Faraday's Ice-Pail Exp: Complete Neutralization of Charge

• Yuqing
In summary, the ball was repelled from the plates because the charge on the ball and the contact plate was the same sign.
Yuqing
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.

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

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?

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

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.

1. What is Faraday's Ice-Pail Experiment?

Faraday's Ice-Pail Experiment is an experimental demonstration conducted by Michael Faraday in the early 19th century to study the phenomenon of electrostatic induction.

2. How does Faraday's Ice-Pail Experiment work?

The experiment involves using a metal pail filled with ice to insulate a charged metal sphere from the outside environment. The metal sphere is connected to a charged electroscope. As the ice melts, the metal pail becomes wet and conducts electricity, causing the charge on the metal sphere to gradually decrease until it is completely neutralized.

3. What is the significance of Faraday's Ice-Pail Experiment?

The experiment demonstrated that the charge on a conductor can be completely neutralized by surrounding it with an insulating material, such as ice. This helped to disprove the popular theory at the time that charged objects could hold an unlimited amount of charge.

4. How is Faraday's Ice-Pail Experiment relevant today?

The experiment is still relevant today as it helped to lay the foundation for our understanding of electrostatics and the principles of charge neutralization. It is also used as a demonstration in physics classrooms to illustrate the concept of electrostatic induction.

5. Are there any real-world applications of Faraday's Ice-Pail Experiment?

The principles behind Faraday's Ice-Pail Experiment are used in various technologies, such as electrostatic precipitators used in air pollution control, and in the design of lightning rods to protect buildings from lightning strikes.

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