About their problems. They try to find answers for their problems together. For example
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114 SOLAR ENERGY Most of the energy we consume comes directly or indirectly from the sun. In 1972, man consumed 99.ttllion horsepower hours of energy. During the same year, the sun emitted. 1,5 million trillion horsepower hours of sunlight on the planet. Only a small fraction of this natural force (40 billion kilowatts) is converted by green plants into all the food and wood that grows on the earth. Over hundreds of thousands of years this vegetation is eventually convened into fossil fuels like coal and oil. Because of the length of time which is required to produce fossil fuels, it would be more feasible to employ solar radiations directly to help meet our energy needs. Presently solar energy can be used to stretch our limited supplies of fossil fuels. Experiments have already shown that we can heat a house, heat water, air condition buildings, cook food, and produce electricity with solar energy~ Scientists have even converted an automobile to run on batteries which are charged by solar cells. In the future, solar energy may be one of our main sources of energy.
115 DESALINATION METHODS Without water there could be no life on Earth. Water is more important than food to us. Over three-quarters of our bodies is water and without it we cannot grow crops, run factories or keep clean. Nearly three-quarters of the earth's surface is water. This, however, is the undrinkable water in the oceans and seas. We cannot use this water because of the amount of minerals it contains. The world is living through a population explosion. In other words, there is a rapid growth in the number of people living on Earth. Therefore, we have a problem: How can we get more water for man's use? One way of doing this is to make sea water drinkable. Many scientists today are trying to find methods to change sea water into fresh water. There are three ways to take the salt from sea water. Scientists prefer electrodialysis to desalt water with a small quantity of salt. In this process , an electric charge passes through the salty water and separates the salt from the water. Another method of desalting is freezing. Ice is pure, or fresh, water. When sea water freezes, the salt separates from the water and then you can wash it off. Finally, you have to melt the ice to use it as fresh water. A very common way to convert sea water into fresh water is distillation. In common distillation, the sun provides the necessary heat. A piece of plastic covers a few centimetres of salt water in a basin, or open container. The water evaporates with the heat of the sun and the vapour rises until it hits the top. Then it condenses into fresh water. That is, it changes from the vapour state into the liquid state. Distilling water by this method is very slow. For this reason, industry is now using an expensive variation of the distillation process. In this process, boiling and condensation are combined. There is a disadvantage of all desalting methods: their high cost. The cost is high because all use a lot of expensive energy coming from sources such as coal, oil and gas.
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