Question

Describe how to create a net positive charge on an electrophorus.

Answer 1

Charging by induction

When the electrophorus is held near the negatively charged Styrofoam plate, electrons within the electrophorus will be repelled by the negatively charged Styrofoam. These electrons will move from the bottom surface of the electrophorus to the rim of the electrophorus. This leaves the bottom of the aluminum plate with an excess of positive charge and the rim of the electrophorus with an excess of negative charge. It could be said that the Styrofoam has served to polarize the electrophorus - i.e., to separate its positive charge from its negative charge. While there may be a separation of charge within the electrophorus, the overall charge of the plate is zero. Once the electrophorus is touched, electrons move from the electrophorus to the ground. It is at this instant that the electrophorus acquires an overall positive charge. This excess of positive charge remains localized near the bottom of the electrophorus due to its attraction to the Styrofoam plate. As the electrophorus is lifted away from the Styrofoam plate, there is a movement of remaining electrons within the electrophorus such that the excess positive charge becomes distributed about the electrophorus.

Charging the Electroscope with the Electrophorus Disk by Conduction

To charge the electroscope by conduction, the electrophorus disk, which has a positive charge, is brought into physical contact with the fixed disk on the electroscope . Since electrons from the electroscope are drawn to electrophorus disk, the electroscope disk and leaf  will be left with a net positive charge, the same charge as that on the electrophorus.