# Thermal Physics

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```					Thermal
Physics
Chapter 13
Temperature
and Heat
Common Temperature
Scales
Temperatures are reported in degrees
Celsius or degrees Fahrenheit.

Temperatures changed, on the
other hand, are reported in Celsius
degrees or Fahrenheit degrees:

9
1C  F
5
Converting from a Fahrenheit to a Celsius Temperature

A healthy person has an oral temperature of 98.6oF. What would this
reading be on the Celsius scale?

degrees above ice point

98.6 F  32 F  66.6 F

 1 C 
         
66.6 F  9    37.0 C

 F 
5 

0 C  37.0 C  37.0 C

ice point
Converting from a Celsius to a Fahrenheit Temperature
A time and temperature sign on a bank indicates that the outdoor
temperature is -20.0oC. Find the corresponding temperature on
the Fahrenheit scale.

degrees below ice point
 9 F 
          
20.0 C  5    36.0 F

1C 
      

32.0 F  36.0 F  4.0 F

ice point
Daniel Fahrenheit
1686 - 1736
Fahrenheit
According to a journal article Fahrenheit wrote
in 1724, he based his scale on three reference
points of temperature. The zero point is
determined by placing the thermometer in
Later, used a other scientists observed
brine: hework by mixture of ice, water, and
that water boils about 180 degrees higher
ammonium chloride. The mixture automatically
than the freezing point at 0 °F.
stabilizes its temperature and decided to
redefine the degree slightly to make it
He then put a thermometer into the mixture and
the liquid in the thermometer is for this
letexactly 180 degrees higher. It descend to its
reason that normal body temperature degree
lowest point. The second point is the 32 is
98.6 on the revised scale (whereas it water
found by putting the thermometer in still was
ice is just forming original scale).
as96 on Fahrenheit's on the surface.
The third point, the 96 degree, was the level of
the liquid in the thermometer when held in the
mouth or under the armpit.
Unsatisfied with the Celsius and Fahrenheit temperature scales, you decide to
create your own. On your temperature scale, the ice point is 77 M and the
steam point is at 437 M, where “M” stands for “my scale.” What
temperature on your scale corresponds to 68 F?

1.       154 M
2.       168 M                                               20%                    20%
3.       140 M
4.       136 M
20%
5.       149 M                                                                20%
20%
154        168        140      136        149

1    2     3   4    5    6    7    8    9    10   11    12    13   14     15    16     17   18    19   20
21   22   23   24   25   26   27   28   29   30
William Thomson,
1st Baron Kelvin (Lord Kelvin)
1824 - 1907

Kelvin
Temperature Scale
T  Tc  273 .15
Temperatures have an
extremely large range,
both on Earth and
throughout the
Universe.
Thermometers
Thermometers make use of the change in some physical property with
temperature. A property that changes with temperature is called a
thermometric property. All thermometers require calibration.
12.4 Linear Thermal Expansion

LINEAR THERMAL EXPANSION OF A SOLID

The length of an object changes when its temperature changes:

L   Lo T

coefficient of
linear expansion

 
1       1
Common Unit for the Coefficient of Linear Expansion:    C
C
12.4 Linear Thermal Expansion

L  Lo
Example: Linear Thermal Expansion
A brass bar and an aluminum bar are each attached to an
immovable wall as shown. At 22.0 oC, the air gap between
the rods is 1.00 mm. At what temperature will the gap be
closed?

2.000 m

1.000 m
for the aluminum                 for the brass
LAL  AL LALT                 LBR  BR LBR T

We want to solve for T where LAL + LBR is equal to
the gap separation distance.

So, add the two equations together and solve for T.

LAL  LBR   AL LALT   BR LBR T
  AL LAL   BR LBR  T
T  T  T0
LAL  LBR
T  T0 
 AL LAL   BR LBR
1.00  103 m
 22.0 C 
 23  10   6
/C    1.000 m   19 10   6
/C    2.000 m 
T  38.4 C

2.000 m

LAL  LBR                                                                       1.000 m
T  T  T0 
 AL LAL   BR LBR
LAL  LBR
T  T0 
 AL LAL   BR LBR
1.00  103 m
 22.0 C 
                                               
23  106 / C 1.000 m   19  10 6 / C  2.000 m            
 38.4 C
12.4 Linear Thermal Expansion
DEFINITION OF HEAT

Heat is energy that flows from a higher-
temperature object to a lower-temperature
object because of a difference in
temperatures.
SI Unit of Heat: joule (J)

The heat that flows from
OTHER UNITS
hot to cold originates in
the internal energy of
1 kcal = 4186 joules
the hot substance.
1 cal = 4.186 joules
1 BTU = 1055 J
It is not correct to say
that a substance
contains heat.
Heat and Temperature Change: Specific Heat Capacity

The heat that must be supplied or removed to
change the temperature of a substance is

Q  mcT
specific heat
capacity

Common Unit for Specific
Heat Capacity: J/(kg·Co)

1 kcal = 4186 J
1 BTU = 1055 J
When you drink cold water, your
body must expend metabolic
energy to maintain normal body
temperature of 37 oC by warming
up the water in your stomach.
Could drinking ice water substitute
for exercise as a way to “burn
calories?” Suppose you expend
430 kilocalories during a brisk one-
hour walk. How many liters of ice
water would you have to drink in
order to use 430 kilocalories of
metabolic energy? Note: the
stomach can hold about one liter.
Heat and Phase Change: Latent Heat
You find that you have let a 2.5 kg stainless steel barbeque grate become
too hot for cooking. You Decide to cool the grate from 296 oC to 185 oC by
spraying water onto the grate. The specific heat of steel = 445 J/kg - K.
Calculate the mass of water at 20 oC you will need, assuming all the water
evaporates to steam at 100 oC.
heat lost by plate = heat gained by water in raising its own temp + heat to
transform water into steam

ms cs Ts  mwcw Tw  mw Lv
 ms cs Ts  mwcw Tw  mw Lv  mw  cw Tw  Lv 
 ms cs Ts
mw 
cw Tw  Lv



  2.5 kg  445      J
 185 C  296 C 
kg  C

   4186      J
 100 C  20 C   22.6 10
kg  C
5 J
kg

 0.048 kg or 48 g
Heat = transfer of energy
   From someplace hot to someplace cold
   Convection
   Conduction
Convection
Convection is the process in which heat is carried from one place
to another by the bulk movement of a fluid.
Concepts in action
Hot water baseboard heating units are mounted on the wall
next to the floor. The cooling coil in a refrigerator is mounted
near the top of the refrigerator. Each location is designed to
maximize the production of convection currents.
Conduction

The amount of heat during a time t through a bar of bar depends on
The heat Q conducted Q that is conducted through the length
L and cross-sectional area A is
a number of factors:

Q
 kAT  t
1. The time during which conduction takes place.
L
2. The temperature difference between the ends of the bar.
3. The cross sectional area of the bar.
4. The length of the bar.
SI Units of Thermal Conductivity: J/(s·m·Co)
Thermal
Conductivity
What thickness of concrete, with a thermal
conductivity of 1.1 J/(smK) will conduct heat at the
same rate as 0.25 m of air, which has a thermal
conductivity of 0.0256 J/(smK), if all other conditions
are the same?

Q
 kAT  t            
Q kAT

L                     t   L
kconcrete AT kair AT

Lconcrete     Lair
kconcrete kair

Lconcrete Lair
kconcrete         1.1 smK
J       
Lconcrete              Lair             J     0.25 m = 11 m
kair            0.0256 smK   
Insulation
spaces are usually
excellent thermal
insulators.
Radiation is the process in which
energy is transferred by means of
electromagnetic waves.

A material that is a good absorber
is also a good emitter.

A material that absorbs completely
is called a perfect blackbody.
Black Robe Mystery
Why do Bedouins wear black robes?

The extra warm the matter        having a
Scientists checked intoair trapped by the black
stand in the hot desert sun first in a equal
man color experiences a buoyancy force white
to then in a black one. They displaces.
robe and the weight of the cool air itfound that
The robe air rises 2.5 times of the robe
the black warmabsorbed up and outmore solar
and was in turn, oC) hotter than the
radiation top. This,11 oF (6 creates a breeze as
cooler
white one. air is drawn in at the bottom. The
breeze evaporates sweat, cooling the robe
Thewearer, and moves thethe same in air out
either robe, but the man felt cooler in the top.
the robe
black one.
The loose-fitting burnoose is a solar air-
conditioning system.
The maximum of the
intensity shifts to shorter
wavelengths as the black-
body temperature increases.

The radiant energy Q, emitted in a time t by an object that has a
Kelvin temperature T, a surface area A, and an emissivity e, is given by

Q  e T 4 At                    The emissivity e is a
dimensionless number between
zero and one. It is the ratio of
Stefan-Boltzmann constant             what an object radiates to what

  5.67  108 J s  m2  K 4        it were a perfect emitter.

Q
Rate of Heat Transfer      e T A
4
t
A cube, 10 cm on a side, of rough steel is heated in a furnace to a
temperature of 400.0 oC. If its total emissivity is 0.97, determine the rate
at which it radiates energy from each face.

Q
 e T 4 A
t
                             
  0.97  5.67  10 J (s  m  K )  400  273 K   0.10 m 
8       2   4                 4           2

 113 J/s = 113 W

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