Physics Lab Activity Answers

Electrostatics - 3

Induction & Polarization

Part 1 - Charging by Induction

You can tell that the electroscope has an electric charge by observing that the vane is repelled from the support inside the electroscope.
The charge on the electroscope is opposite the charge on the charged strip, since when the strip is brought close to the top of the electroscope (but not touching), the vane collapses toward the support.
(1) When the electroscope is initially grounded, any excess electrons on the electroscope can escape to ground, or any electrons that would like to get onto the electroscope can do so. Therefore, the electroscope is neutral after grounding.
(2) Bringing a charged strip near the electroscope polarizes it. If the strip has a negative charge, the electrons in the electroscope are repelled toward the bottom of the instrument. This creates a region of positive charge near the top of the electroscope and a region of negative charge near the bottom - although the electroscope, as a whole, is still neutral. The vane separates from the support since they both have a net negative charge (they are at the bottom end of the electroscope), and like charges repel. If the charged strip is positive, the argument is similar.
(3) When the electroscope is grounded, Any electrons that want to get away from a negatively charged strip. or any electrons that would like to get closer to a positively charged strip, can travel to and from the electroscope.
(4) When you remove your finger, any excess electrons that were attracted to a postive strip are now marooned on the electroscope - giving it an net negative charge. If electrons fled the electroscope to excape a negatively charged strip, they can't get back - giving the electroscope a net positive charge.

Part 2 - Electrical Polarization

The (neutral) pop can is attracted to the charged strip. Suppose the strip has a negative charge. Electrons in the pop can will be repelled to the opposite side of the can from the charged strip. This will leave the side of the can nearest the strip with fewer electrons than normal, and the side farthest from the can with more electrons than normal. The pop can is still neutral, but it is polarized. Since the electric force depends on the distance between charges, even though there are the same number of positive and negative charges in the pop can the positive charges are closer. This means that the attractive force due to the positive charges in the pop can will be greater than the repulsive forces due to the negative charges in the can. There will be a net force on the (neutral) can which will pull it toward the charged strip.
Just as with the pop can, the neutral pith ball is attracted to the charged strip. The charged strip polarizes the pith ball, attracting its unlike charges and repelling its like charges.. Since the unlike charges end up on the near side of the pith ball, their attraction force toward the charged strip is greater than the repulsion force away from the charged strip due to the like charges. There are the same number of positive and negative charges in the pith ball, but the unlike charges are closer.
Charged styrofoam "peanuts" or a charged balloon stick to a wall (or you!) becauss the charged object (peanut or balloon) polarizes the wall (or you) so that the attractive force between the unlike, nearby charges is greater than the repulsive force between the farther away, like charges. Even though there are the same number of positive and negative charges in the wall (or you), there is a net attractive force toward the charged object!

last update February 21, 2001 by JL Stanbrough