chapter1 thermal physics student

8/14/2019 Chapter1 THERMAL PHYSICS Student

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Chapter 1

Thermal Physics


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1.1 Temperature and TheZeroth Law ofThermodynamics


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Thermal Physics Thermal physics is the study of

Temperature Heat How these affect matter


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Thermal Physics, cont Concerned with the concepts of energy

transfers between a system and its

environment and the resultingtemperature variations Historically, the development of

thermodynamics paralleled the

development of atomic theory Concerns itself with the physical and

chemical transformations of matter inall of its forms: solid, liquid, and gas


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Heat The process by which energy is

exchanged between objects because of

temperature differences is called heat Objects are in thermal contact if energy

can be exchanged between them

Thermal equilibrium exists when twoobjects in thermal contact with eachother cease to exchange energy


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Zeroth Law of

Thermodynamics

If objects A and B are separately in thermal

equilibrium with a third object, C, then A and Bare in thermal equilibrium with each other.

Allows a definition of temperature


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Temperature from the

Zeroth Law Two objects in thermal equilibrium

with each other are at the same

temperature Temperature is the property that

determines whether or not an

object is in thermal equilibriumwith other objects


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1.2 Thermometers andTemperature Scales


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Thermometers Used to measure the temperature of an

object or a system Make use of physical properties that

change with temperature Many physical properties can be used

volume of a liquid length of a solid pressure of a gas held at constant volume volume of a gas held at constant pressure electric resistance of a conductor color of a very hot object


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Thermometers, cont A mercury

thermometer is anexample of a common

thermometer The level of the

mercury rises due tothermal expansion

Temperature can bedefined by the heightof the mercurycolumn


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the bimetallic strip.

Thermometers, cont


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Thermometers, cont

Any conductor (such as a metal) is subjected to athermal gradient, it will generate a voltage. This isnow known as the thermoelectric effect or Seebeckeffect. The magnitude of voltage generated

depends on the metal in use.

Thermocouple
http://en.wikipedia.org/wiki/Thermoelectric_effecthttp://en.wikipedia.org/wiki/Thermoelectric_effect


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Thermometers, contThermistor

A thermistor is a type ofresistor used to measuretemperature changes, relying onthe change in its resistance with

changing temperature.Thermistor is a combination ofthe words thermal and resistor.

The Thermistor was invented bySamuel Ruben in 1930.

A thermistor operates throughan electrical resistance changein semi-conductors rather thanpure metal.
http://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Thermal_%28disambiguation%29http://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Samuel_Rubenhttp://en.wikipedia.org/wiki/Samuel_Rubenhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Thermal_%28disambiguation%29http://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Resistor


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Temperature Scales Thermometers can be calibrated

by placing them in thermal contact

with an environment that remainsat constant temperature Environment could be mixture of ice

and water in thermal equilibrium Also commonly used is water and

steam in thermal equilibrium


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Celsius Scale Temperature of an ice-water mixture is

defined as 0 C

This is the freezing pointof water Temperature of a water-steam mixture is

defined as 100 C This is the boiling pointof water

Distance between these points is dividedinto 100 segments or degrees


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Gas Thermometer Temperature

readings arenearlyindependent ofthe gas

Pressure varieswith temperaturewhen maintaininga constantvolume


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Kelvin Scale When the pressure of a gas goes to zero,

its temperature is 273.15 C

This temperature is called absolute zero This is the zero point of the Kelvin scale

273.15 C = 0 K

To convert: TC

= TK

273.15

The size of the degree in the Kelvin scale isthe same as the size of a Celsius degree


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Pressure-Temperature

Graph All gases

extrapolate to the

same temperatureat zero pressure

This temperature isabsolute zero


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Modern Definition of

Kelvin Scale Defined in terms of two points

Agreed upon by International Committee on

Weights and Measures in 1954 First point is absolute zero

Second point is the triple pointof water Triple point is the single point where water

can exist as solid, liquid, and gas Single temperature and pressure Occurs at 0.01 C and P = 4.58 mm Hg


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Modern Definition of

Kelvin Scale, cont The temperature of the triple point

on the Kelvin scale is 273.16 K

Therefore, the current definition ofof the Kelvin is defined as

1/273.16 of the temperature of

the triple point of water


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Some Kelvin

Temperatures Some

representative

Kelvintemperatures

Note, this scale islogarithmic

Absolute zero hasnever beenreached


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Fahrenheit Scales Most common scale used in the US

Temperature of the freezing point

is 32 Temperature of the boiling point is

212

180 divisions between the points


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Comparing Temperature

Scales


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Converting Among

Temperature Scales

( )

273.15

932

5

532

9

9

5

C K

F C

C F

F C

T T

T T

T T

T T

=

= +

=

=


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1.3 Thermal Expansion ofSolids and Liquids


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Thermal Expansion The thermal expansion of an object is a

consequence of the change in the averageseparation between its constituent atoms or

molecules At ordinary temperatures, molecules vibrate

with a small amplitude As temperature increases, the amplitude

increases This causes the overall object as a whole to

expand(eg: wall, roadways, railways) How about human?


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Linear Expansion For small changes in temperature

, the coefficient of linearexpansion, depends on thematerial

See table 10.1 These are average coefficients, they

can vary somewhat with temperature

( )o o oL L T or L L T T = =


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Applications of Thermal

Expansion Bimetallic Strip

Thermostats

Use a bimetallic strip Two metals expand differently

Since they have different coefficients of expansion


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Area Expansion Two dimensions

expand accordingto

is the coefficientof area expansion Active figure

,

2

o A A t

=

=


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Volume Expansion Three dimensions expand

For liquids, the coefficient of volumeexpansion is given in the table 10.1

=

=

3,solidsfor

tVVo


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More Applications of

Thermal Expansion Pyrex Glass

Thermal stresses are smaller than for

ordinary glass Sea levels

Warming the oceans will increase the

volume of the oceans


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Unusual Behavior of Water

As the temperature of water increases from0C to 4 C, it contracts and its densityincreases

Above 4 C, water exhibits the expectedexpansion with increasing temperature

Maximum density of water is 1000 kg/m3 at 4

C


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1.4 Macroscopic Descriptionof an Ideal Gas


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Ideal Gas A gas does not have a fixed

volume or pressure

In a container, the gas expands tofill the container

Most gases at room temperature

and pressure behaveapproximately as an ideal gas


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Characteristics of an Ideal

Gas Collection of atoms or molecules

that move randomly

Exert no long-range force on oneanother

Each particle is individually point-

like Occupying a negligible volume


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Moles Its convenient to express the amount of

gas in a given volume in terms of thenumber of moles, n

One mole is the amount of the substancethat contains as many particles as thereare atoms in 12 g of carbon-12

massmolar

massn =


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Avogadros Number The number of particles in a mole is

calledAvogadros Number

NA=6.02 x 1023 particles / mole Defined so that 12 g of carbon contains

NA atoms

The mass of an individual atom canbe calculated:

A

atomN

massmolarm =


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Avogadros Number and

Masses The mass in grams of one Avogadro's number

of an element is numerically the same as themass of one atom of the element, expressed

in atomic mass units, u Carbon has a mass of 12 u

12 g of carbon consists of NA atoms of carbon

Holds for molecules, also


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Ideal Gas Law PV = n R T

R is the Universal Gas Constant

R = 8.31 J / mole.K R = 0.0821 L.atm / mole.K Is the equation of state for an ideal gas P = 1.013 x 105 Pa/atm

(standard atmospheric pressure)


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Ideal Gas Law, Alternative

Version P V = N kB T

kB is Boltzmanns Constant

kB = R / NA = 1.38 x 10-23 J/ K

N is the total number of molecules

n = N / NA

n is the number of moles N is the number of molecules


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1.5 The Kinetic Theory ofGasses


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Kinetic Theory of Gases

Assumptions1. The number of molecules in the

gas is large and the average

separation between them is largecompared to their dimensions

2. The molecules obey Newtons

laws of motion, but as a wholethey move randomly


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Kinetic Theory of Gases

Assumptions, cont.3. The molecules interact only by

short-range forces during elastic

collisions4. The molecules make elastic

collisions with the walls

5. The gas under consideration is apure substance, all the moleculesare identical


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Pressure of an Ideal Gas The pressure is

proportional to thenumber of molecules

per unit volume andto the averagetranslational kineticenergy of a molecule

= 2mv

2

1

V

N

2

3P


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Pressure, cont The pressure is proportional to the

number of molecules per unit volumeand to the average translational kineticenergy of the molecule

Pressure can be increased by Increasing the number of molecules per unit

volume in the container Increasing the average translational kinetic

energy of the molecules Increasing the temperature of the gas


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Molecular Interpretation of

Temperature Temperature is proportional to the

average kinetic energy of themolecules

The total kinetic energy is

proportional to the absolutetemperature

Tk2

3mv

2

1B

2 =

nRT2

3KE

total =


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Internal Energy In a monatomic gas, the KE is the only

type of energy the molecules can have

U is the internal energyof the gas In a polyatomic gas, additional

possibilities for contributions to theinternal energy are rotational andvibrational energy in the molecules

nRT2

3

U =


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Speed of the Molecules Expressed as the root-mean-square

(rms) speed

At a given temperature, lighter molecules

move faster, on average, than heavierones Lighter molecules can more easily reach

escape speed from the earth

MTR3

mTk3v Brms ==


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Some rms Speeds


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Maxwell Distribution A system of gas

at a giventemperature will

exhibit a varietyof speeds

Three speeds areof interest: Most probable Average rms


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Maxwell Distribution, cont For every gas, vmp < vav < vrms As the temperature rises, these

three speeds shift to the right The total area under the curve on

the graph equals the total number

of molecules

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