The Physical World I

Spring 2005
MWF 11:30

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

REVIEW QUESTIONS OF Home Work 12

R17.28. A liquid absorbs thermal energy when it turns into a gas. This absorbed thermal energy causes the molecules to move more rapidly thus break the bonds between the liquid molecules as the liquid changes into a gas. When the liquid turns into a solid, thermal energy is given off. As less thermal energy is available to the molecules, they vibrate less and hence solidify into a solid.

R17.29. A gas gives off thermal energy as it turns into a liquid. A solid absorbs thermal energy as it turns into a liquid.

R17.32. The latent heat of vaporization for water is large. Therefore if you finger is wet a lot of the heat energy from the hot clothes iron goes into vaporizing the water on your finger. (If your finger was dry then you would be more likely to burn your finger in touching the hot iron.)

EXCERISES OF Home Work 12

E17.39. The wood, because its greater specific heat capacity means it will release more energy in cooling.

E17.40. For ice to melt, heat must be flowing into the ice from the surrounding air. Therefore the surrounding air must be getting colder as it is losing heat.

E17.45. Every gram of water that undergoes freezing releases 335 J of thermal energy to the cellar. This continual release of energy by the freezing water keeps the temperature of the cellar from going below 0^oC. Sugar and salts in the canned goods prevents them from freezing at 0^oC. Only when the water in the tube freezes will the temperature of the cellar go below 0^oC and then freeze the canned goods.

E17.46. Water possesses a large specific heat capacity and a large latent heat of freezing, therefore, a lot of heat energy must be removed from the water in order to freeze it. As the temperature of the surrounding air drops below 0C, heat flows from the warmer water on the fruit trees into the colder air. This warms the air and moderates the air temperature around the fruit trees. The fruit will not be damaged (by dropping to very low temperatures below 0C) provide the water on the fruit trees does not freeze.