When a glass rod is rubbed with a piece of silk, it acquires a positive charge. Measurements show that an equal but opposite i.e. negative charge appears on the piece of silk. This suggests that the rubbing process does not create any new charge, it merely transfers some charge from one body to another.
Many examples can be quoted where it is found that the sum of the charges before and after an event remains the same. This important fact is known as the law of conservation of electric charge, which states that the algebraic sum of the electric charge within a system does not change.
Some interesting examples of charge conservation, like pair production, and charge annihilation, are observed in nuclear physics. In the event known as pair production, a high-energy photon ends its existence with the creation of two particles- an electron and a positron. Since an electron carries a charge -e and a positron carries a charge +e, the net charge within the system before and after the event remains zero.
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