Geysers are among the
most popular attractions at Yellowstone National Park. A geyser is a vertical
hole in the ground that gradually fills with water and leads down to a source
of volcanic heat. After a dormant period of minutes or hours, water comes
rushing up out of the hole and sprays vigorously into the sky. A geyser is a
spectacular heat engine and it functions in part through the use of water's
material phases.
1. After it erupts, the geyser's hole refills with water so that the water forms a tall, narrow, vertical column in a solid rock "pipe." The rocks at the bottom of this pipe are volcanically heated and heat flows from those rocks into the water. The temperature of water at the base of the pipe soon exceeds 100 °C, yet the water doesn't boil. Explain why the water doesn't boil.
2. Eventually water at the base of the pipe does begin to boil. When this occurs, the steam bubbles push a little of the water out of the pipe's top, where it trickles down around the opening of the geyser. After a few seconds of this trickling, the geyser erupts violently, spraying water high into the air. Why did losing just a little bit of water from the top of the pipe cause this sudden and dramatic eruption?
3. The geyser is certainly a heat engine--it uses the flow of heat from hot rocks to cold air to do the work of tossing water high into the air. Evidently, some thermal energy from the rocks is being converted into work. Explain the process in which that thermal energy is converted to work, showing both that work is done on the water and that thermal energy is used up while doing that work.
4. When a geyser has finished spraying out water, it releases a great deal of an invisible gas that quickly turns into white clouds once it enters the air. What is that invisible gas?
Most ski resorts have
snowmaking equipment to carry them through periods of low precipitation. While
there are a number of ways to produce snow particles out of water, the most
common scheme involves mixing water and compressed air as they spray out
through nozzles into the cold night air. The compressed air helps to break up
the water into tiny droplets and project those droplets high into the air, but
it has other effects as well.
5. To make the compressed air, a ski resort squeezes ordinary outdoor air to approximately 8 times its normal density. How does this process affect the air's pressure and temperature?
6. The compressed air then passes through a heat exchanger that allows it to reach thermal equilibrium with the outdoor air. In the process, a great deal of liquid water condenses out of the compressed air and is removed so that it doesn't ice up the pipes as the compressed air travels up the mountains. Why does water condense out of the compressed air as that compressed air passes through the heat exchanger?
7. In the snowmaking machine itself, a nozzle sprays a jet of liquid water into the outdoor air. The water enters the nozzle at about 0 °C--its freezing temperature. Although the outdoor air is slightly below 0 °C, it takes a while for the water to freeze into ice. Why doesn't 0 °C water turn into ice immediately when exposed to air that's colder than 0 °C?
8. To speed the freezing process, the jet from the water nozzle mixes with the jet from a compressed air nozzle. Air spraying out of this second nozzle not only breaks up the water into tiny droplets, but it helps to cool and freeze those droplets as well. However, the compressed air and the water both enter their nozzles at the same temperature--about 0 °C. Explain how this expanding jet of compressed air is able to help chill the water jet.