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Temperature and Heat

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and. Heat. Chapter 12. Temperature Scales. Water freezes at: Water boils at: Celsius: 0 C 100 C ... Fahrenheit: 32 F 212 F. Kelvin: 273.15 K 373.15 K ... – PowerPoint PPT presentation

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Title: Temperature and Heat


1
Temperature andHeat
  • Chapter 12

2
Temperature Scales
Water freezes at Water boils at Celsius 0
ºC 100 º C Fahrenheit 32 º F 212 º
F Kelvin 273.15 K 373.15 K The SI unit
of temperature is Kelvin.
3
Temperature Conversions
4
Example
How would you dress if the temperature outside
was (a) 70 degrees Fahrenheit? (b) 70 degrees
Celsius? (c) 70 degrees Kelvin?
5
Why 273.15?
We believe that 0 K (-273.15 ºC) is the absolute
coldest that anything can get.
6
Linear Thermal Expansion
When a substance is heated, it tends to expand.
If we examine the behavior of a long, thing
cylinder we will observe the relation
? is the coefficient of linear
expansion Units 1/Cº
7
Example
A concrete roadbed is divided into sections that
are 2 meters long. How much will each section
expand when the temperature changes from 0 ºC to
37 ºC (100 ºF)? A bridge is 400 m (1/4 mile)
long. How much will the steel support structure
expand under the same temperature change?
8
Bimetallic Strips
Usually made out of brass (?19 x 10-6 1/Cº) and
steel (?12 x 10-6 1/Cº) . The brass
expands and contracts more than the steel, and so
the strip bends when the temperature
changes. Used to make outdoor dial thermometers.
9
Question?
Does a hole expand, contract, or stay the same
size when the material around it is heated?
10
Volume Thermal Expansion
If we consider three-dimensional materials, we
see the same behavior as in Linear Thermal
Expansion, except that now expansion is occurring
in all three dimensions! ? is called the
coefficient of volume expansion.
11
Example
You place 1 cup of water (0.000237 m3) in a Pyrex
measuring cup and place it in the microwave on
high for 3 minutes. As the temperature of the
water (and cup) changes from 21 ºC to 100 ºC, how
does the measured amount of water change?
12
Water at 4 C
Water has a very strange property. As water is
heated from 0 to 4 C, its volume DECREASES,
making it more dense. This prevents ponds
from freezing over in the winter. The 4 C
water is more dense, and so it sinks to the
bottom of the pond. Once the whole pond is at 4
C, the water on the top begins to freeze.
However, 0 C water is less dense, and so it
floats on top of the 4 C water. Ponds freeze
just at the top!
13
Example
A metal rod sits on end on a table. As the rod
is heated, will the pressure it exerts on the
table increase, decrease or remain the same?
Explain.
14
Group Problem Solving
A circular hole is cut through a flat aluminum
plate. A spherical brass ball has a diameter
that is slightly smaller than the diameter of the
hole. The plate and the ball have the same
temperature at all times.
Should the plate and ball both be heated or both
be cooled to prevent the ball from falling
through the hole? Explain your reasoning in
detail.
15
Heat
Definition Heat is energy that flows from a
higher temperature object to a lower temperature
object because of the difference in
temperatures. If two objects come into contact,
and one is at a higher temperature than the
other, energy will flow from the hotter one to
the cooler one. This flowing energy is called
heat. When an object is hot, we say that it has
a lot of internal energy. Unit Joule
16
calories or Calories?
calorie (cal) the amount of heat needed to raise
the temperature of 1 gram of water by 1
C. kilocalorie (kcal) the amount of heat
needed to raise the temperature of 1 kilogram
of water by 1 C. Calorie (Cal) the same as 1
kcal. Used as a measure of the amount of
energy in foods. 1 kcal 4180 Joules
17
Specific Heat Capacity
A measure of how much heat you need to change
the temperature of a substance. Q heat
added or removed J c specific heat capacity
J / (kg C) m mass kg ?T change in
temperature C
18
Example
The specific heat capacity for a human body is
about 3500 J/kg C. A half-hour run will burn
about 800,000 J of heat. (a) If the runner does
not sweat, how much will his temperature
increase? (The runner has a mass of about 60
kg.) (b) How many Calories does the runner burn?
19
Phase Changes
The three main phases are solid, liquid, and
gas. Moving from one phase to another
absorbs (or expels) heat but does not change
the temperature of the substance. S L G
20
Phase Changes
T Q
21
Latent Heat
Latent Heat of Fusion, Lf heat that must be
added or removed when a substance changes phase
between solid and liquid. Latent Heat of
Vaporization, Lv heat that must be added or
removed when a substance changes phase between
liquid and gas. Units J/kg Q mL Table 12.3
22
Example
Ice at 0 C is put into an equal mass of water at
100 C in an insulated container. They reach
equilibrium. (a) Which has the greater
magnitude, the heat transferred into the ice or
the heat transferred out of the water? (b) Will
the equilibrium temperature be greater than, less
than or equal to 50 C ?
23
Group Problem Solving
A quantity of pure substance is enclosed in a
container and its temperature is monitored.
Describe the conditions under which each of the
following behaviors can occur as heat is added to
the substance. (a) As heat is added the
temperature does not change. (b) The temperature
rises in proportion to the amount of heat
added. (c) As heat is added, there is no
temperature change at first, but as more heat is
added the temperature begins to rise.
24
Group Problem Solving
A certain gas is enclosed in a container and the
temperature is monitored as a function of time.
Heat is extracted from the container at a
constant rate. The temperature versus time graph
is shown.
T
t
(a) For each of the 5 straight line portions of
the graph, explain what is happening to the
material in the container. (b) Which is
greater, the latent heat of vaporization or the
latent heat of fusion? Explain your
response. (c) Compare the specific heats of the
gas, liquid, and solid phase. (Hint the
specific heat controls the rate of temperature
change.)
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