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B I am trying to understand energy, and its relation to (matte

  1. May 28, 2018 #1
    1- Let’s say we have 1,000 hydrogen atoms in empty space.
    2- And for this case only, let’s say 1,000 atoms is the point of fusion under their own gravity.
    3- Fusion will create energy.
    4- And it will continue until the star explodes or collapses, so in this case it will explode after all hydrogen was converted to let’s say carbon.

    The thing that is puzzling me is, how many of the 1,000 atoms will be converted to carbon, and how many will be converted to energy?

    Will we lose part of these atoms forever as energy?

    And let’s say that the new carbon is unstable, just for that example, like uranium, it can explode and split back to hydrogen atoms, will this mean that we will get the 1,000 hydrogen atoms again? And what about the energy, how much of these become energy again?

    The full question:

    Is the process of fusion a process of releasing or storing of energy? And what about fission? I can see energy generated in both cases.

    Is matter a one-way street to energy?
  2. jcsd
  3. May 28, 2018 #2


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    Staff: Mentor

    Both fission and fusion convert energy from one form (mass) to other forms (kinetic, electromagnetic radiation, heat).

    In principle these reactions are reversible, so the kinetic/heat/radiation energy could turn back into matter. However, the probability of this happening is so low that realistically it cannot happen - the nuclear bomb inside the shiny metal box may turn into a mushroom cloud, but the mushroom cloud will never turn into a shiny metal box. Similar considerations apply if we scramble an egg: once scrambled the egg cannot be unscrambled, and if we film the entire process from breaking the egg into a bowl, stirring it, pouring it into a skillet, cooking it until it is done.... no one will be fooled if we try playing the film backwards.
  4. May 28, 2018 #3
    Thank you for the explanation
    But what about without reversing them back in time, time is one way as it is today
    - Today, our sun is fusing the hydrogen to Lithium I think.
    - Then this will be fused to the next level and so on.
    - The sun will explode
    - The explosion will generate a small quantity of even heavier metals (gold, uranium, etc)
    - Some of these, like uranium, can be converted back to less heavier atoms, not sure if they go back to hydrogen, but they can be split and that crates more energy.
    Anyway, in fusion, we lost matter to energy, and on fission, we lost matter for energy, when do we gain matter?

    I am trying to understand the gravity relation to energy, is gravity the energy generator? It brings atoms together and creates energy, the atoms split to smaller elements (in the case of uranium) and if these smaller elements go back a star under gravity can generate more energy.
    Is matter fully convert to energy? Or is energy a pure result of gravity and matter stay the same?

    Some companies are working on fusion reactors, and that is a very good idea, but they are doing it without gravity, and the only example of fusion so far in the universe is related to gravity (stars, etc), so businesses are working to create a magnetic field that needs a lot of energy to sustain the fusion, and once it starts they are expecting it to create the energy required to sustain the magnetic field + additional energy to be used for us, and that just does not compute for me.
  5. May 28, 2018 #4


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    Staff: Mentor

    Not that the reaction is actually possible, but a carbon atom has an atomic mass of 12, so 1,000 hydrogen atoms could make about 83 carbon atoms -- and release energy. The energy doesn't come from annihilating atoms it comes from the bonds between the nuclear particles.
    Fusion is exothermic up to iron and fission is exothermic down to iron (since they are opposite processes).
  6. May 28, 2018 #5
    Thank you for the explanation, I now understand that there is more than one type of energy, and I finally understand E=MC2

    As a computer programmer, it will translate to me as something like:
    Gravitational Energy = Gravitational Weight * Speed of Light * Speed of Light

    Is electro magnetic energy the same as kinetic energy?
  7. May 28, 2018 #6


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    Staff: Mentor

    That makes no sense because there is no such thing as "gravitational energy" and all weights (weight and mass are different things) are "gravitational". There is a concept of gravitational potential energy, but it is unrelated to the ##E## in ##E=mc^2##.
    they are different things, but the one can be converted into the other. Think of the relationship between euros and dollars - they aren't the same thing, but they both have purchasing power and you can convert the one into the other while retaining that value.
  8. May 28, 2018 #7


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    Here's a quick fusion and fission primer:

    There are two forces at play here - the electrostatic force between electrically charged particles, and the strong force between protons and neutrons. The former is relatively weak, but long-range. The latter is much stronger at very short ranges, but quickly tapers off to negligibility.

    Let's say we have two hydrogen nuclei - not atoms; the electrons are already stripped off. A hydrogen nucleus is just a single proton, carrying a (+) charge. If it gets close to another proton, the electrostatic force between the two (+) charges will repel them.

    So the closer you want them to get, the faster they have to be going (=have more kinetic energy). In a gas or plasma, higher kinetic energy of its constituent particles means higher temperature. If you then want the protons to get close together, you need to provide the initial extra energy by heating the plasma. You'll also want to compress it, so that the protons have a reasonable chance of encountering one another.

    In stars, the compression and initial heating up is done by gravity. In the experimental fusion reactors, it is achieved by application of magnetic fields. (as a note, proton-proton fusion is not what is actually used in reactors, but for the sake of the argument let's keep talking about p-p)
    In fusion bombs, it is achieved by detonating a fission bomb first, which momentarily creates conditions for fusion to occur.

    Once you have the hot protons get close enough, where the strong nuclear force overcomes the electrostatic repulsion, the two protons snap together (and one of them decays into a neutron - but let's ignore it here). This snapping together releases potential energy of the strong force field, much in the same way as dropping water from a height releases gravitational potential energy in hydroelectric power plants.
    Since the strong force is so much stronger than the electrostatic force, the energy released is much higher than the initial investment in kinetic energy, and there is nett energy production.

    The problem with controlled fusion in reactors is keeping the whole system of confining the hot plasma and extracting the produced useful heat in such a way so as not to waste more energy than the nett production from fusion. In a star, gravity does all the work for you. But gravity is just a way of getting the conditions right - not a requirement.

    Fission happens as a result of the same interplay between the electrostatic repulsion of protons and the strong force holding the nucleons together. If you have a very large nucleus, there's a lot of protons in it = strong repulsion. At the same time, the size starts to skirt the limited range of the strong force.
    If you then bombard the nucleus with an extra neutron (being electrically neutral, it doesn't have to be very fast to get close), it'll put the nucleus over the size limit, and the repulsion wins, making it more energetically favourable for the nucleus to split = splitting releases energy.
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