Download Stacey Carpenter - University of Hawaii

PPT
Energy Conservation
Developer Notes
1. This should probably be presented before the section on work, KE, and PE. Or at least it
should be introduced. I don't know that heat needs to be given yet. Conservation of energy
(Fd = mv2/2) could come right on the heels of conservation of momentum (Ft = ∆mv). The
transition from Ft to Fd is pretty straightforward, as is mv to mv2/2. Done that way, you
wouldn't have to get into heat yet, just force times time or distance, and mass times velocity.
Heat could be introduced later with the whole conservation idea.
2. The use of the terms "thermal energy" and "heat" could be confusing. It confuses me. Why
isn't the equation E = KE + PE + thermal energy (instead of Q)? I'm going to write it that
way because I think it will be easier for the students, and easier for me to explain. In this
equation it seems to me that we're looking at a state of energy rather than a change of energy.
If we are interested in ∆E, then ∆E = W + Q works better. We already have W = ∆E.
3. Then, of course, there's "internal energy," which includes chemical, nuclear, etc. That would
be more accurate. But worth it? Maybe.
Goals
1. Students should understand the concept of a system.
2. Students should understand that the amount of energy in a system doesn't change from
actions within the system.
3. Students should understand that energy changes forms.
4. Students should understand that there is a limited amount of energy in the universe.
5. Students should understand the formula E = KE + PE + TE
Concepts & Skills Introduced
Area
physics
physics
physics
Concept
system
Law of Conservation of Energy
thermal energy
Time Required
About 1-2 class periods
Warm-up Question
Presentation
Activities - Look at something like a hot plate boiling some water. Name all the different types
of energy involved. Trace the energy transitions.
Use a Newton's demonstrator hanging balls thing to discuss conservation of momentum and
energy.
Re-do the momentum activity but record speed somehow to show that energy is conserved. The
PE in the spring is converted to KE. Could load the carts differently and stop them car after 1
time unit (second? but it could be any consistent beat) to compare distances - I haven't tried this
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yet. In the other lab, the carts were tied together with a string so that they traveled the same
distance as each other for each setup, but each setup was a different amount of time. Could just
do it with one cart pushing off a wall. Could use a beeper/ metronome to synchronize the time THIS WOULD HAVE THE GREAT ADVANTAGE OF ELIMINATING THE REACTION/
ANTICIPATION DIFFERENCE THAT PLAGUES SO MANY OF OUR LABS WHEN WE
USE STOPWATCHES. It could lead to a noisy classroom if there are a lot of beep/ clicks going
on - could have one master clock for the whole class.
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Energy Conservation
Reading
Rub your hands together. They get warm. Why? Friction, of course! But…
Use a foot-powered scooter as an example. Call the scooter and the person riding it a system (a
system is any group of objects you're interested in). The person lives at the bottom of a hill, and
takes the bus to school, carrying the scooter. The bus provides an outside force that increases the
scooter+person system's potential energy as they go up the hill. When the scooter+person system
heads home after school, they don't take the bus, but coast down the hill for fun, and their
potential energy turns to kinetic energy. When they get home, they stop. They don't have kinetic
energy any more. And they don't have potential energy. What happened to the system's energy?
Where did it go?
Anyone who's ridden a scooter with a brake on it knows. The brake gets hot. That's where the
energy went. The potential energy turned into kinetic energy, which turned into thermal energy.
(Thermal is like thermometer, it relates to temperature or heat.) The thermal energy is still part of
the system, since it is the scooter that gets hot. This is an example of the law of conservation of
energy. The system's energy wasn't lost, it just changed forms. As the scooter brake cools, the
thermal energy will radiate back outside the system.
Law of Conservation of Energy
Energy can't be created or destroyed. It can change forms or transfer from one object to
another, but the total amount never changes.
The combination of different kinds of energy can be written in an equation:
E = PE + KE + TE
The total energy of a system is potential energy plus kinetic energy plus thermal energy.
The law of conservation of energy is one of the most fundamental laws of physics. It is like the
law of conservation of momentum. It simply means that in any system (and the universe is the
biggest one), there is only a limited amount of energy. The energy can't be increased or
decreased from within the system. The total energy before equals the total energy after, or:
E = (PE + KE + TE)before = (PE + KE + TE)after
Energy can come into the system from the environment, and energy from the system can go to
the environment (except for the universe, of course), but the system's change in energy is always
exactly equal and opposite to the change in energy of the environment.
A roller coaster starts at the highest point, then coasts down and back up hills and around curves.
At the end, it doesn't have enough energy to reach the starting point again. Why? Friction. How
does friction compare to thermal energy? Friction is a force. When that force is applied for a
distance, work is done, in this case changing the kinetic energy of the roller coaster. The work
done slowing the coaster down is exactly equal to the thermal energy created by friction.
If you look at the whole roller coaster and track as a system, the potential energy of the roller
coaster at the top of the first hill is equal to the thermal energy created in the coaster and the
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track by friction. The friction times the distance it was applied equals the work done on the roller
coaster, and it also equals the thermal energy created.
In all of our labs, friction has played a part. In many of our exercises, we say things like "Ignore
friction," or "Ignore air resistance." In an ideal world, you can ignore it. In the real world you
can't, so it is necessary to include friction and thermal energy when thinking about the total
energy of a system. When you rub your hands together, the work you do is just fighting friction,
and at the end, there is no PE or KE, so the thermal energy you create in your hands is equal to
the total work (frictiondistance) you did on your hands.
Exercises
1. If you drag a box across a floor, how much thermal energy (from start to stop) will you create
in the box and floor?
2. If you hit a penny with a hammer repeatedly, will it get warmer? Explain in terms of the
hammer's KE.
3. In our lab on work, KE, and PE, the ball rolled down the ramp, went into the cup, and slid to
a stop. How much thermal energy was created by the cup and ball sliding?
4. Explain why the space shuttle gets so hot when it re-enters the atmosphere. Include PE, KE,
and friction in your explanation.
5. When a 1,000 kg car is going 10 m/s and brakes to a stop, how many joules of thermal
energy are created?
Vocabulary
1. System - A system is a group of objects that you are interested in.
2. Law of Conservation of Energy - Energy can't be created or destroyed. It can change forms
or transfer from one object to another, but the total amount never changes.
E = KE + PE + TE (Total energy equals kinetic energy + potential energy + thermal energy.)
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