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Featured I Are charged batteries heavier?

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  1. Feb 27, 2018 #1
    Since you can add energy to a battery then extract it later, considering mass energy equivalence, it should be more massive, no?
    Somehow that doesn't make sense though.
     
    Last edited: Feb 27, 2018
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  3. Feb 27, 2018 #2

    phyzguy

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    It is more massive. Let's calculate how much. The battery on my laptop stores about 50 W-h = 180,000 Joules. Δm = ΔE/c^2 ~= 2 picograms. Hard to measure!
     
  4. Feb 27, 2018 #3
    Thanks and yes it would be hard to measure but not impossible.
    I expect somebody will be able to confirm that this has been checked out.
     
  5. Feb 27, 2018 #4

    phyzguy

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    Of course mass-energy equivalence has been checked in many different ways. Here's one for example. But I don't think there is any scale accurate enough to measure the mass difference between a charged and uncharged battery.
     
  6. Feb 27, 2018 #5
    Interesting link.
    I heard also about the planes flying in different directions as well, although I think that is GR rather than SR.
    Back to batteries though, if they become more massive when charged, even by an immeasurably small amount, where is the extra mass?
    Do electrons get more massive?
     
  7. Feb 27, 2018 #6

    Nugatory

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    Are you thinking of Hafele-Keating? https://en.wikipedia.org/wiki/Hafele–Keating_experiment
    The battery as a whole is more massive; you can't assign the extra mass to any one part of it. The mass of the charged battery is greater than the sum of the masses of its constituent parts.
     
  8. Feb 27, 2018 #7
    1 Yes that is the experiment I heard of.

    2 Aggregated mass of constituents is something I need to think about,.
     
  9. Feb 27, 2018 #8

    Dale

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    The molecules in a charged battery are different than the molecules in an uncharged battery. The ones in the charged battery are more massive than the ones in he uncharged battery. You cannot assign the mass to any part of the molecules, just the molecules as a whole.
     
  10. Feb 27, 2018 #9
    How?
     
  11. Feb 27, 2018 #10

    Dale

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    By being in a configuration with more potential energy.
     
  12. Feb 27, 2018 #11
    OK I get the idea.
    But what exactly is configuration?
    Something to do with Higgs field?

    I am puzzled, but hey that's why I am here,
     
  13. Feb 27, 2018 #12

    Dale

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    No. For a chemical reaction it is just electromagnetic. The Higgs is not relevant.

    Configuration means the various distances between nuclei and electron orbitals etc. That is what gives chemicals their energy.
     
  14. Feb 28, 2018 #13

    DrClaude

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    Slightly off-topic comment: this is something that irks me about many sci-fi stories. Some devices, such as weapons, have an incredibly big energy supply, but are still lightweight. Not possible, ##E=mc^2## rules!
     
  15. Feb 28, 2018 #14

    CWatters

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    Not sure if it's possible to weigh something like a smartphone battery to within 2 pico grams? I know you can weigh much smaller things more accurately but i don't think the method used for that can be applied to something big like a battery.

    According to Wikipedia the international standard kilogrammes gains around about 1 micro gram a month by absorbing contaminants from the air despite being under two nested bell jars. That works out at about about 33 Pico grams a day.
     
  16. Feb 28, 2018 #15

    mjc123

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    An exception would be the lithium-air battery, which uses oxygen from the atmosphere to oxidise lithium in the discharge reaction:
    2Li + O2 → Li2O2
    As the oxygen comes from the atmosphere on discharge and is released to it on charge, it is not part of the battery, which therefore weighs more in the discharged state than the charged state.
     
  17. Feb 28, 2018 #16

    CWatters

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    I did wonder is some sort of balance could be created using an ink jet cartridge....

    Ink jet cartridges create pico litre droplets of ink so I was thinking that perhaps you could use one to squirt ink onto a counter weight until it was in balance with a discharged battery. Then charge the battery using a wireless charger and count how many additional drops are required to bring it back into balance again.

    Then I realised that the tiny drops produced by an ink jet printer would be three orders of magnitude too big! (pico Liters >> pico grams)
     
  18. Feb 28, 2018 #17

    Andy Resnick

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    Minor quibble (since I'm an experimentalist): I think you mean 'precise', not 'accurate'. This measurement, for example, requires about 14.5 significant figures (to reliably measure 1 pg changes in a 100g battery). I don't know any existing balance that can do this (maybe the Watt balance). The standard kilgoram calibration campaign quotes about 11 digits (1 ug/kg) using one of these:

    https://www.mt.com/dam/P5/labtec/08...tion/03_Datasheet/DS_Vacuum_M_one_M_10_EN.pdf
     
  19. Feb 28, 2018 #18

    256bits

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    For absolute mass measurement.
    But what about mass difference.
    Using a Cavendish torsion balance, the period of oscillation should be comparable to the square root of the difference in mass, or rather 10-6 seconds, and that 'should be' fairly easy to measure with a current time clock.
    Just wondering.
     
  20. Feb 28, 2018 #19

    Khashishi

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    Check out https://en.wikipedia.org/wiki/Binding_energy

    The binding energy can sometimes be interpreted as a reduction of field energy. For example, an electron and a proton are bound as a hydrogen atom. The electric field has energy ##\frac{1}{2}\epsilon E^2##. When the electron and proton are close together, the E fields overlap and mostly cancel out, reducing the field energy. By rearranging the atoms in matter, you can change the energy even though the number of atoms is equal.
     
  21. Feb 28, 2018 #20

    Andy Resnick

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    Why go through the effort? The uncertainty/statistical spread of a discharged battery's mass is likely many times larger than a pg.

    Edit- I just realized, how did you get 10-6 seconds fractional difference for the oscillation period?
     
    Last edited: Feb 28, 2018
  22. Feb 28, 2018 #21

    CWatters

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    Why is that a problem? Wouldn't you would measure the frequency of oscillation of _a_ battery while it was being discharged looking for the change in mass.
     
  23. Feb 28, 2018 #22

    Andy Resnick

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    How would you discharge it while in the device? How much of an effect would you expect to see?
     
  24. Feb 28, 2018 #23

    DaveC426913

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    So, to bring this back to the OP's question:
    the inflow of electricity, in the form of electrons, causes molecules to change shape, and some of the electrons' orbital energy is converted back to mass ... by way of muons?

    Or am I word salading here?
     
  25. Feb 28, 2018 #24
    See a battery on table, put a table mat underneath the battery and wa la, battery weighs more. Not a lot more, I'll give you that!
     
  26. Feb 28, 2018 #25

    phyzguy

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    If you think this measurement is feasible, why don't you go ahead and give it a try? I concur with Andy Resnick. No measurement technique exists that can measure a change in mass of 1 part in 10^14.
     
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