Dear Christiana--
When you bring a negatively charged rod near an electroscope, the electrons
in the electroscope are repelled down into the two leaves making each leaf
negatively charged due to the elxtra electrons (the top of the electroscope
now has a positive charge due to the ansence of the electrons that travelled
down into the leaves). The leaves. each with a negativce charge, repel each
other and spread apart. If you use a positively charged rod instead, the
electrons in the leaves are attracted to the top of the electroscope, making
each leaf positively charged. The leaves again repel each other and spread
apart. It's always the electrons that do the moving, and in each case the
leaves spread apart because they are charged alike -- both negative, or both
positive.
If you touch a negatively charged rod to the electroscope, electrons move
into the electroscope, charging the leaves negatively, and again they spread
apart. If you use a positively charged rod, electrons move from the
electroscope onto the rod, and again the leaves spread apart, but this time
because they are each left with a net positive charge.
Hope this helps.
Don Rathjen
Exploratorium Teacher Institute
donr@exploratorium.edu
or
donrath@aol.com
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