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Physics 262/266George Mason University

Prof. Paul So

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Course Info

Website:http://complex.gmu.edu/www-phys/phys262http://complex.gmu.edu/www-phys/phys266

Mastering Physics (automated web-based

assignment system):http://www.masteringphysics.comCourse ID: MPSOSPRING13

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Course Info

First 5 weeks: Thermodynamics (PHYS 262 & 266)

Mid 5 weeks: Optics (PHYS 262)

Last 5 weeks: Modern Physics (PHYS 262)RelativityQuantum Mehcanics

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Chapter 17: Temperature & Heat

meaning of thermalequilibrium

thermometers andtemperature scales absolute zero and the Kelvin

scale

thermal expansion meaning of heat calorimetry calculations mechanisms of heat transfer

Topics for Disscusion

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Description of Physical Systems

microscopic properties ofatoms/molecules thatmake up the systemnot directlyassociated with sense

perceptions

macroscopicbulk properties of thesystem

directly associated

with sense perceptions

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Description of Physical Systems

microscopicvariables:

Theory: Classical/QuantumMechanics

Newtons Eqs, Maxwells Eqs, Schrodingers Eq (later)

macroscopicvariables:

Theory: Thermodynamics 0th Law of Therm. 1st Law of Therm.

2nd Law of Therm.

, , , , , ,...KE PE x v a p

, , , , ,...T P V U S

Statistical Mechanics(e.g. Kinetic Theory)

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Temperature ( T )

Common usage : a measure of hot & coldPhysics definition : average KE of molecules (more

on this next chapter)

Physical changes associated with T :Most materials expand when heatedPressure of gas (in a closed container) with T Electrical resistance changes with T Materials radiate at different at different T State of matter change with T

Ice water steam

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Measuring Temperature/Thermometer

A small amount of liquid will seean increase in volume astemperatures rise. Mercury waschosen early on because itsdense, a small volume can recorda large temperature range.

The pressure of a fixed volume ofgas will rise if temperature rises.

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Measuring Temperature/Thermometer

Another commonly usedmeasuring device relies on thedifferential expansion of

bimetal strips.

D bimetal stripD infrared sensor

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Temperature Scales

Celsius ( oC) & Fahrenheit (oF)are traditionally defined withtwo readily reproduciblereference states:

Freezing point & Boiling pointof water at 1 atm.

532

99

325

oC F

oF C

T T

T T

(new)

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Absolute Zero and the Kelvin Scale (K)

A Gas Thermometer with sufficiently diluted gas (~Ideal Gas)

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Absolute Zero and the Kelvin Scale (K)Important Experimental Observations:1. P vs. T relationship is linear for all dilute gases.2. All curves extrapolate to a single zero point with zero pressure.

These implyThere exists a unique absolute zero reference point and anabsolute temperature scale.

273.15K C T T

At this absolute zero point,

273.15 oC T C

Kelvin T Scale (K)

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Thermal Equilibrium

Observation:When two objects at different T are broughttogether, they will eventually reach the

same temperature and the system reaches anequilibrium state when no further physicalchanges occur in the system.

e.g. warm soda cans in a coolerfilled with ice

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Thermal EquilibriumAdditional concepts

Heat (more on this later): the transfer of energy between objects with different T .

Thermal Contact : two objects are in thermal contactif heat can transfer between them (not necessary in physical contact).

Thermal equilibrium : the situation in which twoobjects in thermal contact cease to exchange energy bythe process of heat .

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The 0 th Law of Thermodynamics

Note: this transitive property is not true for all physical process, e.g. two pieces of ironwith a magnet but the 0 th Law have been shown to be true experimentally.

No heat flow between A-C and B-C. No heat flow between A-B.

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Thermal Expansion (linear)Objects such as theserailroad tracks willthermal expandwhen T increases.The size of thechange will dependon the material.

0

0 0(1 ) L L T

L L L L T

0T T T

is the coefficient of linear expansion

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Thermal Expansion (volume)Molecules can be visualized as bedsprings and spheres. At a giventemperature, these molecules vibrate according to the spring potentialenergy. Because of the asymmetry of this potential energy, the moleculesaverage separation tends to increase with T .

0V V T

One can approx.this volumeexpansion by:

is the coefficient ofvolume expansion

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Linear and Volume Expansion Rates

V WHL

Each of the linear dimensions expands according to the Linear Thermal Expansionequation given previously.

, ,o o o o o oW W W T H H H T L L L T (assuming material to be isotropic same in all directions)

0

( )( )( )

3

o o

o o o

o

o o

o o o o

o

o

o

o o o o

W W T

W W

H H L L T

L L LW T W

V

V V V

T

H H T H L T H

T

(only terms up to T )

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Thermal Expansion of Water

Coolerwaterfalls

warmerwaterrises

4oT C 0 4o oC T C cooler waterstays on top

warmer water

stays on bottom

0oT C Ice forms on top Ice is less dense than 4

oC

water Ice grows from top to bottom

4oC liquid water remainat bottom