Transcript ch. 12 real3 - Physics-YISS

12.4 Linear Thermal
Expansion
Normal Solids
• Ex. Glass jar too tight, hot water over lid.
• Linear expansion – increase in any one
dimension of a solid, linear is the
expansion occurring along a line.
• L – length
• T – temp.
• Length and temp. is directly proportional.
Thermal Expansion
• Change in length is proportional to both
(initial) L and change in Temp.
• Proportionality constant _____, is called
the coefficient of linear expansion.
Linear Thermal Expansion of a Solid
Thermal Expansion
• Coefficient of linear expansion ___ has the
unit of _____.
• Value of the ____ depends on the nature
of the material.
Coefficients of Thermal Expansion
for Solids and Liquids
Example 3: Buckling of a Sidewalk
Expansion Joint
Usefulness
• Anti-scalding device: shuts off the flow of
water when it becomes too hot,
• Higher temp expands the actuator spring
and pushes the plunger forward, shutting
off the flow.
Practice Problem 9.
A steel section of the Alaskan pipeline had a
length of 65 m and a temperature of 18 C
when it was installed. What is its change
in length when the temperature drops to a
frigid -45 C?
Thermal Stress
• Change in temp. causes a force, stress on
the material.
• Can cause structural damage.
Stress
Example 4: The Stress on a Steel
Beam
• http://www.youtube.com/watch?v=JA5nTv
EU3MA&feature=relmfu
Practice Problem 90.
A cylindrical brass rod (cross sectional area
= 1.3 x 10^-5 m2) hangs vertically straight
down from a ceiling. When an 860N block
is hung from the lower end of the rod, the
rod stretches. The rod is then cooled such
that it contracts to its original length. By
how many degrees must the temperature
be lowered?
Homework
Problems:
10
11
13
14
24
The Bimetallic Strip
• Made from two thin strips of metal that have
different coefficients of linear expansion.
• Brass
• Steel
• When the strip is heated, the brass, having the
larger value of ____, expands more than the
steel.
• The strip bends into an arc, with the longer brass
piece having a larger radius than the steel piece.
• When the strip is cooled, the bimetallic strip
bends in the opposite direction.
Video of bimetallic strip
• http://www.youtube.com/watch?v=WzX5tp
C8Bs8
Bimetallic strips used in appliances
• Automatic coffee maker turns off when the
coffee is brewed to the selected strength.
• Brewing cycle is on: electricity passes through
the heating coil that heats the water.
• Flows because the contact mounted on the
bimetallic strip touches the contact mounted on
the “strength” adjustment knob.
• Strip gets hot enough to bend away, electricity
stops because it no longer has a continuous
path along which to flow.
• Brewing cycle is shut off. Pg. 368
• http://home.howstuffworks.com/therm2.ht
m
Conceptual example 5: Do Holes
Expand or contract when the
temperature increases?
Example 6: A Heated Engagement Ring
12.5 Volume Thermal
Expansion
• The volume of a normal material increases
as the temperature increases.
• Change in V is proportional to the change
in T.
Volume Thermal Expansion
• ____ like _____ is _______.
• Values for ____ depend on the nature of
the material.
• Values are larger for liquids than solids.
Example 8: An Automobile
Radiator
• Some substances do not expand when
heated.
• When water is heated its volume
decreases until the temperature reaches 4
C. Above 4 C water behaves normally,
and its volume increases as the temp.
increases.
• The density (mass per unit volume) of
water is greatest at 4 C.
Freezing pond
• When the temp drops the top layer of
water becomes 4 C and denser than the
warmer water below.
• The denser water sinks and pushes up the
deeper and warmer water, which in turn is
chilled at the surface.
• The process continues until the temp. of
the entire pond reaches 4 C.
• Further cooling of the surface water below
4 C makes it less dense than the deeper
layers then the surface layer does not sink
but stays on top.
12.6 Heat and Internal Energy
• An object with a high temperature is said
to be “hot.”
• Heat – flows from a hotter object to a
cooler object when the two are placed in
contact.
• From HOT to COLD.
• Arrival or departure of heat prompts the
brain to identify the coffee cup as being
hot and the glass as being cold.
Heat is a form of Energy
Definition of Heat
Internal Energy
• Sum of the molecular kinetic energy,
molecular potential energy, and other
kinds of molecular energy.
• Energy of a substance decreases when it
is transferred to another object (HEAT).
• Heat is the transfer of energy, and object
doesn’t contain heat.
12.7 Heat and Temperature
Change: Specific Heat Capacity
• Solids and liquids
• Heat = Q
• Heat Q is directly proportional to the
change in temperature and to the mass m.
Metals have a lower c. why?
Put one hand on the metal part of desk and
another hand on the wooden part.
Example 9: A Hot Jogger
Example 10: Taking a Hot
Shower
Example 11: Heating a
Swimming Pool
Example 12: Measuring the
Specific Heat Capacity
H.W. pg. 389
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