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E=MC[s] question

  1. Oct 16, 2004 #1
    I'm sure we are all familiar with the equation E=MC^2, but can someone calrify what it implies? Energy=Mass x Speed of Light ^2. Energy can be made by mass on the speed of light ^2? Mass on the speed of light^2 can be made into energy? Why is mass made out of potential energy, and not matter is made out of potential energy? I dont get it... Thanks in advance
    Last edited: Oct 16, 2004
  2. jcsd
  3. Oct 16, 2004 #2


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    It means that the amount of energy you would get from converting matter to energy is equal to the rest mass of the matter times the speed of light squared.

    example: If you converted 1 gram of matter into energy you would get

    [tex]E= (1g)(3*10^{10}cm/sec)^2 = 9*10^{20} ergs [/tex]

    or 25 million kilowatt-hours of energy.
    Last edited: Oct 16, 2004
  4. Oct 16, 2004 #3
    wow....1 gram into 25 million kilwatt-hours... Was it ever possible to convert it to energy? Btw...wat is the speed of light? [tex]3*10^{10}cm/sec[/tex]? or is there a more specific measurement?
    Last edited: Oct 16, 2004
  5. Oct 16, 2004 #4
    This principle is used every day in nuclear power plants. They convert some of the mass of a uranium atom into energy which is then used to generate electricity.

    This is the principle behind nuclear weapons - a few grams of plutonium is converted into energy.
  6. Oct 16, 2004 #5
    i thought a nuclear bomb was a uranium plutonium fission reaction
  7. Oct 16, 2004 #6
    It is. That's how fission works. When a uranium-235 or a plutonium-239 nucleus absorbs a neutron, it "splits" into two smaller nuclei and releases some free neutrons. If you add up the masses of the smaller nuclei and the released neutrons, you will find that it is less than the masses of the original nucleus and absorbed neutron. What has happened is this "mass defect" has been converted into energy in accordance with Einstein's famous equation.
  8. Oct 17, 2004 #7
    hoe exactly do you CONVERT mass into energy. or how would you make a substance absorb a neutron?
  9. Oct 17, 2004 #8
    Mass to Energy:

    Einstein reasoned that mass and energy must somehow be different manifestations of the same thing. If we can convert kinetic energy into mass, we should also be able to convert mass into energy, right? Well, special relativity in fact gives an express for how mass and energy are related. And such a process was first observed by C.D Anderson in 1932 where he discovered the creation of an electron-positron pair from the energy of a gamma ray photon.
  10. Oct 17, 2004 #9
    Unfortunately, I can't provide you with a step by step explanation as to how mass becomes energy.

    To get a material to absorb a neutron, you put it in a neutron field. Uranium spontaneously fissions emitting neutrons. There are other materials that you can put together that are neutron sources. These materials are all radioactive so they aren't readily available, but this is how a nuclear power plant establishes the initial neutron field necessary to start the fission chain reaction.
  11. Oct 17, 2004 #10


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    Most nuclear reactions, fission and fusion, result in some loss of mass that is converted into energy. A positron (positively charged particle same size as electron) and electron collison results in all the matter being converted into energy.

    Chemical reactions that generate energy do so because the end up with less internal energy afterwards than before the chemicla reaction. Most of the energy is related to the kinetic energy of the electrons "orbiting" around the atoms in a molecule. When the electrons drop down to a lower energy state, energy is released.

    Lasers do a similar thing, except they convert energy into a light. Energy is used to increse the energy state of the electons of a gas or crystal. Then a passing photon triggers the electron to drop it's energy state and produce a photon in the same direction (and phase?) as the original photon. The mirrors in a laser is what controls the ultimate direction. Initially, photons are going off in all directions, but if not lined up with the mirrors they just get absorbed by the walls of the cylinder. Eventually (a very short time, the ones lined up with the mirrors keep being reflected back and forth, generating more photons in the same direction (and phase). One of the mirrors is a partial mirror, and this is where the beam eminates from.
  12. Oct 17, 2004 #11


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    Nuclear bombs - the olds one were either simple enriched Uranium (235) bombs where just 2 halves had to be slammed together, or Plutonimum based bombs whch required multple segements be imploded together at nearly the same time. Hydrogen bombs use tritium (1 proton, 2 neurtons, 1 electron), for the big reaction, and require a conventional fission bomb to initiate the fusion process. The tritium isn't stable and has to be replinished. There are only a few tritium breeder reactors in the world.

    Do web search and you can find out lots of information on this stuff.

    If you look on ebay, you can find the special cetrifuges used to enrich Uranium (the heavier U238 goes to the outside, leaving a layer of U235 on the inside), the special high tech timed triggers needed to implode Plutonium properly for sale. Edmunds scientific catalog will have the rest of the stuff you need to make your own thermal nuclear device (OK, just kidding here).
    Last edited: Oct 17, 2004
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