Rub the acrylic sheet a few times with wool cloth. (This will put a small positive charge on the acrylic sheet.) Ask yourself where the charges are; are they on one side the sheet or in the ‘bulk’ of the sheet? Turn the sheet over. Put the metal part of the electrophorus on the acrylic sheet (but don’t try and rub the metal against the acrylic sheet and don’t touch the metal part yet!) Bring the electrophorus close to an electroscope. Is the electrophorus charged? Now let’s add a step. Put the disk of the electrophorus on the acrylic sheet as before. Touch the metal part of the electrophorus with your finger. Take away your finger while the electrophorus is still on the acrylic sheet. Bring the electrophorus close to an electroscope. Is the electrophorus charged (now that you have touched it when it was on the acrylic sheet)? Discharge the electrophorus by touching it. Charge the electrophorus repeatedly as above. Discharge it each time by touching it. (Notice that whether the electrophorus becomes charged, depends critically on when you touch it.) Can you find a limit to the number of times that you can charge the electrophorus by induction using the same charged acrylic sheet? It is this property of the electrophorus that makes it useful. It will be our source of (more) charge for the next experiments.
When charging by contact, it is clear that the charge is transferred from one object to another so it is clear where the charge came from. When the electrophorus (in the previous paragraph) was charged by induction, where did the charge come from? Explain. (Be careful. The step that was added involved ‘grounding the electroscope’. Think carefully before you conclude that the charge on the electrophorus has its origins in the ground.)
A. Is the electrophorus charged?
B. Is the electrophorus charged (now that you have touched it when it was on the acrylic sheet)?
C. Can you find a limit to the number of times that you can charge the electrophorus by induction using the same charged acrylic sheet?
D. When the electrophorus (in the previous paragraph) is charged by induction, where did the charge come from? Explain. (Be careful. The step that was added involved ‘grounding the electroscope’. Think carefully before you conclude that the charge on the electrophorus has its origins in the ground.)