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Faraday's Ice-pail.

  1. Nov 29, 2009 #1
    In Faraday's ice pail experiment, a positive charged ball is lowered into an ice pail without contact. A charge is registered on the electroscope attached to the outer surface. Next, the ball is allowed to contact the bucket in which the electroscope still registered the same charge. But upon removing the ball, the charge has been completely neutralized.

    I have always thought that charging by contact would never completely neutralize a charge, only leave each object with a partial charge. How is the ice pail able to completely drain the ball's charge. I think it has something to do with charges residing on the outer surface of a conductor but I'm not completely sure.
     
  2. jcsd
  3. Nov 29, 2009 #2
    Once the charged ball is lowered into the ice pail, there are electric fields set up inside the pail between the positive charged ball and an opposite but equal charge on the inside of the pail (it is a capacitor). There may or may not be charges on the outside of the pail (doesn't matter). As soon as contact is made between the charged ball and the pail, the electric fields between the charged ball and the ice pail will be discharged, IF both the charged ball and the ice pail are conducting. There is no charge left on the ball. The capacitor is completely discharged.
    Bob S
     
  4. Nov 29, 2009 #3
    So how does this differ from normal charging by contact? Say for example instead of the pail a normal conducting plate is used. When the ball is brought near the plate the same charge separation is induced with the side closer to the ball becoming negative and the side further becoming positive. Touching the ball to this plate wouldn't completely remove the ball's charge would it?
     
  5. Nov 29, 2009 #4
    Good point. If a normal conducting plate is used, and it has a voltage on it relative to its surroundings, then there is a charge density on it. If you touch a conducting ball to it, some charge will move to the ball, along with field lines, so there will now be same-sign charge on the ball. If the ball is pulled away from the plate (or it could push itself away), there will be same-sign charge left on the ball.

    I once saw the following experiment. A ping pong ball was painted with a conducting paint. It was dangled from a support by a thread using a spot of glue. It was placed between two vertical charged conducting plates at several kV (maybe 5 or 10 kV dc) potential between them. The gap was maybe 5 - 7 cm. The ping pong ball pinged and ponged back and forth between the two plates- It was repelled as soon as it made contact, because the charge on the ball and on the contact plate was the same sign.
    Bob S
     
  6. Nov 29, 2009 #5
    My old college physics textbook illustrates the experiment as a cork ball, with positive charge, lowered on a silk thread into a conducting can, with the can sitting atop an isolator stand.

    Before the cork ball touches the bottom or any part of the can, the inside can surface takes a negative charge (as Bob describes) and the outside can surface takes a positive charge equal to the charge on the ball.

    This is explained by the free charge (electrons) in the conductor rapidly moving to the inside surface in the presence of the positivley charged ball.

    The same electrons vacate sites of metal atoms near the outside surface of the can, leaving a net positive charge at the outside can surface.

    When you touch the ball to the can, the ball takes a neutral charge, which means it must take electrons from the inner can surface via a transfer mechanism. It's atoms are complete.

    Remove the ball and keep the can isolated. The can now has a positive charge on the outside surface. If you "ground" the can it will take electrons from the earth and become neutral.

    Although informal, I think this is best explained by actual electron transfer. The ball gave up electrons to take a positive charge, and it accepted electrons to become neutral. Metals (good conductors) easily lose electrons to materials that tend to attract electrons more forcefully in the outer electron shells.

    By the way, electron transfer and transport is a huge area of research in nanotechnology and may potentially solve problems with providing clean alternative energy.
     
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