# The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

REVIEW QUESTIONS OF Home Work 10

R7. Heat energy travels from your hot hand to the cold surface. This occurs via the process of conduction, where the molecules of your hot hand (which are vibrating faster because they are hotter) bang into and transfer energy to the slower moving molecules of the cold surface.

R10. The temperature difference of two objects. Heat always flows from the hot object to the cold object.

R15. Q = mcDT therefore DT = Q/(mc). Therefore DT is large (and the object heats up quickly) if c is small.

R16. DT = Q/(mc), therefore, DT is large (and the cools down quickly) if c is small.

# E1. Inanimate things such as tables, chairs, furniture etc. have the same temperature as the surrounding air (assuming they are in thermal equilibrium with the air – i.e. no sudden gush of different temperature air). People and other mammals, however, generate their own heat and have body temperatures that are normally higher than air temperature.

## E9. When no more energy can be extracted from a material, it is at absolute zero. But there is no limit, in principle, to how much energy can be added to a material. (It is like kinetic energy, which has a minimum, zero, but no maximum.)

E10. Yes, but the amount of water in the Atlantic Ocean is so large that it is difficult to measure the small increase in temperature of the Atlantic Ocean when a hot rock is dropped into it.

E11. The temperature at the bottom of the falls is expected to be slightly higher than the temperature at the top of the falls because the GPE at the top is converted to KE at the bottom, which in turn is transformed into heat energy when the falling water is stopped.

E13. Q = mcDT therefore DT = Q/(mc). For a given amount of heat (Q) the temperature change DT depends upon the mass m and the heat capacity c of the object. For example, for small m and c then DT will be large, conversely, for large m and c then DT will be small.

E15. Water has a high heat capacity c i.e. it usually takes a long time to heat up or cool down. The water mellon is mostly water; it also has a much larger mass than the sandwiches. Therefore for the water mellon as DT = Q/(mc) where both m and c are large it will require a lot of heat Q in order to change the temperature of the water mellon significantly – the water mellon will therefore stay cool over a longer period of time compared with the sandwiches.

E18. Water has a large heat capacity therefore it takes a long time to heat up or cool down. By contrast land heats up or cools down very quickly. In hot weather, land becomes hot very quickly but a large water mass remains relatively cool compared with the land, hence, a large water mass cools the land. In cold weather, land becomes cold very quickly but a large water mass remains relatively warm compared with the land, hence, a large water mass warms the land.