l8 entropy

8/11/2019 L8 Entropy

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Overview

Physics 10: Lecture 8

EntropyPaul Leonard Atchong C. Hilario, M.Sc.

National Institute of Physics, University of the Philippines, Diliman, Quezon City, Philippines

Email: [email protected]

Conservation of Energy

Entropy



2Physics 10: Lecture 8

Tractor beams

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Optical tweezers

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http://upload.wikimedia.org/wikipedia/commons/b/b7/Optical_Trap_Ray_Optics_Explanation.jpg

http://www.physics.nyu.edu/~dg86/figures/tweezer.png

http://www.physics.nyu.edu/~dg86/figures/tweezer.png

http://upload.wikimedia.org/wikipedia/commons/b/b7/Optical_Trap_Ray_Optics_Explanation.jpg




Optical tweezers

Light has momentum

If propagation of light changes directionthen the momentum changed

If the momentum of light changed then anet force is exerted on the light by the

particle

Light also exerts a force on the particle




Last time

Objects continously move because theyhave energy

Energy comes in many forms

Energy can never be created nor destroyed

Energy can only be transformed

Conservation of energy




Seven ideas that shoo the universe

.!he earth i" not the center o#the uni$er"e.

2.!he uni$er"e i" a %echani"%

run &y rule".3.Energy %a'e" it go.4.Entropy tell" it (here to go.5.!he truth" are relati$e &ut

the la( i" a&"olute.6. )ou can not 'no( or predict

e$erything.*.+unda%entally, thing" ne$er

change!Spielberg" #nderson" !even Ieas that !hoo" the Universe, $%%&!

Energy %a'e" it uni$er"e- go.

Conservation of energy










change!Spielberg" #nderson" !even Ieas that !hoo" the Universe, $%%&!

Entropy tell" it uni$er"e-(here to go.

Entropy



/Physics 10: Lecture 8

The Laws of thermodynamics

'eroth Law of Thermodynamics

The first law of thermodynamics

The second law of thermodynamics

The third law of thermodynamics

Entropy




The Laws of thermodynamics

'eroth Law of Thermodynamics

The first law of thermodynamics

The second law of thermodynamics

Entropy




(eat versus temperature

(eat is a #or% o# energy and is describedin terms of the change" in the total

energy of the molecules

Temperature is the concentration of theinternal energy and is described in termsof the a$erage energy of the molecules

)hen two systems are allowed to exchangeheat" they are said to be in ther%alcontact and &oth of their temperaturechanges

Entropy




(eat versus temperature

)hen in thermal contact" the two systemsexhange energy until their te%perature

i" the "a%e

)hen this happens" they are said to be inther%al e1uili&riu%

Entropy



3Physics 10: Lecture 8 Entropy

'eroth law of thermodynamics

#lso nown as thethermometer law

If a third object C is inthermal e*uilibriumwith both # and $

then # and $ are inthermal e*uilibrium




+evisit, conservation of energy

Entropy

-or an isolated system" energy is conserved

To completely mae sense of this concept" wemust determine,

– )hich is the system and which are parts ofthe system

– )hich is not part of the system but is part ofthe environment

)e must then tae into account, – (ow much energy went in.

– (ow much energy went out.

– (ow much wor was done to the system

– (ow much wor was done by the system




The $st law of thermodynamics

Essentially" a statement of the conservationof energy in a thermodynamic system

The change in the internal energy ise*ual to the energy a&"or&ed orrelea"ed

/hysics textboosInternational union for pure and applied

chemistry 0I1/#23

Entropy

ΔU =Q−W

ΔU =Q+W

h i 0 8




The e*uation of state

One can use anything as a thermometer aslong as a de#inite relation"hip between

te%perature and other para%eter" exists!

These are called the e1uation o# "tate!

Example, Ideal gas law 4 P% & n'(

Entropy

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*Physics 10: Lecture 8

Thermodynamic processes

)hen the process does not in$ol$e heattran"#er" it is said to be adia&atic.

)hen the process is under con"tantte%perature" the process is i"other%al.#diabatic processes are not isothermal

)hen the process is under con"tantpre""ure" the process is i"o&aric.

)hen the process is under con"tant$olu%e" the process is i"ochoric.

Entropy

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Thermodynamic processes

Entropy

⏞

isotherms

Ph i 10 L t 8



/Physics 10: Lecture 8

5nd Law of thermodynamics

In an i"olated "y"te%" it is i%po""i&le tore$er"e the #lo( o# heat from fromhigher temperature to lower temperature

(eat flows spontaneously from a hot to cold

On its own" heat will not flow from cold to hot

Entropy

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6ore on engines

Entropy

Engines convert heatinto mechanical

energySteam engines uses

steam to do this

(eat flows from ahot re"er$oir to acold re"er$oir

Physics 10: Lecture 8 E




+efrigerators and aircons

Entropy

(eat flows from acold re"er$oir to

a hot re"er$oir7ot spontaneous

or' i" re1uired

0done by acompressor3

Physics 10: Lecture 8 E




The perfect heat engine

Entropy

#ssume all of theheat is convertedto wor

)e can use thatwor to bring heatfrom cold to hot"essentially aworless ref

8iolate the 5nd law ofthermodynamics

perfect engine

does not exist

Physics 10: Lecture 8 E t




Entropy

Is a measure of the amount of disorder inthe system

Entropy is a relative value hence absolutemeasurement is difficult! )e are howevermore concerned of the change

2hange is measured by amount of heatchange per a&"olute te%perature

Entropy

Physics 10: Lecture 8 E t




Entropy

#luminum at $9 degree 2elsius is in thermalcontact with water at $99 degrees!

#luminum will get hotter and waterbecomes cooler

2hange in entropy of aluminum will be

roughly :.)* +- while that of water isroughly /.)) +- 0you may consult for thedetails3

Total change is ;!%: +-

Entropy

Physics 10: Lecture 8 Entrop




Entropy

)ater at 5; degree 2elsius is in thermalcontact with water at $99 degrees!

2ool water will get hotter and warm waterbecomes cooler

2hange are e*ual in magnitude but reverse

in signs Total change is 9 +-

Entropy

Physics 10: Lecture 8 Entropy




Entropy

-or re$er"i&le proce""e"" the change inentropy i" al(ay" ero

-or irre$er"i&le proce""e"" the change inentropy i" al(ay" po"iti$e

-or non"pontaneou" processes change

in entropy i" negati$eEntropy in general is increa"ing

The uni$er"e" on it" o(n" will move

to(ard" increa"ing entropy

Entropy


Entropy



2*Physics 10: Lecture 8







change!Spielberg #nderson !even Ieas that !hoo" the Universe $%%&

Entropy tell" it uni$er"e-(here to go.

Entropy

l8 entropy

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