Warm-Up Exercise for lecture 22

Due 8:00 am, Thurs, Nov 13

Physics 105, Fall 2008

The fact that desert sand is very hot in the day and very cold at night is evidence that sand has a
☑ low specific heat or
☐ high specific heat.

Thermal energy that is used to melt or freeze something without a temperature change is called:
☑ latent heat
☐ specific heat
☐ thermal mass

Suppose you add an ice cube to a glass of water. True of False: this will always decrease the temperature of the water
☐ True
☑ False

Ralph recently saw a sign while driving that said: "Caution: Bridge freezes before road". He is wondering how that can be the case when the road and the bridge are in thermal contact with each other. Plus, they are both in the same environment. What can you tell him to help him understand?

There are likely many factors. Here are some: a. The bridge is surrounded by cold air on both sides. (The earth underneath the road is likely much warmer than the air.) b. The air can flow past the bridge. This allows heat transfer by convection, like in an uninsulated attic. c. The bridge is made out of metal, a good heat conductor. This allows rapid temperature changes, since any heat in the interior of the metal can easily be conducted to the surface (and then lost to the air). Asphalt, by contrast, is a poor conductor; thus (for example) warmer asphalt an inch below the surface is more likely to stay warm.

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