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Re: [Phys-L] Energy Concept Inventory


On 2012, Jul 07, , at 20:21, Folkerts, Timothy J wrote:

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


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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.


I'm lost here. The bars do not have a temperature until equilibrium. The contact surfaces have the same temp, the opposite sides, initially, have close to their original temps. Only after equilibrium do the bars have a temp. in proportion to their mass ratio. I suspect an hour is quite sufficient for equilibrium

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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.

Yes, this is the counter intuitive condition where it's ecologically better to use cows instead of fertilizing and importing water. Otherwise, it's VERRRY inefficient to eat cows.




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