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A weird sentence in my book Need clarifcation

  1. Jun 14, 2012 #1
    It is written in my book under the sub-heading Nuclear Fusion... "If we take some light nuclei and force them to join together, the mass of the new HEAVIER nucleus will be LESS than the consistuent parts, as some mass is converted into energy... This is completely contradicting... Is it actually heavier or lighter... If any of them , why have they written both???
  2. jcsd
  3. Jun 14, 2012 #2


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    hi ehabmozart! :smile:

    i don't see the contradiction :confused:

    if two H nuclei fuse to make an He nucleus,

    then the mass of the He is less than twice the mass of an H …

    the "missing" mass has become energy, which is radiated off :smile:
  4. Jun 14, 2012 #3


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    As tiny-tim said, the mass of the new particle after fusion will be less than the combined masses of the particles before fusion. The missing mass has been liberated as energy from the reaction. The amount of energy released depends on the amount of nuclear binding energy of the particles before and after fusion. http://en.wikipedia.org/wiki/Nuclear_binding_energy
  5. Jun 14, 2012 #4


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    We often refer to some elements as being heavier or lighter than others, when what we mean is that a SINGLE nucleus of one element will be heavier than a SINGLE nucleus of the other.

    Using that definition, we say that fusion combines multiple light nuclei into a single heavier one (just as fission splits a single heavier nucleus into multiple lighter ones). And it's possible that the sum of the masses of all the lighter nuclei we're fusing together will exceed the mass of the single heavier nucleus that results - it's heavier than any one of the constituents but not all of them.
  6. Jun 14, 2012 #5


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    E = mc^2 The excess mass is released as energy. It's what makes a H-bomb a bomb.

    It's the power in nuclear fusion.

    The new heavier nucleus doesn't stay heavy for very long - it releases the excess mass as energy - think of a blinding flash and a mushroom cloud.
  7. Jun 14, 2012 #6
    The new nucleus is much heavier than the individual light nucleii that were initially separate. However, The new nucleus is lighter than the group of light nucleii. In other words:
    Let M_Fused be the mass of the new nucleus after fusion.
    Let m_1, m_2,..., m_N be the masses of the light nucleii before fusion.
    M_Fused>m_i for any value of i between 1 and N.
    Of course, the amount of energy, E_Fusion, given off by the fusion is:

  8. Jun 15, 2012 #7
    Well, that one seems absolutely correct, but you may well find a sentence or two sometime,somewhere that isn't. So it's a good idea to ask, even classmates, because most texts are edited by experts to eliminate any errors and clarify explanations to make them as clear as possible. On the other other hand, some correct explanations ARE,well, weird. See my signature below.
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