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PPT
Temperature, Heat and Thermal Energy
Developer Notes
1. Teaching heat and temperature before conservation of energy might make sense. The link
from KE & PE would be "ignore friction." On the other hand, conservation could easily lead
to questions about heat. Darn chickens and eggs!
2. I think we really need to deal with internal energy here at some point - nuclear, chemical…
3. We should add a section on absolute zero and Kelvin. Stacey disagrees with me and says
they can learn it next year in chemistry. It certainly has little relation to physiology!
Goals
1. Students should know that the change in internal energy of an object is equal to the work
done plus the heat transferred.
2. Students should know that heat is energy transfer due to the difference in temperature. (Work
is energy transfer NOT due to the difference in temperature.)
3. Students should know that temperature is a measure of the average kinetic energy of the
molecules in an object.
Concepts & Skills Introduced
Area
physics
physics
physics
physics
Concept
First Law of Thermodynamics
heat
temperature
Celsius
Time Required
About 1 class period
Warm-up Question
Presentation
Put two equal temperature things of different masses into equal masses of water. Which water
will change more in temperature?
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Temperature, Heat and Thermal Energy
Background / History
In the last unit, we learned that energy is conserved, that the total KE and PE before something
happens is equal to the total KE and PE plus the thermal energy generated by friction afterwards.
How does energy get transferred? If you lift a bucket, you raise its gravitational potential energy.
If you throw a baseball, you raise its kinetic energy. The work done on the object is equal to its
change in energy, W = ∆E. We know now that ∆E also includes thermal energy. But if
something increases its thermal energy, it gets warmer, even hot. Check the wheels (the metal
part, not the rubber tires) on your car after a trip - you'll find they are warm because of the
thermal energy from the friction of the brakes. If you put your hand on the wheel, your hand will
get warm, too. So now your hand has more energy than it did. But there was no work done! No
Fd. How did you gain energy when there was no work done? If your hand and the wheel were
the same temperature before you touched the wheel, you wouldn't gain any energy from it.
Because of the difference in temperature, the energy in your hand changed. If your hand was
warmer than the wheel, your hand would lose energy to it. Physicists call the transfer of energy
due to a difference in temperature heat.
Heat is like work. Both of them are energy being transferred.
 Heat is energy transferred due to a difference in temperature.
 Work is energy transferred not due to a difference in temperature.
When you say something has a lot of heat, it means you can feel the energy coming from it,
transferring to you. A stove transfers a lot of heat energy - the pot on the stove absorbs it.
An object can gain or lose energy in two ways, by work and by heat. This leads to a more
complete statement of the Law of Conservation of Energy, the First Law of Thermodynamics.
First Law of Thermodynamics.
The change in internal energy of a system is equal to the work done plus the heat
transferred.
∆E = W + Q
Work on the system is +
Work by the system is Heat added is +
Heat lost is We keep talking about thermal energy and heat, but what are they? They are very similar to
kinetic energy, but at a sub-microscopic level. The molecules of an object are always moving.
When an object absorbs heat, the molecules move faster. When water is heated, the molecules
eventually start moving so fast they fly out of the water as steam. Even though molecules are
very small, they still have mass. They also have velocity. That means they have kinetic energy
(and momentum). Thermal energy is really the total kinetic energy of all the molecules in
something.
How hot is it? Temperature measures how hot something is. Temperature does not measure the
total amount of thermal energy, however. If you get a bowl of steaming hot saimin and fill your
mouth with the broth, you could burn your whole mouth. If you take just one drop on your
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Temperature, Heat and Thermal Energy
tongue, it will do much less damage, even though it is the same temperature. The mouthful of
broth has much more thermal energy in it than the drop because it has a lot more mass. It
transfers much more heat and does more work on your mouth (maybe even raising blisters!).
Temperature is a measure of the average kinetic energy of the molecules in something, the total
amount of energy depends on the temperature and the mass.
Temperature is measured in degrees Celsius (or Centigrade). 0C is the freezing point of water,
and 100C is the boiling point of water at normal atmospheric pressure.
Work and heat are both forms of energy. Work can actually be done by heat. The most common
example is an automobile engine. When the gas/air mixture in the cylinder is ignited by the
sparkplug, it burns quickly (explodes). The temperature of the air inside the cylinder becomes
much higher than the temperature outside. The molecules of the air in the cylinder have more
kinetic energy than those outside. They hit the piston harder and make it move. That's a force and
a distance - work!
Exercises
1. When a pan of water is put on a burner, heat is transferred from the burner to the pan and
from the pan to the water. If thermal energy is continually being added to the water, why
does its temperature stop rising?
2. When you put water in your freezer, it eventually turns to ice. Thermal energy has been
removed from it. Where does that thermal energy go? (Try putting your hand behind your
refrigerator.)
3. Suzy the parachutist was in an airplane. She jumped out and opened her parachute. At first
she accelerated down, but eventually she reached a terminal speed, where she was neither
slowing down nor speeding up. When she was in the airplane, she had a certain amount of
potential energy. As she fell, what happened to her potential energy? At terminal speed was
her kinetic energy changing? If her KE + PE was decreasing, where did the energy go?
4. If your normal energy intake per day is 2500 C, about 1000 C are available for work. If you
are 10% efficient in converting that energy into work done on outside things, like shoveling
dirt, how many Joules of thermal energy do you produce? (1 cal = 4.184 J)
5. Snakes are cold-blooded, which means that their body temperature is the same as the
environment. Why are they more active after having been in the sun?
Vocabulary
1. First Law of Thermodynamics
The change in internal energy of a system is equal to the work done plus the heat transferred.
∆E = W + Q
2. Heat - energy transferred by a difference in temperature
3. Temperature - a measure of the average kinetic energy of the molecules of an object
4. Celsius - a unit for temperature, where 0C is the freezing point of water, and 100C is the
boiling point of water at normal atmospheric pressure.
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