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Mass of energy

  1. Jul 12, 2007 #1
    Is a fully charged battery heavier than a discharged one? Would any difference in weight be measurable using ordinary lab equipment, such as a balance or an electronic scale?

  2. jcsd
  3. Jul 12, 2007 #2


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    I can't imagine there being any difference with a sealed unit. The electrons that leave from one end come back in the other. A vented battery such as the older style automotive ones gets marginally lighter because hydrogen escapes during operation, as well as some evaporation of the water/acid mixture.
  4. Jul 12, 2007 #3
    Technically, I think one should be able to argue that the work done by the battery in discharging reduces the total energy, which in turn reduces the total mass. But we are really splitting hairs here since this quantity of energy is so extremely small. I would not be surprised if it was unmeasurable with any kind of equipment
  5. Jul 13, 2007 #4


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    The energy in this case is an excess of electrons at the negative pole and a lack of electrons at the positive pole. I don't know if the energy states of this distribution of electrons creates more mass than a discharged battery where there's no excess or lack of electrons at either pole. I assume "charge" has no mass.
  6. Jul 13, 2007 #5
    Thanks for the replies. I was in fact imagining a hypothetical sealed unit. I guess we all agree there are no more electrons in a charged cell than a discharged one? The question, then, I think, is answered pretty well. I would like some mentor confirmation to be sure. But it seems to me that the seperated charges are an energy which should warp space, at least a little, and so be an effective mass, by Einstein's equation. I guess the mass in any real cell would be extremely small.

    Thanks for the comments.
  7. Jul 13, 2007 #6
    Hmmm. Is this technically correct?
  8. Jul 13, 2007 #7


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    No, it isn't; I was just simplifying to say that there is no net loss of electrons. They don't actually travel around the whole circuit. Sorry for the sloppiness. :redface:
  9. Jul 13, 2007 #8


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    I do not think that chemical reactions are accompanied with a change in mass. Essentially what Danger said is right, the excess of electrons at the negative terminal are transfered to the positive terminal, but they start in a chemical bond and end in a different chemical bond. Any change in mass would show up in the difference between the initial and final bond energies. Not clear to me that this exists, it certainly is not measurable with standard lab equipment.
  10. Jul 14, 2007 #9

    Let W(0) be the energy of the charged battery and W its energy after a given time of functioning. In accordance with the mass energy relationship the battery undergoes a change in mass DM=(W(0)-W)/cc
    independent of the way in which the energy goes lost. Of course the measurement of that change is not possible at the actual technical level.
  11. Jul 14, 2007 #10
    There's energy 'in' the electron configurations of the reagents in the battery. The reaction that produces a current must be releasing some energy to drive this current since the circuit has resistance. Therefore, the energy released by the battery over an operating period t is P*t where P is the mean power of the circuit.

    This energy really has been 'lost' from the reagents - and specifically, it's lost from the internal energy of the electron configurations of the molecules that liberate charge. So yes, in some absolutely miniscule way, a battery will lose mass-energy as it's depleted.
  12. Jul 17, 2007 #11
    Thanks everyone. I learned some things. 1. the energy in a battery is not from additonal stored electrons, as I had previously imagined. 2. potential energy from seperated charges results in a very small but calculable difference in mass, in accordance with the Einstein field equations.

    On to the next thing.
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