Lecture 4 - Lecture 3 - mechanical engineering principle lecture and tutorial . covering basics on...

8/13/2019 Lecture 4 - Lecture 3 - mechanical engineering principle lecture and tutorial . covering basics on distance, velocity, time, pendulum, Hydrostatic Pressure, fluids, solids, etc

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Raymond A. Serway

Chris Vuille

Chapter TenThermal Physics


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Thermal Physics

Thermal physics is the study of Temperature Heat How these affect matter

Introduction


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Thermal Physics, cont

Descriptions require definitions oftemperature, heat and internal energy

Heat leads to changes in internal energy andtherefore to changes in temperature

Gases are critical in harnessing internal energyto do work

Introduction


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Definitions

The process by which energy is exchanged betweenobjects because of temperature differences is calledheat

Objects are in thermal contact if energy can beexchanged between them

Thermal equilibrium exists when two objects are inthermal contact with each other and there is no netexchange of energy between them

Section 10.1


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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 B are in thermal equilibrium witheach other Object C could be the thermometer

Allows a definition of temperature

Section 10.1


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Temperature from the Zeroth Law

Temperature is the property that determineswhether or not an object is in thermalequilibrium with other objects

Two objects in thermal equilibrium with eachother are at the same temperature

Section 10.1


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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 Colour of a very hot object

Section 10.2


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Thermometers, cont A mercury in glass

thermometer is anexample of a commonthermometer

The level of the mercuryrises due to thermalexpansion

Temperature can bedefined by the height ofthe mercury column

Section 10.2


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Temperature Scales

Thermometers can be calibrated by placingthem in thermal contact with an environmentthat remains at constant temperature

Environment could be mixture of ice and water inthermal equilibrium

Also commonly used is water and steam in

thermal equilibrium

Section 10.2


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Celsius Scale

Temperature of an ice-water mixture is defined as 0C

This is the ice point or the freezing point of water Temperature of a water-steam mixture is defined as

100 CThis is the steam point or the boiling point of water at onestandard atmosphere

Distance between these points is divided into 100segments or degrees

Section 10.2


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Gas Thermometer Temperature readings

are nearly independentof the gas

As long as the gas

pressure is low The temperature needs

to be well above thetemperature at whichthe gas liquefies

Pressure varies withtemperature whenmaintaining a constantvolume

Section 10.2


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Pressure-Temperature Graph

All gases extrapolate tothe same temperatureat zero pressure

This temperature isabsolute zero

Section 10.2


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Kelvin Scale

When the pressure of a gas goes to zero, itstemperature 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: T C = T 273.15

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

Section 10.2


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Modern Definition of Kelvin Scale Defined in terms of the triple point of water

Agreed upon by BIPM/CGPM in 1990 (InternationalTemperature Scale 1990) http://www.its-90.com/its-90p4.html

The triple point is the single point where water can exist assolid, liquid, and gas in equilibriumSingle temperature and pressureOccurs at 0.01 C and P = 611.2 Pa

Section 10.2
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Modern Definition of Kelvin Scale, cont

The temperature of the triple point of wateron the Kelvin scale is 273.16 K

Therefore, the current definition of the Kelvinis defined as 1/273.16 of the temperature ofthe triple point of water

Section 10.2


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Some Kelvin Temperatures

Some representativeKelvin temperatures

Note, this scale is

logarithmic Absolute zero has never

been reached

Section 10.2


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Fahrenheit Scales

Most common temperature scale used in theUS/archaic British

Temperature of the ice point is 32 F Temperature of the steam point is 212 F 180 divisions between the points

Section 10.2


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Converting Among Temperature Scales

Section 10.2


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Comparing Temperature Scales

Section 10.2


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Thermal Expansion The thermal expansion of an object is a consequence

of the change in the average separation between itsconstituent atoms or molecules

At ordinary temperatures, molecules vibrate with asmall amplitude

As temperature increases, the amplitude increases This causes the object as a whole to expand

Section 10.3


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Linear Expansion

For small changes in temperature

, the coefficient of linear expansion, dependson the material

See table 10.1These are average coefficients, they can vary

somewhat with temperature

Section 10.3


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Applications of Thermal Expansion BimetallicStrip

Thermostats Use a bimetallic strip Two metals expand differently

Since they have different coefficients of expansion

Section 10.3


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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 ofthe oceans

Section 10.3


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Unusual Behaviour of Water

As the temperature of water increases from 0C to 4C, itcontracts and its density increases

Above 4C, water exhibits the expected expansion withincreasing temperature

Maximum density of water is 1000 kg/m 3 at 4C

Section 10.3


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Ideal Gas

If a gas is placed in a container It expands to fill the container uniformly Its pressure will depend on the

Size of the container The temperature The amount of gas

The pressure, volume, temperature and amount of

gas are related to each other by an equation of state

Section 10.4


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Ideal Gas, cont

The equation of state can be complicated It can be simplified if the gas is maintained at

a low pressure Most gases at room temperature and pressure

behave approximately as an ideal gas

Section 10.4


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Characteristics of an Ideal Gas

Collection of atoms or molecules that moverandomly

Exert no long-range force on one another Each particle is individually point-like

Occupying a negligible volume

Section 10.4


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Moles Its convenient to express the amount of gas in a

given volume in terms of the number of moles, n

One mole is the amount of the substance thatcontains as many particles as there are atoms in 12 gof carbon-12

We use the kmol(e).i.e. 1 kmol = 1000 mol

Section 10.4


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Avogadros Number

The number of particles in a kmole is called Avogadros Number

NA=6.02 x 10 26 particles / kmolDefined so that 12 kg of carbon contains N A atoms

The mass of an individual atom can becalculated:

Section 10.4


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Avogadros Number and Masses

The mass in grams of one Avogadro's number of anelement is numerically the same as the mass of oneatom of the element, expressed in atomic mass

units, u Carbon has a mass of 12 u

12 kg of carbon consists of N A atoms of carbon Holds for molecules, also

Section 10.4


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Ideal Gas Law

PV = n R0 TR0 is the Universal Gas Constant R0 = 8.3143 kJ / kmol .K

n is the number of kmolIs the equation of state for an ideal gas

Temperatures used in the ideal gas law must be in

kelvin

Section 10.4


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Ideal Gas Law, Alternative Version

m = nMn is the number of kmolesM is the molar mass (kg/kmol)

Section 10.4


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Kinetic Theory of Gases Assumptions

The number of molecules in the gas is largeand the average separation between them islarge compared to their dimensions

The molecules obey Newtons laws of motion,but as a whole they move randomly

Section 10.5


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Kinetic Theory of Gases Assumptions,cont.

The molecules interact only by short-rangeforces during elastic collisions

The molecules make elastic collisions with the

walls All molecules in the gas are identical

Section 10 5

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