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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_system8/13/2019 Temperature (Chapter19)
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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/Volume8/13/2019 Temperature (Chapter19)
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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