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Re: [Phys-L] Energy Concept Inventory



I see several other questions in this test with which I could take issue.

"1. Workers from the electric company regularly read the meters on houses. These meters
measure the amount of ..."

The meter measures power (the rate the disk spins), which is integrated to give power. So both are measured, although I expect the "correct" answer is "energy".

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"4. A hot 8-kg copper bar is placed in contact with a 5-kg copper bar at room temperature inside
a good Styrofoam cooler. The hot bar does not melt the cooler. The cooler is tightly closed.
Which statement below best describes the temperatures of the bars after an hour?"

After a few minutes, I would definitely pick "c. The temperatures of the bars are nearly the same, closer to the original temperature of the hot bar." But after an hour, they could be pretty close to half way in between, depending on how "good" the cooler is.

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"9. A tropical rainforest is an example of an ecosystem. Which of the following statements about
matter and energy in a tropical rainforest is the most accurate?"

Some matter is recycled (eg decaying plant material), while other matter must be replenished (eg water)
Some energy is recycled (plants getting eaten by animals), while other energy must be replenished (photosynthesis).
This answer is not one of the choices.

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"11 ... 100 kg of green plants serve as the original source of food for each of
the food chains. ... In which of the three situations is the most energy available to the person?"

If the 100kg is grass, then very little/none of the energy would be available to the person who eats it directly , so feeding it to the cow would (I suspect) be more efficient. If the plants are wheat, then feeding it straight to the person would be more efficient.

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"32. A puck sitting on level ice is pushed back against a spring that is attached to a wall. This
partially compresses the spring. The puck is released, and the spring propels it.
If you have only this spring, but a variety of different pucks, how could another puck be
given more energy?"

If friction is significant, then a lighter puck will will lose less energy due to friction, so it will have more energy.
If friction is neglected, then the heavier puck will leave the spring moving slower when they part, so the heavier puck will have more of the energy.
If both friction and the mass of the spring can be neglected, then the mass of the puck doesn't matter.