Loading...

PPT – Temperature and Heat PowerPoint presentation | free to download - id: 4cf53b-OGRkN

The Adobe Flash plugin is needed to view this content

Chapter 15

- Temperature and Heat

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

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)

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

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

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)

- Consider thermal interactions of systems in (a).
- red slab thermal conductor (transmits

interactions) - blue slab thermal insulator (blocks

interactions)

Demonstration

- 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

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

Demonstration

- 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

- 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

- This means If two systems A and B are in

thermal equilibrium, they must have the same

temperature (TATB), and vice versa

Demonstration

Temperature Scales

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

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)

Temperature Scales Summary

- Relations among temperature scales
- Fahrenheit temperature
- Celsius temperature
- Kelvin temperature

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

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

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

Demonstration

Thermal Expansion

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

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

Thermal Expansion

- Example two different materials have different

DL - They can be used to build a thermometer or a

thermostat

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

Demonstration

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!

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

Thermal Volume ExpansionSolids

- Find a simple relationship between linear and

volume expansion coefficients - b 3a

Thermal Expansion of Water

- unusual state
- a lt 0 if0o C lt T lt 4o C
- (its why lakes freeze from the top down)

Thermal Stress

- Thermal stress stress required to counteract

(balance) thermal expansion - Tensile thermal stress

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)

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

Heat and Heat Transfer

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

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)

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

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)

Absorption of Heat

- If Q and DT positive heat absorbed by m
- If Q and DT negative heat leaves m

Do Exercise 15-35

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!

Phase Changes in Water

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

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)

Heat Transfer

Heat Transfer

- dQ/dt rate of heat flow

heat current - Three mechanisms for achieving heat transfer
- Conduction
- Convection
- Radiation

Heat Transfer Mechanisms

- Conduction Collisions of molecules, no bulk

motion - ConvectionBulk motion from one region to

another - RadiationEmission of electromagnetic waves

Conduction

Conduction

- k thermal conductivity of material unit

W/(mK) - A cross sectional area of material
- L length of material

Conduction

Do Exercises 15-57, 15-58

Notes on a composite conducting rod

Convection (usually complicated)

Radiation (e.g. emitted by the sun)

Radiation Electromagnetic Waves

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

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