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Separating charge to produce positive and negative charge

Where does the electricity come from when we rub materials such as a rubber rod against fur or a glass rod against silk? The following phenomena allow an interesting interpretation.

A metal sphere can be charged if we pull a sticky tape off the surface of the sphere (Photos 1 and 2). The electrometer demonstrates the presence of charge on the sphere. If we touch a different sphere connected to a second electrometer with the sticky tape, the electrometer shows charge on the tape. If we then bring a rubber rod rubbed with fur close to the electrometers, we notice that the reading on one of the meters increases whereas on the other one it decreases. Moreover, if we let the two spheres touch, the readings of both electrometers go back to zero. (See equilibration: Phenomenon 1.)

Interpretation

Charge is not produced when materials rub against each other. Rather, positive and negative charge is separated by the process (Figure 1). When we bring two bodies carrying equal amounts of positive and negative charge in contact, the two amounts neutralize each other—we have again zero charge. When we bring a charged rubber rod (or any other charged insulator such as glass or amber) close to oppositely charged electrometers (or touch the electrometers), the reading of the meter carrying charge of the same sign as the rod will increase (addition of charge of equal sign) whereas the reading of the other meter will decrease (addition of charge of opposite charge or neutralization).

Today we believe that electric charge can neither be created nor destroyed. When we “produce electricity,” positive and negative charge is separated. When electricity “disappears,” positive and negative charge neutralize each other.

Positive and negative signs of charge have to be defined once. After that, signs can be established by comparison. Amber rods carry negative charge.