Physics Lab Activity Answers

Electrostatics - 2

The Electroscope

The electroscope vane moves away from the support.
You can tell that the electroscope is charged because the vane separates from the support. It does that because like charges repel. If the electroscope got extra electrons from the charged strip, then both the vane and the support have a negative charge. If the electroscope lost electrons to the charged strip, then both the vane and the support have a positive charge.
If you bring the charged strip near the top of the charged electroscope, the vane separates further from the support. When the charged strip is removed, the vane goes back to where it was.

Suppose the strip and the electroscope have a negative charge. The vane of the electroscope separates from the support because there is a repulsive force between the excess electrons on the vane and the excess electrons on the support. When the negatively charged strip is brought near the top of the electroscope, it repels the electrons in the top of the electroscope more than the electrons in the bottom, because the electric force depends on distance. Therefore, additional electrons will be pushed out of the top of the electroscope into the vane and support. This effectively increases the negative charge on the vane and the support, and more charge means more force, and more force means more separation.

It is important to note that no electrons are transferred from the strip to the electroscope during this process - the charge that was already on the electroscope is merely rearranged.

The analysis if the strip and the electroscope are both positively charged is similar.
If you bring an oppositely charged strip near the top of the charged electroscope, the vane gets closer to the support. When the charged strip is removed, the vane goes back to where it was.

Suppose that the strip has a positive charge and the electroscope has a negative charge (the opposite situation can be analyzed in a similar manner). The vane is separated from the support due to the fact that they both have an excess of electrons, and like charges repel.

When you bring a positively charged strip near the top of the electroscope, the electrons in the electroscope are attracted toward the top of the electroscope, since unlike charges attract. This leaves fewer excess electrons in the vane and support of the electroscope. Smaller charge means smaller force. Smaller force means smaller separation.

It is important to note that no electrons are transferred from the strip to the electroscope during this process - the charge that was already on the electroscope is merely rearranged.
The vane and support of the electroscope behave like the two pith balls in the previous activity. They both receive the same charge, so they repel one another.

last update February 20, 2001 by JL Stanbrough