Period 24 Solutions: Energy and Water 24.1 The Earth s Water Cycle 1) Components of the Earth s water cycle a) What can happen to some of the water in lakes, rivers, oceans, and in the soil as the Sun heats the Earth? Soil moisture and water from rivers, lakes, and oceans is evaporated by the sun s energy into water vapor. b) What happens to this water vapor? This vapor rises due to convection and moves due to wind currents. c) How do clouds form? When the water vapor in the atmosphere cools sufficiently due to rising, it condenses to form clouds. d) What happens to the moisture in clouds? Precipitation from clouds brings the water back to the Earth s surface. e) Evaporation of water represents a phase change from liquid to gas. Why does evaporating water remove energy from the atmosphere? Energy from the atmosphere must be added to liquid water molecules to cause them to change to water vapor. This is the latent heat of vaporization of water. f) How does condensing water vapor add energy to the atmosphere? When water vapor condenses into a liquid, the latent heat of vaporization of the water vapor is released into the atmosphere as thermal energy. This process of evaporation and condensation is one of the principal means of transferring solar energy from warmer regions near the Equator to higher latitudes. g) What energy drives the water cycle? Energy from the sun h) Group Discussion Question: As water moves through the Earth s water cycle, does the total amount of water in all forms (solid, liquid, and gas) change? No. Over time, the proportion of water in each of the three forms may change, such as increased ice during glacial periods, but the total amount of water molecules is approximately the same. 1
2) Movement of water through the Earth s water cycle a) Each of the water cycle cubes represents one portion of the water cycle. Roll one of the water cycle cubes (you can start with any of the cubes). Follow the directions on the face of the cube that lands up to show the movement of a water molecule. On the above diagram, draw an arrow from your starting point of your water molecule to the location that the cube directs you to. This is your second point. Roll the cube that represents your second point and follow its directions. Draw an arrow on the diagram to represent this roll. Continue to roll cubes until you have rolled 10 times. Through how many different locations in the water cycle did your water molecule pass? b) To find the contamination of the water in your cycle, use the chart to determine whether each movement of your water molecule added or removed contaminants. Write the total contamination points for your water cycle: c) Group Discussion Question: How long do you think it could take a molecule of water to move through the Earth s water cycle? It can take 250 or more years for water to move through the water cycle. 2
24.2 Hydroelectric Power 3) Generating Hydroelectric Power a) How does the process of generating electricity in a hydroelectric power plant differ from the process of a coal-burning power plant? The blades of a turbine turn due to the pressure of moving water rather than the pressure of steam. b) Which of the four fundamental forces provides the energy for hydroelectric power? Gravitational force c) What advantages to the environment result from hydroelectric power when compared to power from fossil fuels? No emission of carbon dioxide ( a greenhouse gas), sulfur oxides (a cause of acid rain), or particulate pollution. No thermal pollution results from returning heated water to its source (a river or lake) d) What environmental concerns result from hydroelectric power generation? A constant supply of moving water usually involves damming a river. A dam causes environmental changes to the aquatic life downstream. 4) Tidal Generation of Hydroelectric Power a) What causes the tides? Tidal energy comes from the gravitational interactions between the Earth and moon and, to a much smaller extent, between the Earth and the Sun. Because in relation to tides the Sun s gravitational force is much less than the moon s force, tidal energy is not considered to originate from solar energy. b) Why do most locations on Earth have two high tides per day? The moon s gravitational force causes the oceans, which are an easily deformed fluid, to form two bulges, one on either side of the Earth. As the Earth spins on its axis, land bordering the oceans (the shore) passes through both bulges each day, producing two high tides and two low tides per day. c) How can the energy of tides be used to generate electricity? When the tide is rising, there is a flow of water from the ocean into the harbor. As the tide falls, water flows from the harbor back into the ocean. If the opening of a harbor is dammed, 3
water entering or leaving the harbor may be diverted to a water turbine to generate electricity. d) How can river currents be used in addition to or instead of tides to generate electricity? Instead of damming a harbor, underwater turbines can make use of river and ocean currents or ocean waves to generate electricity. This reduces the environmental consequences of damming a harbor. e) Group Discussion Question: Is the source of tidal energy ultimately the Sun? No. While the Sun does exert a gravitational force on the Earth, the moon s gravitational force plays a much greater role in the formation of the tides. 5) Geothermal Generation of Power a) What produces heat in the core of the Earth? The decay of unstable radioactive isotopes b) How does this thermal energy reach the outer layers of the Earth? Thermal energy is conducted through rock layers to the outer layers of the Earth c) How can geothermal energy be used to generate electricity? Hot water and steam near the Earth s surface can be used to produce the steam needed to turn the turbines that spin magnets near coils of wire. d) What are some limitations of the use of geothermal energy to generate electricity? Underground water hot enough to generate electricity is present only in geologically unstable areas, usually volcanically active areas. e) What, if any, environmental disadvantages are there to the use of geothermal energy? There are often sulfur compounds present near geothermally active areas that have an unpleasant rotten egg smell. 4
24.3 Climate Change and Water Resources 6) Retreating Glaciers a) What is the predicted fate of glaciers on mid-latitude and tropical mountains? Mountain glaciers will disappear in this century, many before 2050. b) Why is this a problem for people, especially those living on the rain shadow side of a mountain range? Billions of people depend on glacial melt water for drinking, irrigation, and hydroelectric power. c) Initially, glacial melting will result in increased melt water. How could this be a safety problem for millions of people living near mountain glaciers? Many glacial lakes are impounded by earthen dams. As lake volumes swell, the added pressure could cause these dams to fail, flooding valleys downstream. d) Eventually, glacial melt water will diminish. What problems will this cause for agriculture? Much irrigation water comes from lakes that are replenished by glacial melt water. This is particularly true in South America. e) What effect will reduced melt water have on the generation of electricity? Glacial melt water is the primary source of the river water used to generate hydropower, especially in the South America Andes Mountains, the European Alps, and the mountain ranges of the Pacific Northwest. Other energy sources will be needed to replace this hydropower. 7) Fresh Water Supplies a) What effect will reduced glacial melt water have on the Earth s population? The availability of fresh water for drinking will be reduced. b) About how many people may be affected? An estimated one to two billion people. c) What options are there for providing fresh water for this population? i) Pipe water in from other locations ii) iii) Desalinize sea water Purify and recycle used gray water d) Group Discussion Question: What costs are involved in providing this water? All of these options require energy (and money)! 5