Notes - Thermodynamics & Entropy

Example

A series of thermodynamic processes is shown in the pV-diagram. In process ab 150 J of heat is added to the system, and in process bd , 600J of heat is added. Fill in the chart.

150 600 750

0 240 240

150 J 840 J 990 J

90

0

Thermodynamic Processes -

Isothermal

constant both the pressure and volume change to compensate. (Volume goes up,

Thermodynamic Processes -

Isobaric

Heat is added to the gas which increases the

Internal Energy (U) Work is done by the gas as it changes in volume.

The path of an isobaric process is a horizontal line called an isobar.

Thermodynamic Processes -

Adiabatic

"impassable") In other words, NO

Second Law of Thermodynamics

warmer body AND heat energy cannot be transformed completely into mechanical work.”

The bottom line:

Engines

C H

output

C H

Q

Q

W

Q

W

Q









Engine Efficiency

In order to determine the

thermal efficiency of an engine you have to look at how much ENERGY you get OUT based on how much you energy you take IN. In other words:

H C

H C H

hot thermal

Q Q Q

Q Q

Q W

Is there an IDEAL engine

model?

what’s the most work we can

possibly get for a given amount of fuel?

The efficiency question was first posed—and solved—by Sadi Carnot in 1820, not long after steam engines had become

efficient enough to begin replacing water wheels, at that time the main power sources for industry. Not surprisingly, perhaps, Carnot visualized the heat engine as a kind of water wheel in which heat (the “fluid”) dropped from a high temperature to a

low temperature, losing “potential energy” which the engine turned into work done, just like a water wheel.

Carnot Efficiency

Carnot temperatures must be expressed in KELVIN!!!!!!

The Carnot model has 4 parts •An Isothermal Expansion •An Adiabatic Expansion •An Isothermal Compression •An Adiabatic Compression

Example

A particular engine has a power output of 5000 W and an efficiency of 25%. If the engine expels 8000 J of heat in each cycle, find (a) the heat absorbed in each cycle and (b) the time for each cycle

Example

The efficiency of a Carnot engine is 30%. The engine absorbs 800 J of heat per cycle from a hot temperature reservoir at 500 K. Determine (a) the heat expelled per cycle and (b) the temperature of the cold reservoir                 C C H C C C C C H H T T T T e Q Q W Q Q W W J W Q W e 500 1 30 . 0 1 800 800 30 .

0 240 J

560 J

Second Law of Thermodynamics

 _{Impossible processes consistent with the First}

Law

 _{The spontaneous (i.e. without the action of}

another agent) transfer of thermal energy from a cold body to a hotter body

 _{The air in a room suddenly occupying just one half}

of the room and leaving the other half empty

 _{A glass of water at room temperature suddenly}

Second Law of Thermodynamics

 _{Order and Disorder – Consider a bowling ball}

rolled down the hall in between classes

Second Law of Thermodynamics

 _{Once the ball comes to a stop, the ordered}

kinetic energy can’t be recovered because it has been degraded.

 _{All that is left is the disordered heat energy}

Second Law of Thermodynamics

 _{The Second Law of Thermodynamics deals}

with the limitations imposed on heat

Second Law of Thermodynamics

 _{Reversibility}

 All natural processes are irreversible

 They lead to a state of increased disorder

 _{Measure of Disorder – Entropy}

Entropy

 _{During an isothermal}

expansion of a gas, the gas must receive heat

 _{During an isothermal}

compression of a gas, the gas expels heat

 _{The net change in entropy is}

zero

T

Q

S





Entropy

 _{The entropy of heat}

flowing from a hot object to a cold one

 _{The hot object loses}

heat, the cold one gains heat

 _{Entropy increases}

T

Q

S





Entropy

 _{The entropy of heat}

flowing to a hot object

from a cold one

 _{The hot object gains}

heat, the cold one

loses heat

 _{Entropy decreases}

Ain’t Gonna Happen!

T

Q

S





Second Law of Thermodynamics

(finally!)

 _{General Statement:}

 The entropy of an isolated system never decreases.

 It is impossible for thermal energy to

Energy Degradation

 _{The flow of energy from a hotter to a colder}

object tends to equalize the two

temperatures and deprives us of the opportunity to do work.

W

Q

U





Energy Degradation

 _{It is a consequence of the second law that}

energy, while always being conserved, becomes less useful – this is energy

Energy Degradation of the Universe

 _{Consider the universe to be a big bag of gas}  _{As far as we know, the universe is a closed}

system – no heat is entering the universe

 _{Since the universe is expanding, positive work is}

being done

0

0











U

Energy Degradation of the Universe

 _{Thus the internal energy of the universe, i.e. its}

temperature, is constantly decreasing

 _{This aligns with the idea that the temperature}

during the BIG BANG was millions of degrees and today, wavelength measurements of

electromagnetic radiation left over from the BIG BANG show the temperature

of the universe to be about

2.7K

0

0











U

Energy Degradation of the Universe

 _{The inevitable result of the constant cooling of}

the universe is,

0

0











U

Summary

 _{Do you understand the meaning of internal}

energy?

 _{Can you calculate work when a gas expands}

or compresses using?

V

P

W





Summary

 _{Can you state the relationship between}

changes in the internal energy, the work done and the thermal energy supplied through the

first law of thermodynamics?

 _{Can you define the terms adiabatic,}

isothermal, isobaric, and isochoric and show these on a pressure-volume diagram?

W

Q

U





Summary

 _{Do you understand what is meant by}

irreversibility and disorder?

 _{Do you understand that entropy is a measure}

of disorder?

 _{Can you state the second law of}

thermodynamics?

 _{Do you understand the meaning of energy}