Heat and Thermal
Energy
Iceland
Volcanoes!
The Thermal Energy of a substance is the
total energy of all of its molecules and
atoms. It consists of both the potential and
kinetic energy of the molecules of the
substance as they vibrate back and forth.
Physics people usually call this internal
energy.
Temperature Scales
Fahrenheit
freezing at 32o, boiling at 212o
Celsius
freezing at 0o, boiling at 100o
Absolute
C + 273o
There is not an absolute upper limit for temperature….the
temperature inside of many stars is over 30 million
degrees, and even higher temperatures are likely inside
supernovas and black holes…
The is an absolute lower limit. Gas samples (like the air
inside a balloon) change their volume based on the
temperature. At –273oC the volume of a gas sample will
go to zero – this means basically that all molecular energy
stops! We have been within a few tenths of a degree of this
temperature, but will have difficulty actually reaching it.
-273oC = 0o Kelvin = Absolute Zero!
piston
gas
Zero
volume!
When pressure is held constant, the volume of a gas
changes (decreases) by 1/273 for each degree C the
temperature decreases.
An assortment of absolute
temperatures
Some Definitions….
Matter can contain thermal energy (or internal energy).
Heat is the flow of thermal energy – from a hotter object to a
cooler one. Heat is thermal energy in transition.
Heat!
High Thermal Energy
Low Thermal Energy
There is no such physical property as
Cold…..
Cold is simply the absence of heat!
How do we measure Heat??
Heat is a form of energy. A physics person measures heat in
a unit called a Joule.
We use a different unit:
A calorie is the amount of heat required to to raise the
temperature of 1 gram (1 cm3 or 1 ml) of water by 1oC.
A Calorie (like we look at on the candy bar wrapper!) is
really 1000 calories!
Specific Heat Capacity
The crust and the filling of the pie come
out of the oven at the same temperature…..
but the crust cools much more quickly. If
you bite into a big bite of pie, you’ll
probably burn your mouth!
The part of the pie that has a lot of liquid (which is water!)
“holds” its heat much longer than the crust.
Some substances take more heat to warm up than others, and,
once warmed, take longer to cool (give up their heat!)
It takes much more energy to warm up a
kilogram of water than it does a kilogram of
oil! (or wood, air, glass, etc…!)
The Eastern Shore has cooler
summers and milder winters
than the western shore
because the Bay stores and
gives up energy…!
“Day after
Tomorrow” web
link
Ocean current provide the major mechanism to transfer
the excess of solar thermal energy from the tropics to
the polar regions.
When molecules of a substance are heated, they gain
energy and “move” faster. This makes them push
apart from each other and the volume of the
substance expands. (volume is how much space
something takes up!)
This is called Thermal Expansion, and must be
considered by engineers and architects!
Not allowing
for thermal
expansion can
cause
problems…..
What’s wrong
with this
picture??
The base of a landbased bridge has
one side which can
move!
Most bridges have
expansion joints which
allow the sections of
the bridge to move
relative to each other
“bi-metallic”strips can be used in
thermostats to turn a device on or
off as temperature changes
An old-ish
thermostat!
Unique Properties of Water
As water cools to near the freezing point, ice
crystals start to form in it – decreasing the density!
The water molecules (H2O) in ice are farther
apart than they are in water. This makes the
density of ice LESS
Density = mass/volume!
Normally,
substances become
more dense as they
cool, and water
does until it gets to
4oC – at which
point ice crystals
start to form and
density decreases!
This makes fish very happy…