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Van de Graaf generator?

  1. Oct 8, 2006 #1
    Hi, I'm still confused on how a Van de Graaf generator works

    We did a lab involving it and my observation is as listed:

    Rubber band: the rubber band would stick for a short while, but then would bounce off momentarily, and would repeat this process.

    Paper clip with string: The paper clip seem to repell when near the generator. Part of the string would stick to the generator, but some part of the string also seem to repell and float in air.

    Plastic ruler: no change

    Here's what I think is happening. The van de graf creates a huge electric field with points away from the generator. Induction would take place. Materials that are conductors allow the electrons to move around inside the material, which allow them to react with the generator. Non conductors such as the plastic ruler does not react.

    Do I have the right idea?

    Can someone guide me through this?
     
  2. jcsd
  3. Oct 8, 2006 #2

    OlderDan

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    Non-conductors like rubber bands do not have free electrons that move around, but many of them do have mollecules that are or can be "polarized." Think of the rubber band as being full of little barbells that are positive on one end and negative on the other end that are free to rotate around their midpoint, but not move within the rubber band. These are called "dipoles." When the rubber band gets close to the generator, what will happen to these dipoles? Which charges get closer to the generator? What effect will that have on the rubber band? If the band touches the generator, it can get excess charges from the generator on its surface. This is like the static electricity you can add to a balloon by rubbing it on a cloth. It is also what is going on inside the generator on the moving belt. It picks up charge at the bottom, and deposits that charge to metal points inside the sphere. That charge then flows to the surface of the shpere. What will happen to the rubber band when it has the same kind of net charge as the sphere?

    The bouncing you observe probably has to do with where the string was supported. Gravity was strong enough to pull the rubber band back after a bounce. Eventually, the rubber band should stay away from the sphere.

    Your observation of the paper clip is incomplete. If it were initially neutral, the conduction charges should have rearranged themselves within the clip to make the clip like one big dipole. It would be attracted to the sphere until it touched and gained extra charge. It would be repelled once enough extra charge was added. If you can, try to repeat the experiment without allowing the string to touch the sphere.
     
  4. Oct 8, 2006 #3
    I think I understand a bit better how it works. But whats bugging me is the transfer of charge to the material. Can you explain how that works?
     
  5. Oct 8, 2006 #4

    OlderDan

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    In chemistry we talk about various kinds of bonds that hold atoms together to form compounds. One such bond is called an ionic bond. It is the bond that holds NaCl (salt) together. Basically what happens is that when a Na atom gets close to a Cl atom, the Cl nucleus has a stronger attraction to the outer electron of Na than the Na nucleus and steals it away. The result is you have Na+ ions and Cl- ions that arrange themselves into a nice orderly array of alternating positive and negative ions. A similar thing happens when a dielectric touches your highly charged sphere. Either the sphere pulls some charge off the ends of some of the dipoles, or the dipoles pull some excess charge off the sphere.

    For metals it is even easier to transfer charge. The sphere has an excess charge of one kind because it is very easy to pull the conduction electrons out of a metal or add excess electrons to it. If one charged piece of metal touches a neutral piece of metal, the excess charges get distributed across both objects. Both objects wind up with the same type of excess charge; if nothing is holding the objects together, they will separate becuase of the repuldive forces between like charges.
     
  6. Oct 8, 2006 #5
    Oh, ok. Just to make sure,
    to sum it up, that repelling force that i saw was because the electrons from the paper clip is being transfered to the van de graf. Then that electron is removed from the van de graf and eventually, there will be more proton repelsion than electron/proton attraction?

    The paper clip would become neutral due to the air eventually?

    and the same concept applies to everything that is repelled?

    Oh and I forgot one item, I dont really know what its called but its the thing used in packages so that they dont get damaged. Its white and kinda fluffy, very soft. When we threw some of that on the generator, it stuck. Why is that?
     
    Last edited: Oct 8, 2006
  7. Oct 8, 2006 #6

    OlderDan

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    Yes Yes and Yes. I think you've got it. The white fluffy stuff is styrofoam. It is a non-conductor. The fact that it sticks to the sphere indicates that some polarization is taking place, but the styrofoam is not transferring charge to the sphere or vice versa. There are lots of different insulating materials and they behave differently with regard to polarization and acquiring excess charge.
     
  8. Oct 8, 2006 #7
    Ohh ok I think I thought it now. THXXX SOO MUch. You've been a great help.
     
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