Thermal Physics IB Physics

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Thermal PhysicsIB Physics

• Topic 3 Option C

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Thermodynamics

• Understanding the words
• Temperature
• Heat
• Heat capacity
• The 0, 1, 2 laws of thermodynamics
• (one of) Kelvins legacys

WilliamThompson (Lord Kelvin)
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What is Heat?

• Perception as to hot and cold defined relative to
  our own body temperature, i.e. object is hotter
  or colder than oneself
• Objective measurement of temperature
• Macroscopic, display of temperature gauge
• Microscopic behaviour of atoms and molecules

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Measuring temperature

• Properties of materials change with temperature
• Length
• Volume
• Resistance

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Hotter things become longer

• All(?) solids get bigger when they get hot
• A 1 metre long bar heated by 1 degree gets bigger
  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 with different coefficient of
  thermal expansion

e.g. fire alarm
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Hotter things take up more volume -1

• Most materials get bigger when they get hot (but
  not water 0C -gt 4C gets smaller!)
• Thermometer relies on a thermal expansion of a
  liquid (e.g.mercury)

Thin tube (Gives big length change for small
increase in volume)
Large volume of reservoir
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Hotter things take up more volume -2

• Gases (as we will see) can behave near perfectly

Hotter
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Hotter things change their resistance

• All hotter metals have a higher electrical
  resistance or conductivity
• Digital thermometer
• All hotter semiconductors have a lower electrical
  resistance
• key definition between to distinguish metals and
  insulators!

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Example

• You 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 leave a thermometer in
your mouth?

• Hot things stay hot if you insulate 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 the
  same temperature as you Thermal Equilibrium!!

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Insulation

• Example of good (thermal) insulators
• A vacuum, polystyrene, fibreglass, plastic, wood,
  brick
• (low density/foam structure, poor electrical
  conductors)
• Examples of poor insulators, i.e. good conductors
• Most metals (but stainless steel better than
  copper) e.g. gold contact used within IC chips
  to prevent heating
• 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 not burn)
• 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 in thermal equilibrium with C
  then A and B are in thermal equilibrium with each
  other
• If Alfred and the Bread are the same temperature
  as Cliff then Alf is the same temperature as the
  Bread.

Temp
Temp?
Temp
Cliff
Alf
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Temperature and scales

• Temperature scales (melting boiling of water)
• 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 is constructed.

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The Common Temperature Scales

• 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, born Belfast 1824
• Student in Natural Philosophy
• Professor at 22!
• Baron Kelvin of Largs in 1897
• A giant
• Thermodynamics, Foams, Age of the Earth, Patents
  galore!

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Converting between scales
State the relation between Kelvin Celsius
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)

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Example

• Convert the following temperatures into F and 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 between the
  terms.
• Temperature
• Heat
• Internal Energy

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Temperature Absolute Temperature

• Temperature is a property that determines the
  direction of thermal energy transfer between two
  bodies in thermal contact.
• Absolute temperature is a measure of the average
  kinetic energy of the molecules of a substance.
• Average kinetic energy is proportional to
  absolute temperature in Kelvin.

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Example

• What is the kinetic energy of an oxygen molecule
  at room temperature ( 21C)?
• (k 1.38x10-23J/K)
• Since we know the kinetic energy, how is it
  travelling?
• This is called the root mean squared speed or rms
  speed.

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
  difference
• Bigger 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

• Two objects of different temperature when placed
  in contact will reach the same temperature

Hot black coffee
Cold milk
Light brown coffee
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Heat transfer energy transfer

• Energy measured in Joules but heat often measured
  in Calories
• One cal raises one gram of water from 14.5C to
  15.5C
• 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 temperature proportional to work done
• Showing equivalence of heat and mechanical energy
• Also that electrical current flow through a
  resistor causes heating

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Joules Experiment
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Internal Energy

• Is the total potential and kinetic energy of the
  molecules in a substance.
• Potential energy is associated with
  intermolecular forces.
• Kinetic energy includes both translational and
  rotational 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
• Liquid
• Gas Weakest molecular forces
• When the kinetic energy of the molecules become
  comparable 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 quantity of the
  substance whose mass in grams is numerically
  equal to the substances molar mass, µ.
• 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 are there in a quantity of oxygen
  gas containing 1.2 x 1025 molecules?

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 material
• Convection
• 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 vibrating
  atom passes this vibration to the next
• Conduction in metal
• Heat energy causes electrons to gain energy,
  electrons travel through metal (conduction) and
  carry 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 higher
  temperature and will oscillating more strongly
  than those at the top.

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Rate of heat flow

• Heat flow (H) is energy transfer per unit time,
  depends on
• 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)

A
TH
TC
L
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Example

• Two rods of the same cross-sectional area are
  joined together. The right rod is a better
  conductor of heat than the rod at left. The ends
  are kept at fixed temperatures.
• In which rod is the rate of heat transfer the
  largest?
• Is the temperature at the joining point lower are
  higher 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 thermal resistance

• Also can use thermal resistance, cf
• Can make equation of heat flow more general

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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 16-11Alternating Land 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 light carries heat (e.g. the
  sun heats the earth)
• Anything above absolute zero radiates heat
• P a AT4 Stefan-Boltzmann law.

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Radiation of heat

• involves the generation and absorption of
  photons. Unlike conduction or convection,
  however, radiation requires no intervening medium
  to transport the heat.
• All objects radiate energy continuously in the
  form of electromagnetic waves
• The hotter an object the more power it radiate
  sand the shorter the wavelength of the peak
  emission wavelength

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Not all things emit heat the same

• 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 upper limit to the heat emission of
  the sun
• Suns temperature 7000k
• Suns radius 7x108m

Emission, P AesT4 Area 4pr2 6.2 x 1018 m2
Emissivity 1 H 6.2 x 1018 x 5.6x10-8 x
70004 Suns output 8.3 x 1026 W
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Are heat emitter also good absorbers?

• 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 to methods 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 on
  temperature
• 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
  light hence appears white)
• 2.7K peak in micro-wave (background emission in
  the universe left over from the Big Bang)