Transcript Heat and Temperature

Heat and Temperature
Physics 101
Common Temperature Scales
Name
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Fahrenheit
Rankine
Centigrade
Kelvin
Conversion between
Celsius and Fahrenheit
C
5

F  32 9
Absolute 0
FP water
-459.67
32
0
491.67
-273.15
0
0
273.15
BP Water
212
671.64
100
373.13
Between K and Celsius
Kelvin (K) = oC + 273
Common Temperatures
• Photosphere of Sun is
6000 oC or 11,000oF
• Sun Spots are cooler
at 4000oC or 7,000oF
• 16 to 20 million oC at
the center of the Sun
which sustains
nuclear fusion
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Dry Ice -78.5 oC
Liquid N2 77.2 K
Kelvin (K) = oC + 273
What is Liquid N2 in
oC
• Absolute 0 is 0 K
• What is Absolute 0 in
oC
• Room Temperature is
20 oC
Question for Thought
At what temperature is the Fahrenheit and
Celsius temperature the same?
X
5

X  32 9
X = -40o C or -40o F
Temperatures continued
• Temperature which
sustains life 37 oC
• Outer Space 3 K
• Dwarf Star 35,500 K
surface
• 100 billion K is the
temperature of a
supernova
• See
http://en.wikipedia.org
/wiki/Supernova for
more information on a
supernova
• Temperature of ice is
0 oC
• Water boils at 100 oC
Types of Thermometers
A Thermometer is a device for measuring temperature
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Galileo thermometer
Mercury thermometer
Thermistor
Thermocouple
Transistor (a solid state device)
Bi-Metalic Strip
The higher an object’s temperature the faster its atoms or molecules move
Temperature is not a measure of the total kinetic energy of atoms and
molecules in a substance
Atoms can move into a state from which it is impossible to extract more
energy—this is Absolute Zero
Thermal Expansion
• The tendency for a
heated object to expand
is expressed in terms of
the coefficient of linear
expansion 
L  L0 (1   T )
Note the units of 
Water is an anomaly since it actually shrinks slightly when its
temperature is raised between 0 and 4o C. This property of water
may have been very important in the evolution of life.
If a 50 m steel footbridge experiences extreme
temperatures between -15 C and 45 C, what is the range in
size of the bridge if it measures exactly 50 m at 20 C (Steel
has a coefficient of linear expansion of 0.000011/0C
-15 C – 20 C
L  L0 (1   T )
20 C – 45 C
Range is 50.014 to 49.981 m
50(1+0.000011 * -35) = 49.981 m
50(1 + 0.000011*25) = 50.014
Heat and Internal (thermal) energy
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Thermal energy resides in an object because of the motion and interactions
of its molecules and atoms
Heat is reserved for energy that is transferred from one body or substance
to another due to a temperature difference
Under special circumstances absorbed heat may break inter-atomic bonds
while the temperature remains constant (change of phase)
Heat is measured in a unit called the calorie (defined as the amount of heat
required to raise the temperature of 1 gram of water by 1 degree Celsius.
Heat (calories) is like joules in the m.k.s. system, in fact 1 calorie is = 4.186
joules.
In the English system the unit of heat is the BTU (British Thermal Unit) and
is defined as the amount of heat required to raise one pound of water, 1
degree Fahrenheit.
Do not confuse calorie with a Food Calorie (C). One food Calorie = 1000
calories.
Specific Heat
• Heat capacity is a measure of how much heat an
object can absorb. A large mass has more heat
capacity than a small mass.
• Specific heat is a measure of the ability of a
material to absorb heat. It is defined as the
quantity of heat required to raise the
temperature of one gram of that material by 1
degree Celsius. Water displays the highest
specific heat of any familiar substance.
(examine question #9, “If water had a lower
specific heat, would your chances of enjoying a
long hot bath be greater or less?”
Change of Phase
• Specific Heat gives us a good indication of
how well any substance absorbs heat.
However a change of phase to another is
a different situation.
• The amount of heat needed to rearrange 1
gram of molecules from their conversion in
a solid to a configuration in a liquid is
called the latent heat of fusion. This value
is 80 calories for each gram of ice.
Latent Heat of Vaporization
• The amount of heat required to convert
one gram of liquid to a gas is called the
latent heat of vaporization.
• It is 540 calories per gram for water.
• Other substances have different latent
heats of vaporization and latent heats of
fusion.
Thought question for the day?
• Many fruit growers even in southern
climates face the dangers of frost harming
fruit trees and crops. An “old wives tale” is
to sprinkle your crops with water
(especially in the wee hours of the
morning) to protect them from freezing. A
similar tale has you putting soaked
newspapers between the flower and plant
rows. Does this have any scientific
validity?
Forms of Heat Transfer
• Heat transfer occurs by three mechanisms,
conduction convection and radiation.
• Insulation acts as to slow the conduction of heat.
(insulation is measured by R values)
• Conduction requires atoms or molecules that
vibrate and collide with each other.
• Convection requires atoms or molecules of bulk
fluid
• Radiation can occur in a vacuum like outer
space. (Radiation is transfer by electromagnetic
radiation)
Copied from Nick Strobel “the sun and stellar structor”, www.
Astronomynotes.com
Our Sun
Design of a solar home