thermal physics cbse

7/30/2019 Thermal Physics CBSE

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Thermal PhysicsIB Physics

Topic 3 & Option C


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Thermodynamics

Understanding thewords

Temperature Heat

Heat capacity

The 0, 1, 2 laws of

thermodynamics (one of) Kelvins

legacysWilliamThompson

(Lord Kelvin)


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What is Heat?

Perception as to hot andcold defined relative to ourown body temperature, i.e.

object is hotter or colderthan oneself

Objective measurement oftemperature Macroscopic, display of

temperature gauge

Microscopic behaviour ofatoms and molecules

I feel hot


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Measuring temperature

Properties of materials change withtemperature

LengthVolume

Resistance


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Hotter things become longer

All(?) solids get biggerwhen they get hot

A 1 metre long barheated by 1 degree getsbigger by

Steel 0.01 mm

Glass 0.001 mm

Zerodur 0.0001mm

Rails expand and

may buckle on a

hot summer day


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A bimetallic strip

Join two metals withdifferent coefficient

of thermalexpansion

e.g. fire alarm


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Hotter things take up morevolume -1

Most materials getbigger when they get

hot (but not water 0C -> 4C gets smaller!)

Thermometer relies on

a thermal expansion ofa liquid (e.g.mercury)

Large volume of

reservoir

Thin tube

(Gives big

length change

for small

increase in

volume)


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Hotter things take up morevolume -2

Gases (as we will see)can behave near

perfectly

Hotter


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Hotter things change theirresistance

All hotter metals have a higher electricalresistance or conductivity

Digital thermometer All hotter semiconductors have a lower

electrical resistance key definition between to distinguish metals

and insulators!


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ExampleYou have a (glass) jar and you cant get the

metal lid off.What should you do:

a) ask your girlfriend

b) run the jar & lid under cold water

c) run the jar & lid under hot water


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Solution:

a) ask your girlfriend

b) run the jar & lid under cold water

c) run the jar & lid under hot water

Because the metal has a

substantially higher

coefficient of thermal

expansion than the glass,

heating them will make both

of them bigger, but the metal

will be more bigger.


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How long do you have to leavea thermometer in your mouth?

Hot things stay hot if youinsulate them, e.g.

coffee in a vacuum flask(keeps things cold too)

an explorer in a fur coat

The mercury in the

thermometer must reach thesame temperature as youThermal Equilibrium!!


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Insulation

Example of good (thermal) insulators A vacuum, polystyrene, fibreglass, plastic, wood,

brick

(low density/foam structure, poor electricalconductors)

Examples of poor insulators, i.e. goodconductors Most metals (but stainless steel better than copper)

e.g. gold contact used within IC chips to preventheating

Gases, liquids

(high density, mobile, good electrical conductors)


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Ask a friend if its cool enough

to eat

Your friend eats the hot loaf and says it

cool enough to eat (i.e it is close enough

to their own temperature that it does notburn)

Is it safe for you to eat too

If it is safe for him, its safe for you!


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The 0th law of thermodynamics

If A and B are each inthermal equilibrium with Cthen A and B are in

thermal equilibrium witheach other

If Alfred and the Breadare the same temperatureas Cliff then Alf is thesame temperature as theBread.

=Temp

=Temp

L

=Temp?

Alf

Cliff


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Temperature and scales

Temperature scales (melting & boiling ofwater)

Degrees Celsius (MP 0C 100C)Degrees Kelvin (MP 273.15 K BP 373.15 K)

Degree Fahrenheit (MP 32 F BP 212F)

Explain how a temperature scale isconstructed.


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e ommon empera ureScales Fahrenheit &

Celsius Celsius &

Fahrenheit


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Absolute zero

Ideal gas has zero volume

Resistance of metal drops to zero

(actually superconductivity cuts in above 0K)

Brownian motion ceases(kinetic energy due to thermal excitation)

But lowest temperature attained is 10-9K


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Absolute zero, 0K


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Lord Kelvin

William Thompson, bornBelfast 1824

Student in Natural

Philosophy Professor at 22! Baron Kelvin of Largs in

1897

A giant Thermodynamics, Foams,

Age of the Earth, Patentsgalore!


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Converting between scales

Kelvin to Celsius

K = C + 273.15

C = K - 273.15

Fahrenheit to Celsius (Not IB)

F = C x (9/5) + 32

C = (F - 32) x (5/9)

State the relation between Kelvin & Celsius Scales.


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Example

Convert the followingtemperatures into Fand K

Boiling water, 100C

Freezing water, 0C

Absolute zero,

-273.15C

212F, 373.15K

32F, 273.15K

-460F, 0K


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Type of thermometer

Change in electrical resistance(convenient but not very linear)

Change in length of a bar (bimetallic strip) Change in volume of a liquid

Change in volume of gas (very accurate

but slow and bulky)


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Heat and internal energy

Can you describe the difference betweenthe terms.

TemperatureHeat

Internal Energy


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Temperature & Absolute Temperature

Temperature is a property thatdetermines the direction ofthermal energy transfer between

two bodies in thermal contact.

Absolute temperature is ameasure of the average kineticenergy of the molecules of a

substance. Average kinetic energy is

proportional to absolute temperaturein Kelvin.

kTKEavg2

3


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Example

What is the kineticenergy of an oxygenmolecule at room

temperature ( 21C)?(k = 1.38x10-23J/K)

Since we know the

kinetic energy, how is ittravelling?

This is called the root meansquared speed or rmsspeed.

KE = 3/2 kT

= 3/2(1.38x10-23 x 294)

= 6.09 x 10-21 Joules

We could equate

KE = 1/2 mv2 = KE = 3/2 kT

and get v2 = 3kT/m

mass must be in kg!!!! Not u.


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Heat (Energy)

Is the flow of energy in or out of a system.

Heat (energy) flows because of

temperature differenceBigger temperature difference bigger heat

flow

Less insulation give more heat flow for the

same temperature difference Heat will not flow between two bodies of

the same temperature


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Equilibrium

+ =Hot black coffeeCold milk Light brown coffee

Two objects of different temperature whenplaced in contact will reach the same

temperature


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Heat transfer = energy transfer

Energy measured in Joules but heat oftenmeasured in Calories

One cal raises one gram of water from 14.5C to15.5C

1 cal = 4.186J

Doing work on something usually makes it hot

Joules Experiment! 1st law of thermodynamics heat and work are

both forms of energy


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Sir James Joule

James Prescott Joule 1818-1889

Stirring water made it warm

Change in temperatureproportional to work done

Showing equivalence of heat andmechanical energy

Also that electrical current flowthrough a resistor causesheating


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Joules Experiment


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Internal Energy

Is the total potential and kinetic energy ofthe molecules in a substance.

Potential energy is associated withintermolecular forces.

Kinetic energy includes both translational androtational motion.


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Three Phases Atomic Model

Three States of Ordinary Matter

Solid liquid gas


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Atomic Model of Matter

Comparing Molecular Forces

Solid Largest molecular forces

LiquidGas Weakest molecular forces

When the kinetic energy of the molecules becomecomparable to the energy required for separation

the molecules change there position and separate(PE increase). This is a phase transition

Melting or vaporizing


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Avogadro constant

One mole a any substance is that quantityof the substance whose mass in grams is

numerically equal to the substances molarmass, .

EX: The moloar mass of O2 is 32 g mol-1

NA = 6.02 x 1023

molecules mol-1


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Example

How many grams arethere in a quantity ofoxygen gascontaining 1.2 x 1025molecules?

The number of moles is

(1.20 x 1025)/6.02 x 1023

= 19.93 mol

Since the molar mass is 32 g mol-1

The mass is 19.93 x 32

= 638 g or 0.638 kg


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Example

So, how fast is that O2

molecule traveling? O2 = 32 g/mole

v2 = 3kT/m (rms speed of a molecule)

m = 0.032/(6.023 x 1023) = 5.3x10-26 kg

v2 = 3(1.38 x 10-23J/K)(294)/(5.3x10-26 kg)

v = 479 m/sec


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Transferring heat energy

3 mechanisms

Conduction

Heat transfer through materialConvection

Heat transfer by movement of hot material

Radiation

Heat transfer by light


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Conduction of heat

Conduction in solids

Heat energy causes atoms to vibrate, a vibratingatom passes this vibration to the next

Conduction in metal

Heat energy causes electrons to gain energy,electrons travel through metal (conduction) andcarry heat energy with them

Metals are good conductors of both heat and electricity


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Conduction of heat

The atoms at the bottom are at a highertemperature and will oscillating more stronglythan those at the top.


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Rate of heat flow

Heat flow (H) is energy transfer per unit time, dependson

Temperature difference

Thermal conductivity (k) k (copper) = 385 W/(m K)

k (glass) = 0.8 W/(m K)

k (air) =0.02 W/(m K)

ATH TC

L

L

TTkA

t

QH CH


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Example

Two rods of the samecross-sectional area are

joined together. The rightrod is a better conductorof heat than the rod atleft. The ends are kept atfixed temperatures. In which rod is the rate of

heat transfer the largest?

Is the temperature at thejoining point lower arehigher than 54 C?

Heat entering the joint must equal the

heat leaving the joint. (Conservation

of Energy). Hence, the rate of heat

transfer is the same.

Since the second conductor is poor a

much larger temperature gradient can

be maintained. Thus, the temperature

at the junction will be larger.


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Thermal conduction vs thermalresistance

Also can use thermal resistance, cf

Can make equation of heat flow more

general

RTT

LTTkA

tQH CHCH

x

TkA

t

QH

i.

e.

R L

kA


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Convection of heat

Hot air rises (and

takes its heat with it!)

Radiators


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Convection of heat

Hot air rises (and takes its heat with it!)Cumulus clouds


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Figure 16-11AlternatingLand and Sea Breezes


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Convection of heat

Hurricanes

Plate tectonics


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Radiation of heat

Dont confuse with

radioactivity

Instead realize that lightcarries heat (e.g. the sunheats the earth)

Anything above absolute zero

radiates heat P aAT4 Stefan-Boltzmann law.


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Radiation of heat

involves the generation andabsorption of photons. Unlikeconduction or convection, however,radiation requires no interveningmedium to transport the heat.

All objects radiate energycontinuously in the form ofelectromagnetic waves

The hotter an object the morepower it radiate sand the shorterthe wavelength of the peakemission wavelength

N t ll thi it h t th


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Not all things emit heat thesame

Heat emission from an object area A

P = AesT4

s= Stafans constant = 5.6x10-8

W/(m2

K4

) e = emissivity of a body, 0 -1

ecopper = 0.3

ecarcoal 1


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Example

Estimate the upperlimit to the heatemission of the sun

Suns temperature7000k

Suns radius 7x108m

Emission, P = AesT4

Area = 4pr2 = 6.2 x 1018 m2

Emissivity 1

H = 6.2 x 1018 x 5.6x10-8x70004

Suns output = 8.3 x 1026 W

A h t itt l d


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Are heat emitter also goodabsorbers?

Black and dull on the surface Best emitter/absorber

Charcoal

Blackbody radiators

perfect absorber & emitter

White and polished/shiny Good Reflectors

Stay cool in the summer


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Figure 16-12The Thermos Bottle

Discuss the operation of a thermos making reference tomethods of heat exchange.


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Assignment

Questions from Packet.1, 2, 3, 5, 7, 10, 11, 12, 14, 18, 19.


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The colour of heat

Peak wavelength of light emitted depends ontemperature

Spectrum includes all wavelength longer than

the peak but not many above 20C - peak in infrared (need thermal imaging

camera to see body heat)

800C - peak in red (electric coil, fire glows reds)

3000 - peak in blue (but includes green and red lighthence appears white)

2.7K peak in micro-wave (background emission inthe universe left over from the Big Bang)

thermal physics cbse

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