1.2TEMPERATURE AND HEAT for college physics.ppt

pfe

1. Temperature and Heat

2. Temperature – an objective property of matter; it is related to the KE of
the molecules
Thermal equilibrium – two systems that have the same temperature
Zeroth law of thermodynamics – If system C is in thermal equilibrium
with both A and B, then A and B are in thermal equilibrium with each
other.

3. Thermometers make use of the change in some physical property with
temperature. A property that changes with temperature is called a thermometric
property.
liquid-in-tube thermometer
b.) bimetallic strip
c.) resistance thermometer
d.) constant volume gas
thermometer

4. Common Temperature Scales
a.) degrees Celsius b.) degrees Fahrenheit c.) kelvin

5. Change in the dimension(s) of
a substance due to change in
temperature.

6. Temperature
Higher change in temperature (∆T), greater expansion.
Kind of Material
Quantified by the Coefficient of Thermal Expansion
Higher Coefficient, greater expansion
Original Dimension
Greater original dimension (L, A,or V), greater
expansion

7. NORMAL SOLIDS

8. o
LL
TL

9. LINEAR THERMAL EXPANSION OF A SOLID
The length of an object changes when its temperature changes:
TLL
o
coefficient of
linear expansion
Common Unit for the Coefficient of Linear Expansion: 
1
C
C
1 




11. A surveyor uses a steel measuring tape that is exactly
50.000 m long at a temperature of 20
0
C. What is its
length on a hot summer day when the temperature is
35
0
C?
Note: αα for steel is 1.2x10 for steel is 1.2x10
-5 -5
/C/C
0
Answer=
50.009 m

12. Example 3 The Buckling of a Sidewalk
A concrete sidewalk is constructed between
two buildings on a day when the temperature
is 25
o
C. As the temperature rises to 38
o
C,
the slabs expand, but no space is provided for
thermal expansion. Determine the distance y
in part (b) of the drawing.

1
6
C1012





13.   m 00047.0C 13m 0.3C1012
1
6



 
TLL
o
   m 053.0m 00000.3m 00047.3
22
y

14. THE BIMETALLIC STRIP

16. Conceptual Example 5 The Expansion of Holes
The figure shows eight square tiles that are arranged to
form a square pattern with a hole in the center. If the
tiled are heated, what happens to the size of the
hole?

17. A hole in a piece of solid material expands when heated and contracts
when cooled, just as if it were filled with the material that surrounds it.

18. Volume expansion
V
o
∆V

19. VOLUME THERMAL EXPANSION
The volume of an object changes when its temperature changes:
TVV
o
coefficient of
volume expansion
Common Unit for the Coefficient of Volume Expansion: 
1
C
C
1 




20. A copper cylinder with a volume of 23.00 cm
3
is
initially at 20
o
C. What is the final volume if the
temperature is increased to 80
o
C?
Note: αα for copper is 1.7x10 for copper is 1.7x10
-5 -5
/C/C
0
Answer=
23.07 cm
3

21. http://upload.wikimedia.org/wikipedia/commons/8/88/Auckland_Harbour_Bridge_Details_02.jpg

22. A glass flask with volume of 200 cm
3
is filled to the brim
with mercury at 20.0
o
C. How much mercury overflows
when the temperature of the system is raised to 100
o
C? the
coefficient of linear expansion of the glass is 0.40x10
-5
/C
o

and 6.00x10
-5
/C
o
for mercury.

23. Example
The beam is mounted between two
concrete supports when the temperature
is 23
o
C. What compressional stress
must the concrete supports apply to
each end of the beam, if they are
to keep the beam from expanding
when the temperature rises to 42
o
C?
(for the beam, Y=2x10
11
N/m
2
)

24.    
27
1
6211
mN107.4C19C1012mN100.2
Stress





 
TY
L
L
Y
A
F
o

TLL
o

25. Expansion of water.

26. A steel string guitar is strung so that there is
negligible tension in the strings at a temperature of
24.9 °C. The guitar is taken to an outdoor winter
concert where the temperature of the strings
decreases to –15.1 °C. The cross-sectional area
of a particular string is 5.5 × 10
–6
m
2
. The distance
between the points where the string is attached
does not change. For steel, Young's modulus is
2.0 × 10
11
N/m
2
; and the coefficient of linear
expansion is 1.2 × 10
–5
/C°. calculate the tension in
the string at the concert.

27. 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)

28. The heat that flows from hot to cold
originates in the internal energy of
the hot substance.
It is not correct to say that a substance
contains heat.

29. SOLIDS AND LIQUIDS
HEAT SUPPLIED OR REMOVED IN CHANGING THE TEMPERATURE
OF A SUBSTANCE
The heat that must be supplied or removed to change the temperature of
a substance is
TmcQ 
specific heat
capacity
Common Unit for Specific Heat Capacity: J/(kg·C
o
)

31. GASES
The value of the specific heat of a gas depends on whether the
pressure or volume is held constant.
This distinction is not important for solids.
OTHER UNITS
1 kcal = 4186 joules
1 cal = 4.186 joules

32. CALORIMETRY
If there is no heat loss to the surroundings,
the heat lost by the hotter object equals the
heat gained by the cooler ones.

33. THE PHASES OF MATTER

34. During a phase change, the temperature of the mixture does not
change (provided the system is in thermal equilibrium).

35. Conceptual Example 13 Saving Energy
Suppose you are cooking spaghetti for dinner, and the instructions
say “boil pasta in water for 10 minutes.” To cook spaghetti in an open
pot with the least amount of energy, should you turn up the burner
to its fullest so the water vigorously boils, or should you turn down
the burner so the water barely boils?

36. HEAT SUPPLIED OR REMOVED IN CHANGING THE PHASE
OF A SUBSTANCE
The heat that must be supplied or removed to change the phase
of a mass m of a substance is
mLQ
latent heat
SI Units of Latent Heat: J/kg

38. Two spheres, labeled A and B, have identical masses, but are made of
different substances. The specific heat capacity of sphere A is 645
J/(kg · C°) and that of sphere B is 240 J/(kg · C°). The spheres are
initially at 21 °C; and the same quantity of heat is added to each sphere.
If the final temperature of sphere A is 74 °C, what is the approximate
final temperature of sphere B?

39. How much heat is required to increase the temperature of ice
at -5
o
c to steam at 130
o
c ?

40. A 2.00-kg metal object requires 1.00 × 10
4
J of heat to raise its
temperature from 20.0

°C to 40.0

°C. What is the specific heat
capacity of the metal?

41. A 200.0-kg object is attached via an ideal pulley system to paddle wheels that are
submerged in 0.480 kg of glycerin at 20.0 °C in an insulated container as shown.
Then, the object falls through a distance of 5.00 m causing the paddle wheel to
turn. Assuming all of the mechanical energy lost by the falling object goes into
the water, determine the final temperature of the glycerin. The specific heat
capacity of glycerin is 2410 J/kg K

42. A gold sphere has a radius of 1.000 cm at 25.0 °C.
If 7650 J of heat is added to the sphere, what will the
final volume of the sphere be? Gold has a density of
19 300 kg/m
3
at 25.0 °C, a specific heat capacity of
129 J/(kg · C°), and a coefficient of volume
expansion of 42.0 × 10
–6
/C°.
A 0.030-kg ice cube at 0 °C is placed in an insulated box
that contains a fixed quantity of steam at 100 °C. When
thermal equilibrium of this closed system is established,
its temperature is found to be 23 °C. Determine the
original mass of the steam at 100 °C.

44. CONDUCTION
Conduction is the process whereby heat is transferred directly through
a material, with any bulk motion of the material playing no role in the
transfer.
One mechanism for conduction occurs when the atoms or molecules
in a hotter part of the material vibrate or move with greater energy than
those in a cooler part.
By means of collisions, the more energetic molecules pass on some of
their energy to their less energetic neighbors.
Materials that conduct heat well are called thermal conductors, and
those that conduct heat poorly are called thermal insulators.

45. The amount of heat Q that is conducted through the bar depends on
a number of factors:
1.The time during which conduction takes place.
2.The temperature difference between the ends of the bar.
3.The cross sectional area of the bar.
4.The length of the bar.
CONDUCTION

46. CONDUCTION OF HEAT THROUGH A MATERIAL
The heat Q conducted during a time t through a bar of length
L and cross-sectional area A is
 

L
TT
kA
dt
dQ
H
L
tTkA
Q
cH




SI Units of Thermal Conductivity: J/(s·m·C
o
)
thermal
conductivity

47. Materials with dead air spaces
are usually excellent thermal
insulators.

48. CONVECTION
Convection is the process in which heat is carried from one place
to another by the bulk movement of a fluid.
convection
currents

49. “Thermals” can be used by glider
pilots to gain considerable altitude.
NATURAL CONVECTION - heat flows due to a difference in
density at in the different parts of the fluid.

50. Forced Convection – heat flows with the aid of an external
pump or device.
The forced convection generated
by a pump circulates radiator fluid
through an automobile engine to
remove excessive heat.

51. The amount of heat Q that is transferred through convection depends
on a number of factors:
1.The heat or heat current is directly proportional to the surface area.
2.The viscosity of fluids slows natural convection near a stationary
surface.
3. The heat or heat current is approximately proportional to the 5/4
power of the temperature difference between the surface and the main
body of the fluid.
CONVECTION

52. RADIATION
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.

53. THE STEFAN-BOLTZMANN LAW OF RADIATION
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
ATe
dt
dQ
H
AtTeQ
4
4




The emissivity e is a dimensionless number between zero and one. It
is the ratio of what an object radiates to what the object would radiate if
it were a perfect emitter.
 
428
KmW1067.5 


Stefan-Boltzmann constant

54. A thermos bottle minimizes heat
transfer via conduction, convection,
and radiation.

55. The halogen cooktop stove creates
electromagnetic energy that passes
through the ceramic top and is absorbed
directly by the bottom of the pot.