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Can charge be held in a vacuum?

  1. Aug 29, 2015 #1
    It's my understanding that matter holds charge on earth from the insulation of the atmosphere, since the atmosphere is a bad conductor and essentially blocks current due to the gaseous molecules' atomic structure and their covalent bonds. Does this mean it's impossible for matter to hold charge in a vacuum since the charges would repel one another with nothing to stop them from moving?
     
  2. jcsd
  3. Aug 29, 2015 #2

    BvU

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    Hello Steve, welcome to PF :smile: !

    You may have to adjust your understanding: the vacuum is an even better insulator than the atmosphere ! So something charged in space holds on to this charge quite happily! [edit] well, I'll have to withdraw that ! (See below) Good thread !
     
    Last edited: Aug 29, 2015
  4. Aug 29, 2015 #3
    Thanks for your response but it doesn't seem correct to me. How is a vacuum an insulator? What is stopping these charges from repelling each other off the atoms? The only net force present is the repulsion force between the charges. I know textbooks define a vacuum as an ideal insulator, but to me this has no logic to it, and parallel plate capacitors use a dielectric, not a vacuum as textbooks say, between the plates.
     
  5. Aug 29, 2015 #4

    BvU

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    You are right that in practice capacitors (do parallel plate capacitors still exist ? I haven't seen one for a while) use a dielectric, and with good reason: it increases their capacity and they are easier to produce because dielectrics are usually good insulators.

    And your sensible questioning my offhand statement forces me to think this through a bit better. Vacuum gets a zero for conductivity (there's nothing to do the conducting), but you put a good case.

    I agree that there's nothing stopping the charges and I remember there's currents flowing in vacuum tubes (things like transistors in the days when there were no transistors yet -- hehe transistors are microscopic parts of integrated circuits; they used to be actual components in the days when there were no integrated circuits yet). In the nineteenth centrury they discovered x rays and cathode rays with the grandparents of such tubes.

    Of course in those cases besides something repelling the charges there was something attracting the charges as well.

    So I'm speechless at this point ! I have to wonder what keeps charges on conductors, in vacuum (if at all) as well as in the atmosphere. I wonder how, in the atmosphere, this blocking mechanism you describe actiually works.
     
  6. Aug 29, 2015 #5

    mfb

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    The energy of the charges is not sufficient to leave the material (unless your material is really highly charged). In most materials, the highest filled energy state is well below the energy level of free electrons. You have to add energy (with light or heat, for example) to get electrons out of the material. High electric field strengths are another option, so pointy things emit electrons more easily.
    Getting positive charges out needs even more energy.

    This applies to air and vacuum in the same way.
     
  7. Aug 29, 2015 #6
    So it depends on a few things. If the extra electrons are used in metallic bonds, then I would assume they would discharge instantly in a vacuum. However if they are integrated into the atom's electron shell, it would require energy to take it out of the atom? Overall there is still a net force on these electrons so in a vacuum mathematically they should still repel. If I'm incorrect in my reasoning could you point out where I went wrong, or if this contradiction is just something that delves too deep into quantum mechanics to intuitively understand, but has been acknowledged and explained, let me know.
     
  8. Aug 29, 2015 #7

    mfb

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    No, their energy levels are below the energy needed to leave the metal.
     
  9. Aug 29, 2015 #8

    BvU

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    mfb is right, but how do we explain this convincingly without resorting to energy levels and quantum mechanics and all that...
     
  10. Aug 29, 2015 #9
    I appreciate everyone's responses. The way I'm seeing it is just that there is an overall net force repelling electrons from each other. It shakes my understanding of physics to think that this net force is just ignored. Is it that the electrons configure so the forces are at an equilibrium? And only at some parts of the atoms geometry is there a net electric force?
     
  11. Aug 29, 2015 #10

    mfb

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    It is not ignored, but the additional force from some charge imbalance can be weak relative to the attractive forces from the nuclei.
     
  12. Aug 29, 2015 #11
    I was going to say there shouldn't be any attractive forces from the nuclei if the atom is neutral, otherwise it wouldn't be neutral, but then I realized it's more complicated then that, especially when considering that the electrons form a cloud around and away from the nuclei rather than just sticking to the nuclei which raises a hundred more questions. Keeping this model of the atom in mind I can see how an electron can get trapped in the atom from the complicated structure of the electrons and electric field.
     
  13. Aug 30, 2015 #12

    BvU

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    I'm stilll intrigued by this issue. From the photoelectric effect it is known that it takes energy to knock an electron out of a metal surface. So that's why my dear old vacuum tubes need a voltage applied before a current can flow. If there's no voltage applied, only the thermal energy is available to build up an electron cloud that is in equilibrium with the cathode (hence the hot filament in practical tubes).

    Conversely there must be some energy gain from adding an electron to a neutral piece of metal. Can almost only be from the positive nuclei 'benefitting' from sharing electron charges, but I have hard time expressing that simply and quantitatively without a dose of quantum mechanics. Anyone ?

    Like Steve I now have questions. Good :smile:, so thanks for bringing this up and not taking everything for granted!
     
  14. Aug 30, 2015 #13
    It is the nature of the bonding that allows charge to accumulate. A metallic bond can be described as a lattice of positively charged nuclei ( not the full charge as the shielding of the nucleus by the inner electrons ) surrounded by a sea of electrons. While pictures in books show this as a static state ( how else ) the sea is in continual motion with electrons moving about. Add a few more electrons amongst billions and billions and the movie doesn't change much, except now the whole lattice has a negative charge, A particular nucleus doesn't distinguish whether the electron was an original or an inserted.

    Compare that with trying to insert electrons within a lattice formed from ionic or covalent bonds where the electrons are strongly shared so to speak ( very brief summation ). The addition of an electron to the lattice leaves it with few choices to take advantge of an attraction to a nucleus, as the original electrons there have already done so.
     
  15. Aug 30, 2015 #14

    CWatters

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    I can think of at least four different possible scenarios that might amount to the storage of electrons in a vacuum...

    1) A cloud of free electrons in a vacuum (don't ask how they got there). They would repel away from each other (but you could make the vacuum physically very big so they stay in there somewhere.)

    2) An uncharged lump of metal. If the metal is hot enough electrons will jump out of the metal into the space around it by Thermionic Emission (https://en.wikipedia.org/wiki/Thermionic_emission) leaving the metal with a +ve charge that attracts them back again. I suppose you could say electrons are temporarily stored in the vacuum but overall the system has no net surplus of electrons. So does it count?

    3) A charged lump of metal. If the metal is cold enough then no electrons are lost through Thermionic Emission. However you might loose some by Field Emission (https://en.wikipedia.org/wiki/Field_electron_emission).

    4) A set up like a Diode Valve with a hot cathode and an anode and a power supply. Electrons leaving the cathode (see 2 above) will zip through the vacuum to the anode and can be recirculated by the battery. For a short time they are "stored" in the vacuum between the cathode and anode.
     
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