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Thread: How can a grounded conductor repel a charged object?

  1. #1 How can a grounded conductor repel a charged object? 
    nuk
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    I've done this demonstration for years with my HS physics classes, and this was the first class I stood there, looking stupid, while the demo thumbed its nose at me.

    I stick a piece of scotch tape on a table, and then put another piece of tape on top of the first. I peel the top piece off, and then the bottom. This usually gives the two pieces opposite charges, but sometimes one is more or less neutral.
    I stick the two charged pieces on a support, so that they dangle vertically, separated from each other. Then I rub pieces of glass and plastic on different fabrics to charge them, and show the students that each of those will attract one piece of tape and repel the other. So when things go well, I show that the glass and plastic have opposite charges, and the two pieces of tape are also oppositely charged.
    Then I rub my finger in my beard, and show them that my finger will attract both pieces of tape, and make a joke about my finger having "imaginary" charge.

    But in one of my classes today, my finger repelled one of the tapes. Then I took a steel rod and it repelled the tape. Then I grounded myself, and the steel rod still repelled the tape.

    How could this happen? If the tape was charged, I expect it to attract the opposite charge on the conductor and have a net attraction. If the tape is uncharged, I expect no effect.


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  3. #2  
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    maybe your table top had a bias which gave the tapes a bias charge too.


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    Quote Originally Posted by nuk View Post
    I've done this demonstration for years with my HS physics classes, and this was the first class I stood there, looking stupid, while the demo thumbed its nose at me.

    I stick a piece of scotch tape on a table, and then put another piece of tape on top of the first. I peel the top piece off, and then the bottom. This usually gives the two pieces opposite charges, but sometimes one is more or less neutral.
    I stick the two charged pieces on a support, so that they dangle vertically, separated from each other. Then I rub pieces of glass and plastic on different fabrics to charge them, and show the students that each of those will attract one piece of tape and repel the other. So when things go well, I show that the glass and plastic have opposite charges, and the two pieces of tape are also oppositely charged.
    Then I rub my finger in my beard, and show them that my finger will attract both pieces of tape, and make a joke about my finger having "imaginary" charge.

    But in one of my classes today, my finger repelled one of the tapes. Then I took a steel rod and it repelled the tape. Then I grounded myself, and the steel rod still repelled the tape.

    How could this happen? If the tape was charged, I expect it to attract the opposite charge on the conductor and have a net attraction. If the tape is uncharged, I expect no effect.
    Were there thunder clouds forming?
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  5. #4  
    nuk
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    Quote Originally Posted by dan hunter View Post
    maybe your table top had a bias which gave the tapes a bias charge too.
    How would that affect it? A charge on the tape should induce a charge separation on the conductor, and lead to attraction.

    There may have been a storm rolling in at the time - how would that matter?
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  6. #5  
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    If there was a storm in the area the whole area under the storm can become highly charged.
    The end of a pointy conductor just concentrates the charge.

    If the area was charged the table top was charged and both strips end up carrying the same charge just to different degrees.
    The electric field of the ground under a lightning storm is usually + while the cloud's field is -.

    These fields can be present under other conditions too, it does not need to be just thunderstorms.

    http://en.wikipedia.org/wiki/Atmospheric_electricity
    Last edited by dan hunter; March 3rd, 2014 at 09:25 PM.
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  7. #6  
    nuk
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    Best idea I've heard. Thanks, Dan.

    Now I'm going to have to wait for another rare storm in the desert to see if I can duplicate this.
    dan hunter likes this.
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    Hey, I am just guessing. Don't go getting zapped by a bolt out of the blue.
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    Quote Originally Posted by nuk View Post
    Then I grounded myself, and the steel rod still repelled the tape.
    This can happen only ONE way: you did not ground yourself correctly.
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    Nuk, you have not said how you got ground.

    As a former ARRL member (radio "ham"), I can tell you that especially in the desert unusual electrical phenomena are more common due to the dryness and consequent low conductivity of the air. I knew fellow enthusiasts from Arizona and Texas and New Mexico who had to sink holes in their yards ten feet or more, then emplace a copper rod and salt the earth with copper sulfate as they put it back in the hole and tamped it down, then water heck out of it for a month. All this just to get a good enough ground for a 2kW amateur station doing, say, 10m WAC.

    ETA: In fact I'd be a little careful to get good ground and know what you're messing with before you, say, touch it to your tongue. You could wake up in an ambulance. Or not wake up. Be cautious. 150 mA will turn off your pacemaker muscle. 200 mA will burn your finger. 100 mA will make you feel a little shaky. If your pacemaker stops you are fibrillating. It's a narrow window but it's decisive. I never touch anything I haven't put a meter to first, personally.

    You sound careful and wise but I wouldn't want someone else to get in trouble.

    OTOH I was the one drawing one foot sparks off peoples' bedsprings with a tesla coil, which I absolutely guarantee with wake you up straight out of a sound sleep and two feet off the mattress. So never mind me.
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  11. #10  
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    I'm doing this indoors, and I didn't take particular care with the ground. I have a whiteboard bolted to a wall, and I held on to its aluminum tray. It's served well as a ground when I was discharging the Van de Graaf, but that may just be because it's large, and not because it's well connected to the earth.
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    There's your answer.

    But if you don't have good ground then if you start using a good one make sure you use a meter too so you don't get zapped too hard.

    I wouldn't trust the aluminum edges of a whiteboard; it's likely bolted to two-by-fours, not anything grounded. You're almost certainly correct in your conjecture, it's acting as a charge reservoir and then the charge slowly bleeds off as it will over time, into the environment.

    Just be careful; we are already short of science teachers, we don't need to lose any!
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  13. #12  
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    Hi I'm at Lcc science student currently and wanted to know what is the benefit of such research for students?
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