temperature (chapter19)

8/13/2019 Temperature (Chapter19)

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Even though both

are at the same

temperature

Our senses are unreliable and often

mislead usexample : the tile feel

colder the

the carpet

Because ti le

transfers energy

by heat at a higher

rate than carpetdoes


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How are temperature and heat related ?

In our daily conversations, we often use the words

temperature and heat. In physics, these two wordshave very specific meaning

Temperature refers to how hot or cold an object is. Heat

refers to the amount of thermal energy that is being

transferred from a hotter to a colder region.

When we say, Heat the soup in the oven, we are

implying that thermal energy is to be transferred from

the hot to the cold soup. In this way, the soup will be

warmed and taste better!

However, how can we ensure that the oven is hot enough to

warm the soup in the first place? You must have noticed

that ovens usually have temperature controls. This helps

us to pre-set the oven to the desired degree of hotness.


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Difference?


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How is the degree of hotness and coldness

(i.e temperature) measured ?

Our sense of touch is a poor estimate of temperature

We need an instrument,

the thermometer to measure

temperature accurately


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TypeMeasured

Property

Temper

ature

Range

Features

Liquid-in-glass thermometerThermal expansion of

the liquid

-100 C to

300 CCan break!

Electrical resistance thermometer

Platinum resistance

Standard Platinum Resistance

(SPRT)

Industrial platinum resistance

(IPRT, PT100s, RTD (resistance

temperature detector)

Thermistor

Electrical resistance -250 C to

600 C

Very accurate

Laboratory use

Industrial use

Small probes, fast

response, but limited

temperature changes.

Thermocouple thermometer

Voltage generated by

2 wires made out of

different metals

-200 C to

2000 C Cheapest and most

common

Radiation thermometersIntensity of infrared(heat) radiation given

-400 C to3000 C No need to contactthe object to measure


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Physical properties that change with temperature are :

1) the volume of a liquid,

2) the length of a solid,

3) the pressure of a gas at constant volume,4) the volume of a gas at constant pressure,

5) the electric resistance of a conductor, and

6) the color of an object.

For a given substance and a given temperature range

a temperature scale can be established on the basis of

any one of these physical properties.


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An easy-to-read scale

Safe to useResponsive to temperature

Sensitive to small temperature changes

Able to measure a wide range of temperatures

Radiation

thermometer thermocouplesPlatinum

resistance thermistor


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Constructing a temperature scale


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Constructing a temperature scale

mercury

alcohol

Accuracy

problems


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Problems :

1) when extremely accu rate readin gs are needed .

2) l im ited range of temperatures over which it can be used.

Need a un iversal thermometer whose readings are

independent of the substance used in i t the gas

thermometer.


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How It Works:

The pressure of a fixed volume gas varies directly with

temperature according to the gas constant (which is

independent of the nature of the gas). The volume of thegas is kept constant by raising or lowering the reservoir

B to keep the mercury level at A constant.


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The pressu re in the gas thermometer inc reases l inearly

w ith temperature.

When th is pressu re is extrapo lated to zero, a un iversal

temperature is ob tained independent of the gas used.

This

corresponds

to -273.15C,

and is cal led

abso lute zero


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A constant-

Volume gas

thermometer


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0 K


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0K =

273oC


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273K = 0oC

The conversion will be :

TC= T

273.15

TC= the Celsius temperature

T = the absolute

temperature.


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373K = 100oC

The conversion will be :

TC= T

273.15

TC= the Celsius temperature

T = the absolute

temperature.


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= the single

combination oftemperature

and pressure

at which liquid

water, gaseous

water,

and ice (solid

water) coexistin equilibrium.


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The Kelvin (K), unit of

thermodynamic temperature, isdefined as the fraction 1/273.16

of the thermodynamic

temperature of the triple point of

water.


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K = C + 273

K = + 2735 x [F 32]

9


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32oF 212oF

0oC 100oC

ice Water

boiling

F = 180o

C = 100o

F=?

C=?

Equate the

Fractions :

F

32180

= C 0

100

100

180(F

32) = C

C = (F 32)5

9


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Th Z th L f Th d i


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The Zeroth Law of Thermodynamics= if object A and B are separately in

thermal equi l ibr ium with a thi rd object

C, then A and B are in thermalequi l ibr ium with each o ther


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Thermal contact

A thermodynamic systemis said to bein thermal contactwith another system if it

can exchange energy with it through the

process of heattransfer.

Thermal equilibrium

When two objects A and B are in thermalcontact and there is no net transfer of

thermal energy from A to B or from B to A,
http://en.wikipedia.org/wiki/Thermodynamic_systemhttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Thermal_equilibriumhttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Thermodynamic_system


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Two objects in thermal equilibrium with each other

are at the same temperature.

Two objects have different temperatures

are not in thermal equilibrium with each other.


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Have you ever seenrail road tracks thatwere buckled (bentout-of-place) and

wondered if therewhere elephantskicking them wellno. Believe it or notit is called thermalexpansion, its whenthe tracks areexposed to heat (thesun) and the metalsmolecules get hotand move slightly

apart. This occursbecause themolecules in solidsvibrate more fastwhen they are at

higher temperatures.


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Without these jointsto separate sections

of roadway on

bridges, the surface

would buckle due tothermal expansion

on very hot days or

crack due to

contraction on verycold days


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Thermal expansion

is the tendency of matter to

change in volumein response to achange in temperature.
http://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Volume


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To understand this :

Imagine that the atoms are connected by stiff springs

At ordinary temperatures

The atoms in a solid oscillate about their equilibrium positions with an

amplitude of approximately 10-11m and a frequency of approximately 1013Hz

The average spacing between the atoms is about 10-10m

As the temperature of the solid increases

The atoms oscillate with greater amplitudes

The average separation between them increases

As a result

The object expands

Consequently


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Suppose that an object has an initial length L along


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Suppose that an object has an initial length Lialong

some direction at some temperature and that the

length increases by an amount L for a change in

temperature T.

T

L/L i

is constant for

small changes in

temperature

= The averagecoefficient of linear

expansion

Li L

TLL i

)TT(LLL ifiif

Lfback

[ X10-6 (oC-1) ]


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Average

expansion

coefficients for

some materials

at room

temperature

[ X10 6(oC 1) ]


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All dimensions

increase,including the

radius of the

hole


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Because the linear dimensions of an object change

with temperature surface area and volume change

The change in volume of a solid at constant pressure is

proportional to the initial volume Vi and to the change

in temperature according to the relationship

TVV i = the average

coefficient of

volume expansion.

For a solid, the average coefficient of

volume expansion is approximately

three times the average linear

expansion coefficient : = 3

T f th t 3 f lid


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To proof that = 3for a solid

Consider a box of dimensions , and h.

h

Volume at Ti : Vi= h.

i VV hh ThhTT

3T1h

32i TT3T31V

32

i

TT3T3V

V

Div ide bothsides by Vi

and then

isolate the

term V/Vi


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Because

T


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In a similar way, you can show that the change

in area of a rectangular plate is given by :

A = 2AiT

A b imetal l ic s tr ip


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A b imetal l ic s tr ip

bends as the

temperature changes

because the two

metals have dif ferent

expansion

coeff ic ients.

A bimetal l ic str ip

used in a thermostatto b reak o r make

electr ical con tact

temperature (chapter19)

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