Solar Energy |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) If scientists can utilize nuclear fission to create energy, such as in nuclear electric power plants, why can't they use nuclear fusion? <answer>
2) Does E=MC2 work only in the Sun or in other examples of fission and fusion? Are there any instances of its operation in everyday life? <answer>
3) If all matter is so full of energy, why doesn't everything explode? <answer>
4) How do we really know that it is really nuclear fusion that fuels the stars, or even or Sun? <answer>
5) If the Sun is losing mass at the rate of 5 million tons a second, how can it possibly have lasted 5 billion years? Wouldn't it have burned out? <answer>
ANSWERS
1) It would be very desirable to use nuclear fusion to produce power on Earth. The fuel source, the hydrogen in water, is highly abundant, and the process is very clean and environmentally friendly. The fuel supply for nuclear fission is not abundant, must be highly refined, and the process yields a great deal of dangerous byproducts. So definitely fusion would be a great way to produce power on Earth.
Unfortunately, there are a couple of very big problems that may nuclear fusion completely impractical, at least for now. First, it takes very high pressures and very high temperatures to initiate a fusion process. Despite the aspirations of "cold" fusion proponents, temperatures in the order of billions of degrees are needed to start nuclear fusion. Although this can be achieved, it is difficult and certainly is not possible for any large scale commercial venture. The high temperature makes the whole thing exceedingly difficult to deal with, simply because there is no material that can be used to withstand such temperatures. And the process itself, once initiated, is likewise difficult to control. Minor examples of the fusion process have been achieved in laboratories, but nothing feasible for useful power. production.
2) If you could bring together equal masses of matter and antimatter, the resulting energy release would be 100 percent effective applications of Einstein's Equation without fusion or fission. However, antimatter appears to be rare in the Universe. It can be produced in the laboratory, but currently it is very expensive and obviously difficult to handle. As of the late 1990's only small amounts of anti-particles and one simple atom, anti-hydrogen, have been produced in labs.
But in fact, Einstein's Equation works in everyday life. Anytime there is a net change in energy, there is a corresponding change in mass. In any chemical reaction there is also a change in the mass and energy, according to Einstein's Equation. It is so small, however, that even many chemistry texts will say that there is no mass change at all -- that the mass at the end of a reaction is exactly the same as it was in the beginning. But in fact there is a very small mass change, calculable but typically not measurable, corresponding to the amount of energy involved.
Thus in any activity that produces or uses energy actually is an example of Einstein's Equation at work. Getting out of bed in the morning, or even blinking your eyes are examples. But in everyday life, the effects of the equation are far too small to be of any consequence at all.
And just one more point. Einstein's Equation shows that mass and energy can be changed into one another. It forms the basis of the Law of Conservation of Mass and Energy, which states that while mass and energy can be converted, the total amount of mass and energy combined in the Universe must stay the same. In other words, mass and energy can be converted, but neither can be created or destroyed.
3) For some questions, the best answer is sometimes, "That's just the way the Universe works." We don't actually know why the Universe is set up such that matter "froze" out of the energy of the young universe. We don't know why, but we do know the process and that matter will not release its energy except under extreme conditions of heat and pressure. If you think of matter as "frozen" energy, then think of the Cosmos as a very cold place in which nothing thaws except in the interiors of stars or in certain places like a tiny planet called Earth.
4) The simple answer is, "We don't know." We cannot probe the depths of the Sun or stars directly. And there are some unanswered questions, especially regarding neutrinos. Section 11.3 in your text discusses this. On the whole, however, the theoretical groundwork for solar nuclear fusion is strong, and after decades of research, no other reasonable theory has been put forth to explain the Sun's energy. In addition, the fairly recent development of "helioseismology," in which astronomers probe the interior of the Sun much as geologists probe the Earth with earthquake waves, has provided confirmation of our overall model of the Sun's innards. Modern science didn't have to send astronauts to the Moon to prove that our nearby satellite was not made of green cheese. They already had reasonable theories that turned out to be substantially correct. The same is no doubt true for the Sun.
5) These are thinking questions, but thinking cannot always be done with words alone. This is an example of why you must use math in science. Get yourself a calculator and work it out. There are about 60 X 60 X 24 X 365 seconds in a year. (I say about because the year is not exactly 365 days long, which is why we have leap years.) Multiply that product by 5 million tons to find out how much mass the Sun loses in a year. Multiply that to see how much mass the Sun has lost in 5 billion years. Compare that to the Sun's present mass (see table 11.1 on page 326 of your text). Be sure you do the multiplication correctly, and then tell me, could the Sun still be in existence after 5 billion years of mass loss? (Hint: a (metric) ton is 1000 kg, and the answer is . . . yes.)
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