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Temperature and Heat

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Chapter 15 Temperature and Heat Mechanics vs. Thermodynamics Mechanics: obeys Newton s Laws key concepts: force kinetic energy static equilibrium Newton s 2nd Law ... – PowerPoint PPT presentation

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Title: Temperature and Heat


1
Chapter 15
  • Temperature and Heat

2
Mechanics vs. Thermodynamics
  • Mechanics
  • obeys Newtons Laws
  • key conceptsforce kinetic energy static
    equilibriumNewtons 2nd Law
  • Thermodynamics
  • will find new laws
  • key conceptstemperature, heatinternal energy
    thermal equilibrium2nd Law of Thermodynamics

3
Temperature (T)
  • Temperature a macroscopic quantity
  • (see later T is related to KE of particles)
  • many properties of matter vary with T (length,
    volume, pressure of confined gas)

4
Temperature (T)
  • Human senses can be deceiving
  • On a cold day iron railings feel colder than
    wooden fences, but both have the same T
  • How can we define T ?
  • Look for macroscopic changes in a system when
    heat is added to it

5
Two Thermometers
  • Add heat to (a) and (b).
  • (a) liquid thermometer
  • liquid level rises
  • T is measured by L
  • (b) constant volume gas thermometer
  • gas pressure p rises
  • T is measured by p

6
Using Thermometers
  • put the bulb of (a) in contact with a body
  • wait until the value of L (i.e. T) settles out
  • the thermometer and the body have reached thermal
    equilibrium (they have the same T)

7
  • Consider thermal interactions of systems in (a).
  • red slab thermal conductor (transmits
    interactions)
  • blue slab thermal insulator (blocks
    interactions)

Demonstration
8
  • Let A and C reach thermal equilibrium (TATC).
  • Let B and C reach thermal equilibrium (TBTC).
  • Then are A and B in thermal equilibrium (TATB)?

Demonstration
9
  • In (a), are A and B in thermal equilibrium?
  • Yes, but its not obvious!
  • It must be proved by experiment!

Demonstration
10
  • Experimentally, consider going from (a) to (b)
  • Thermally couple A to B and thermally decouple C.
  • Experiments reveal no macroscopic changes in A, B!

Demonstration
11
  • This suggests the Zeroth Law of Thermodynamics
  • If C is in thermal equilibrium with both A and
    B,then A and B in thermal equilibrium with each
    other.

Demonstration
12
  • This means If two systems A and B are in
    thermal equilibrium, they must have the same
    temperature (TATB), and vice versa

Demonstration
13
Temperature Scales
14
Temperature Scales
  • Three scales Fahrenheit, Celsius, Kelvin
  • To define a temperature scale, we need one or
    more thermodynamic fixed points
  • fixed point a convenient, reproducible
    thermodynamic environment

15
Temperature Scales
  • Both Fahrenheit and Celsius scales are defined
    using two fixed points
  • freezing point and boiling point of water
  • Kelvin scale defined using one fixed point
  • triple point of water (all three phases
    coexist ice, liquid, vapor)

16
Temperature Scales Summary
  • Relations among temperature scales
  • Fahrenheit temperature
  • Celsius temperature
  • Kelvin temperature

17
Temperature ScalesKelvin vs. Celsius
  • triple point of water
  • we measure TC, triple 0.01oC
  • we define TK, triple 273.16 K
  • (DT)K (DT)C so the unit of DT is K or oC
  • the scales differ only by an offset, so
    TK TC 273.15

18
Kelvin Temperature Scale
  • Fixed point triple point of water TK, triple
  • p pressure of ideal (i.e. low density) gas
    (on a constant volume gas thermometer)
    (has value ptriple at TK, triple)
  • We define

19
  • At low density, see same graph for all gases
  • Extrapolate to p0 (at T absolute zero K)

Demonstration
20
Thermal Expansion
21
Thermal Expansion
  • Empirical law for solids, valid for small DT
  • (simple case all directions expand equally)
  • For a gt 0
  • If DT gt 0 DL gt 0 , material expands
  • If DT lt 0 DL lt 0 , material compresses

22
Thermal Expansion
  • a coefficient of linear expansion gt 0
    (almost always)
  • characterizes thermal properties of matter
  • varies with material (and range of T)
  • unit 1/K, or 1/oC since (DT)K (DT)C

23
Thermal Expansion
  • Example two different materials have different
    DL
  • They can be used to build a thermometer or a
    thermostat

24
  • Atomic explanation of thermal expansion!
  • Recall spring model for diatomic molecule
  • Van der Waals potential energy, U

Demonstration
25
Thermal Expansion
  • Similar for a solid made of many atoms
  • Each pair of atoms has a potential energy U
  • The asymmetry of U explains thermal linear
    expansion!

26
Thermal Volume ExpansionSolids and Liquids
  • b coefficient of volume expansion
  • varies with material (and range of T)
  • unit 1/K, or 1/oC since (DT)K (DT)C

27
Thermal Volume ExpansionSolids
  • Find a simple relationship between linear and
    volume expansion coefficients
  • b 3a

28
Thermal Expansion of Water
  • unusual state
  • a lt 0 if0o C lt T lt 4o C
  • (its why lakes freeze from the top down)

29
Thermal Stress
  • Thermal stress stress required to counteract
    (balance) thermal expansion
  • Tensile thermal stress

30
Announcements
  • Midtermswill probably be returned Monday
  • Homework 5 is returned at front
  • Homework Extra Credit is on record (but not yet
    listed on classweb if it brings a score over the
    maximum)

31
Temperature ScalesKelvin vs. Celsius
  • triple point of water
  • we measure TC, triple 0.01oC
  • we define TK, triple 273.16 K
  • (DT)K (DT)C so the unit of DT is K or oC
  • the scales differ only by an offset, so
    TK TC 273.15

32
Heat and Heat Transfer
33
Quantity of Heat (Q)
  • Heat energy absorbed or lost by a body
    due to a temperature difference
  • Heat energy in transit
  • SI unit J
  • other units 1 cal 4.186 J
    1 kcal calorie on food labels

34
Quantity of Heat (Q)
  • Q gt 0 heat is absorbed by a body
  • Q lt 0 heat leaves a body
  • (we will see several expressions for Q)

35
Quantity of Heat (Q)
  • Conservation of energy (calorimetry)
  • For an isolated system, the algebraic sum of all
    heat exchanges add to zero
  • Q1 Q2 Q3 ... 0

36
Absorption of Heat
  • Q heat energy required to change the
    temperature of material (mass m) by DT
  • c specific heat capacity of the material
    (treat as independent T) unit J/(kg
    K)

37
Absorption of Heat
  • If Q and DT positive heat absorbed by m
  • If Q and DT negative heat leaves m

Do Exercise 15-35
38
Phase Changes
  • phase state of matter solid,
    liquid, vapor
  • energy is needed to change phase of matter
  • under a phase transition of matteronly its
    phase changes, not its temperature!

39
Phase Changes in Water
40
Solid-Liquid Phase Change Q mLf
  • mLf heat needed for phase change
  • Lf (latent) heat of fusion of the material
    (heat/unit mass) needed for transition
    unit J/kg
  • for melting (solid to liquid) for freezing
    (liquid to solid)

Do Exercise 15-51
41
Liquid-Vapor Phase Change Q mLv
  • mLv heat needed for phase change
  • Lv (latent) heat of vaporization
    (heat/unit mass) needed for transition
    unit J/kg
  • for evaporating (liquid to vapor) for
    condensing (vapor to liquid)

42
Heat Transfer
43
Heat Transfer
  • dQ/dt rate of heat flow
    heat current
  • Three mechanisms for achieving heat transfer
  • Conduction
  • Convection
  • Radiation

44
Heat Transfer Mechanisms
  • Conduction Collisions of molecules, no bulk
    motion
  • ConvectionBulk motion from one region to
    another
  • RadiationEmission of electromagnetic waves

45
Conduction
46
Conduction
  • k thermal conductivity of material unit
    W/(mK)
  • A cross sectional area of material
  • L length of material

47
Conduction
Do Exercises 15-57, 15-58
Notes on a composite conducting rod
48
Convection (usually complicated)
49
Radiation (e.g. emitted by the sun)
50
Radiation Electromagnetic Waves
51
Emission of Radiation
  • all bodies emit electromagnetic radiation
  • A surface area of body
  • T surface temperature of body
  • e emissivity of body (0 lt e lt 1)

Do Exercise 15-67
52
Absorption of Radiation
Example of net radiation and Problem 15-89
  • In general, bodies emit radiation and also absorb
    radiation from their surroundings
  • T surface temperature of body
  • TS surface temperature of surroundings
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