Prevalent student difficulties when applying conservation of energy:
Students will readily identify that energy is conserved in physical processses. Understanding how energy is conserved in a wide array of physical processes is much more challenging. The following is a list of problematic ideas which often arise when students attempt to reconcile energy conservation to specific processes. While these ideas are problematic, they are a positive sign that students are actively engaging with energy conservation. Each problematic idea is followed by physical processes which would be likely to elicit that idea.
· When one event causes or triggers another event the energy associated with the subsequent event is equal to the energy associated with the first event.
o A person pushes over a domino that is standing on a flat table surface.
o A wooden match is struck on a matchbox and then burns until it goes out.
o In the following video a person snaps a small metal disk which causes the sodium acetate inside the bag to begin crystallizing. As the crystallization process occurs the bag becomes noticeably warmer. http://static.howstuffworks.com/mpeg/q290.mpg
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· If kinetic energy and gravitational energy are involved, then the sum of these two types of energy will remain constant.
o Two blocks are tied together with a very light string that extends over a pulley. Initially the heavier block is higher than the lighter block. The heavier block moves downward pulling the lighter block upwards. Atwood Machine.
o A father and daughter are playing on a teeter-totter the father pushes off with his legs and moves upward several feet off the ground, then comes back down. The daughter never touches the ground.
o A bowling ball is sinking to the bottom of a swimming pool. How would this ‘play’ differently from a bowling falling through the air?
o A basketball is released from the bottom of a swimming pool and rises quickly toward the surface.
· If the energy of an object is not changing then there are no energy transfers to or from that object.
If there are energy transfers to and/or from an object, the object’s energy must be changing.
o A person speeds up while riding a bicycle along a level path.
o A person rides a bicycle at a constant speed along a level path.
o Your house guests have just used up all of the hot water in your house. Your electric hot water heater is now on and you are waiting for the water to get warm enough for you to take a shower.
o You leave for a three week summer vacation. While you are gone the hot water in your tank is maintained at a constant temperature.
o You crawl into a cold sleeping bag.
o Hours later, you are sleeping peacefully and the temperature inside the sleeping bag is just right.
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· If one object influences the motion of another object than there must be an energy transfer from one object to the other.
o A person jumps upward off of the ground.
o A pull back car speeds up while moving across a level surface.
o A child pumps a swing at a playground.
· There can be just plain energy, without a particular form and unobservable.
· Students have difficulty reconciling the scientific principle of energy conservation with the socially relevant idea of conserving useful energy through the use of energy efficient devices.
o A regular light bulb and a compact fluorescent bulb both provide the same amount of light but the compact fluorescent uses about 85% less energy.
