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ArrowFaraday's Ice Pail

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Michael Faraday is considered by some historians as the greatest experimentalist in the history of science. It was largely due to his efforts that electricity became a viable technology. Presented in the tutorial is Faraday's ice pail, one of his many groundbreaking experiments exploring the nature of electricity.



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This experiment illustrates the distribution of charge over a metal conductor and principles of electrostatic induction. A metal sphere is charged with some kind of electrostatic generator, giving it a net negative charge. The charge level slider controls the intensity of charge on the metal sphere. The metal cup, or pail – here made of copper – has charges that are evenly distributed (the negative electrons are the yellow particles, the positive particles are denoted by red plus signs), making it electrically neutral.

When the sphere is lowered into a cup (the ice pail) without touching the sides (use the sphere position slider), the negative charge on the surface of the sphere repels free electrons in the copper to the outside surface of the cup. Because opposite charges attract, the inside surface of the metal cup becomes positive. The surplus of free electrons on the outside flows down a wire that connects the cup to an electroscope. This was an early instrument for measuring electrical charge; the farther the electroscope's needle is displaced, the greater the charge. This shows that an induced charge (in the electrocsope) has the same magnitude as the inducing charge (in the metal sphere).

If the metal sphere were touched to the side of the metal cup (not possible in this tutorial), the sphere and the inside of the cup would exchange charges and end up both electrically neutral. The outside of the pail, however, would retain the same charge, and removing the sphere from the pail would then have no effect on the electroscope.

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