I reproduce below two of the best essays from the first midterm exam. Congratulations to Alyssa and to Bethany! The questions were on sunlight as an energy source, and on terrorist nuclear weapons.
1. Describe the
possibility of sunlight as a future energy source. Could it be used for a large
power plant? For an automobile? For an airplane? Explain your
answers, and support them with numbers.
Essay submitted by Alyssa Pskowski:
Sunlight can be used as a future power source, although it doesn't have the versatility of fossil fuels and cannot replace them completely.
Sunlight can be used for a large power plant . One square kilometer of sunlight delivers one gigawatt of power, however, solar cells are only 15-40% efficient, so only 150-400 megawatts can actually be converted to useful energy. A 2.5 square kilometer solar cell facility could produce energy at the same rate as a standard power plant (one gigawatt). Although, this would be a large facility, we would only need to cover 1/1000th of California to provide for our energy needs. In addition, several of these facilities could be placed in open and sunny places like Nevada.
Solar cells stop producing energy once the sun goes down, but hydrogen can be produced to store energy during the day, and then that stored energy could be used once the sun goes down. However, solar cells are expensive, and solar is not likely to become a primary energy source until solar cells become more cost effective.
Solar energy is not a good option for automobiles because only 150-400 watts can be harvested from one square meter of sunlight. One kilowatt equals one horsepower, and cars run on at least 50 horsepower, so a lot of solar panels would be required to power a standard car, but a car is not big enough to have that many solar panels. If solar were used, the car would not be able to go as fast as they do now.
Solar energy is also not a good option for passenger and freight airplanes for the same reasons as the auto -- solar does not provide enough of the power that the plane needs. However, solar can be used for small unmanned planes, such as those used to make weather observations, and so it would be useful for this category of plane.
2. To build a nuclear
bomb, a terrorist faces many challenges.
Describe the kinds of nuclear bombs that they might try to make, and
what they need to do to build such a weapon.
Essay submitted by Bethany Riordan:
There are three main kinds of nuclear bombs, but each of them has its challenges.
A Uranium bomb uses a design so simple that it wasn't even tested before the uranium-type bomb was dropped on Hiroshima. The design is referred to as a "gun-type" bomb because one mass of Uranium-235 is shot at another piece of U-235 to create a critical mass in which the reaction can begin. When U-235 is hit by a neutron, the nucleus gets destabilized and fissions, producing two fission fragments, and two neutrons. The critical mass ensures that there are enough U-235 atoms close enough together to ensure that each of those neutrons hits another U-235 atom to continue the chain reaction. The difficulty, however, is amassing enough U-235 to make a critical mass. Naturally, U-235 has an abundance of only 0.7% and the other 99.3% is U-238, which does not propagate chain reactions. 80% U-235 is required for a nuclear bomb to work.
So the natural Uranium sources must be enriched. The two main methods use Calutrons and centrifuges. Calutrons take advantage of the fact that magnets cause U-235 to bend more than U-238 so they can be separated and the U-235 collected, but it would take decades to accumulate a critical mass by this method. Centrifuges are more practical, combining uranium with fluoride to make uranium hexafluoride in which molecules with U-238 are heavier then withU-235, so they can be separated by spinning, but centrifuges require an extremely strong material called maraging steel, which must be obtained.
Plutonium bombs have the advantage that Pu-239 is available because it is made in the waste product of nuclear reactors and can easily be chemically reprocessed. However, the plutonium bomb design and construction is extremely difficult and precise. Plutonium is often contaminated by Pu-240, which is very reactive and decays before the chain reaction goes to completion. So to prevent this from happening, plutonium bombs use implosion. Explosives are detonated on all sides of a mass of Pu to compress it into a small "blob" where the three neutrons emitted can hit other Pu-239 atoms quickly and continue the chain reaction. But the explosions must be entirely balanced and uniform to implode the Pu properly and successfully detonate a Plutonium bomb.
The final main type of nuclear bomb is a hydrogen or thermonuclear bomb. It uses a three step mechanism in which first, a small plutonium (Pu-239) fission reaction is initiated that creates high enough heat to cause a mass of deuterium and tritium atoms to overcome their nuclear repulsions and fuse, which generates an extreme amount of energy. As a third step, the neutrons emitted from the Pu-239 fission hit the Uranium -238 shell that encloses this entire bomb the Pu and deuterium/tritium mixture), causing the uranium to absorb neutrons and create even more energy. In tests, hydrogen bombs have been proven to be able to generate extreme amounts of energy.