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I am preparing to teach Grade 9 Static Electricity next week and am going crazy trying to figure out what is happening in one of my experiments. I have a short piece of PVC pipe, 4 inches diameter, and I rub it with wool to charge it negatively. I can observe excellent repulsion when I touch it with my foil bit (dangling from a thread). Here is the problem: I am holding down the PVC pipe down on a wooden base with two brass-plated wood screws, and these screws somehow collect an INSANE amount of positive charge, even when I am very careful not to touch them with the wool. The foil bit is strongly attracted to the screws, and when it touches them it bounces off more violently than anything I've seen in any of my other static electricity experiments.

Can anybody explain what I am seeing?

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I would guess that the screws are secretly grounded through a hidden conducting path inside the wood block and the table. Maybe the table is wet, maybe there are metal wires inside the block, I don't know the exact setup, but I think there is some grounding path. This means that they stay at zero potential, no matter what you do to the PVC. So when you charge up the PVC tube, the screws accumulate a large amount of charge to keep their potential zero in the tube background.

The induced charge on grounded screws is much larger than the induced polarization on isolated screws, and the pointiness of the screw will lead to huge local fields near them, and will explain your observation. If this is the case, putting big books beneath the block should de-ground the screws.

For this explanation to work, the screws don't have to be completely grounded. It is enough if you have the screws go all the way through the wood, and rest the block so that the screws touch a metal table. In this case, the metal will act as a not-so-infinite ground, and will supply charge to the semi-grounded screws. You can fix this by mounting the block on top of a dry book.

To test any of these explanations, you can just hold up the block yourself without touching the screws, and rub, and see if you still get the large charge on the screw.

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Ron, I'm going to have to give you credit for the correct explanation. It's not that there is a path to ground: I put a new hole in the PVC that just goes to dead air inside the pipe, and the effect is just as strong. When I bring a ground wire near my pipe, it works just like the screw: the crazy dancing around the wire and everything. It works the same if I ground the screw but I don't have to ground it. What I think is that when it comes to positive charge, the air is almost as good as a ground. I think the air has enough surplus positive charges that it effectively grounds the screw. –  Marty Green Nov 14 '11 at 7:04
    
But then this doesn't explain why the air doesn't just "ground" the whole PVC pipe. Still, this isn't the first experiment I've tried with static electricity where I seem to get positive charge showing up out of nowhere. I also tried the classic electrophorous, rubbing the styrofoam and putting down the metal plate (powder paint coated!): when I picked up the pie plate (by an insulator), the metal plate was full of positive charge! (It's supposed to be neutral until you ground it. But I didn't ground it.) –  Marty Green Nov 14 '11 at 7:23
    
That doesn't make sense... air is not full of unbalanced positive charges floating around, and it's generally a great insulator, which makes for a very bad grounding connection. –  David Z Nov 14 '11 at 8:29
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@David: It is possible that the screws have small metal outjuttings that ionize the air, and allow charges to flow to ground--- this seems implausible, but perhaps this is the grounding path. The PVC is not conducting, and filaments dangling off the PVC will not generate huge fields. You could test this with a smooth metal ball resting in the PVC hole, it should not lead to ionization if it is smooth. –  Ron Maimon Nov 14 '11 at 9:02
    
This just got downvoted! That's a surprise. –  Ron Maimon Nov 14 '11 at 17:45
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Great question. If you can rule out magnetism (based on the material of the foil and the material of the screw under the brass plating, or by demonstrating that there is no force on the foil when the PVC pipe is not charged), you are seeing the effect of induced electric charge on the screws. They are electrically neutral overall, and when they are surrounded by the negative charge of the PVC pipe, electrons in the screw are repelled slightly inward from the surface, leaving positive charge at the surface. This is called polarization. The resulting electric forces are strongly concentrated near the screw, and since the foil is negatively charged, it is attracted to the screw.

Why does the foil bounce when it touches the screw? The electrons of the foil are attracted to the screw, and some of those electrons flow onto the screw at the contact point. You then have a slight positive foil, which is repelled by the screw.

You can test this explanation: The foil should be positively charged after it bounces from the screw, and then it should be attracted to the negatively charged pipe.

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Thanks, JxB, but I can't accept your answer for two reasons. First, I've seen attraction by induced charge and it's not as strong as direct attraction. This is a really strong force. And when it bounces away, it's also super strong, which is really not characteristic of induced charge. Finally, I just tried the pointy end of the screw inside the PVC pipe. Also positive. So the whole screw has the same charge. –  Marty Green Nov 13 '11 at 21:07
    
To your first objection, with induced charges, the forces depend not only on the amount of charge but also on the spatial variability of the force (the force is proportional to the gradient of the electric field). The smallness of the screw compared to the size of the pipe means there will be large forces present. –  JxB Nov 13 '11 at 21:38
    
To your second objection, if the inside of the PVC is negatively charged, the tip of the screw, being on the inside of the pipe, would be induced positively as well. You can test the charging of the inside of the pipe with your negatively charged foil. –  JxB Nov 13 '11 at 21:40
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In any case, it's a good idea not to accept answers right away anyway, to encourage answers from others first.Cheers, JB –  JxB Nov 13 '11 at 21:41
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Thanks, JB. I don't want to seem ungrateful, but it's AWFULLY strong to be just polarization. And I don't know of any kind of polarization where both ends of the screw have the same charge. I'm pretty sure the whole screw is positive. And yes, I checked the inside of the pipe and if there are charges on it, they don't create any forces. By the way, you previously had an upvote on your answer which is now missing; I just want you to know that it wasn't me who cancelled it. –  Marty Green Nov 13 '11 at 22:02
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