Linda F. Malanson
(figure available in print form)
You can feel the sun’s warmth on your skin. Sunshine also warms rocks and walls that are directly exposed to it and they feel warm when touched. In hot climates the packed earth, sand or concrete at midday may be too hot to walk on in bare feet. On the hottest day, though, you can walk barefoot on grass.
Heat is what we feel when the energy radiated by the sun strikes an object. This energy is like light, but it cannot be seen. When heat strikes an object, some of that energy is transferred to molecules in the object.
When touched, these energized molecules transfer some of their energy to molecules in your skin, so the temperature of your skin rises—this is why the object feels warmer. Warmed objects lose energy. They may radiate it, like a fire; pass it to another object by conduction; or warm a gas or liquid, which carries heat away by convection. All these different sorts of heat help to produce our weather.
EXPERIMENT: CONVECTION: WARMING FLUIDS
When a fluid a gas or a liquid is warmed, its molecules move apart. The fluid takes up more space, but the number of molecules remain the same, so it becomes less dense than its cooler surroundings. This means it weights less, so it rises through the fluid surrounding it until it reaches a level where the substance above such as the air above a liquid is less dense than it is. The fluid then cools and sinks. You can demonstrate convection with hot and cold water.
MATERIALS NEEDED:
1. a large jar
2. water
3. a small cup
4. food coloring
5. a rubber band
6. plastic wrap
7. a small stick
DIRECTIONS:
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1. Fill a cup with colored hot water. Cover it with plastic wrap, secured with a rubber band. Put the cup in the jar.
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2. Now fill the jar with cold water so that the water goes over the level of the cup and nearly to the top of the jar.
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3. Poke a hole in the plastic wrap with the stick, and watch the warm colored water rise. What happens next?
EXPERIMENT: HEAT CONDUCTION
(figure available in print form)
The molecules that make up a solid object are touching. If one molecule vibrates faster because it has absorbed energy, it vibration make its neighboring molecules vibrate so the energy spreads. This is conduction.
Different materials have different structures, and this affects how their molecules pass energy. Materials also vary in the amount of heat they can absorb before their temperature rises. This is called their specific heat capacity. Grass feels cooler than stone because grass contains water which has a high specific heat capacity.
MATERIALS NEEDED.:
1. a metal tray
2. a wooden board
3. a piece of plastic
DIRECTIONS:
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1. Put the metal tray, a wooden board and a piece of plastic on a table.
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2. Leave them there for an hour so that they equalize in temperature. Now feel them. Which object feels warmest?