Sheri wrote:Hi GS,

Could you explain q12?

I don't get the explanation there. "Thus, when competing with shapes of equal volume, it will have a smaller surface area than any of the rest. Minimizing the area translates into reducing the size of the numerator while, by hypothesis, the denominator stays the same. This should produce a decrease in Q – an increase in energy efficiency."

which equation is the question/answer referring to when stating numerator and denominator?

Thanks!

The equation being referred to is in the explanation to the previous question. Basically, the first paragraph of the passage was translated into the following equation (yes, even on the real exam, sometimes you will need to produce an equation based on your understanding of the problem):

"From the opening paragraph in the passage, the thermal resistance R = x/(A•k) where x is thickness, A is area and k is thermal conductivity. "

Now thinking in reverse, if we want to have the highest thermal resistance (so this would prevent the loss of heat making the structure most energy efficient), then we want the denominator to be very low (given that there is no change in x and k). A sphere is the smallest surface area relative to volume (this is why droplets of water form spheres; see PHY 6.1.5).

The explanation to this question was a bit confusing, so it has been improved based on the information above.