iceland volcanoes! heat and thermal energy. the thermal energy of a substance is the total energy of...
TRANSCRIPT
Iceland Volcanoes!
Heat and Thermal Energy
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!
When pressure is held constant, the volume of a gas changes (decreases) by 1/273 for each degree C the
temperature decreases.
gas
piston
Zero volume!
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.
High Thermal Energy Low Thermal Energy
Heat!
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…!
Ocean current provide the major mechanism to transfer the excess of solar thermal energy from the tropics to
the polar regions.
“Day after Tomorrow” web link
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 land-based 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…