Electroscope question?

Question

How can I identify a charge whether it is positive or negative through an electroscope? I know that this instrument only identifies whether an object has a charge but how can I know if its positive or negative? Please give we websites or references.

Answer 1

This is an excerpt from the website http://www.answers.com/topic/electroscop...

If an electroscope has a charge of known sign, as in illus. b, it can be used to test the sign of an unknown charge, as in illus. c, where the metal test ball (T), with its insulating handle (J), has the unknown charge. In the situation pictured, L moves farther away from P as T is brought slowly up toward K, showing that T has a positive charge. If T had a negative charge, L would move toward P, as T slowly approaches K. The converse situation, if the leaf system in illustration c had a negative charge initially, can be readily visualized.

Basically you get a "test ball" with a known positive or negative charge. If the charge is the same as the unknown charge the parts in the electroscope will repell since same charges repell. However if you have a "test ball" with a known charge and it is placed it on a electroscope an the parts in the electroscope don't repell each other (no angle) then the charges are opposite since like charges attract each other. The electroscope measures  the repulsion of two forces not the attraction.