increasing (continually running “downhill”).

• We say net because there are some regions in which energy is actually being organized and concentrated.– This occurs in living organisms, which survive by

concentrating and organizing energy from food sources.

– Energy must be transformed into the living system to support life. When it isn’t, the organism soon dies and tends toward disorder.


Entropy

The second law of thermodynamics is a probabilistic statement.

• Given enough time, even the most improbable states may occur; entropy may sometimes decrease.– For example, the haphazard motions of air molecules

could momentarily become harmonious in a corner of the room.

• These situations are possible, but they are not probable.

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© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 18: THERMODYNAMICS