More Thermodynamics?

Question

a marble is sitting on a level brass bar. the bar is supported by 2 rods of equal length but one is made of iron and the other aluminium. The structure is cooled to a lower temperature-will the marble roll? if so which way?

Answer 1

I think it'll roll towards the aluminum supported side since it has a more sensitive specific heat.

You should have some reference table to help you here, but basically the concept is that aluminum requires less heat to expand (or contract). So if the same amount of heat is applied to both a bar of iron and a bar of aluminum, the aluminum should shrink more since it requires less heat to shrink any given amount.

I'm saying aluminum also because aluminum foil can melt in a microwave whereas you need a blast furnace to smelt iron. ;-)

Answer 2

The quantity you need to look up is called the coefficient of thermal expansion. The quantity that has the greater coefficient will shrink more, so the marble will roll down that way. I believe aluminum has a greater coefficient than iron, so the aluminum bar will sag--look it up yourself, though, to be sure.

Answer 3

Toward the aluminum rod.

Coefficients of thermal expansion:
Fe 11.8 µm·m−1·K−1
Al 23.1 µm·m−1·K−1