ELECTRIC CURRENT AND CURRENT DENSITY

An electric current is charge in motion. The carriers of the charge can be physical particles like electrons or protons, which may or may not be attached to larger objects, atoms or molecules. Here we are not concerned with the nature of the charge carriers but only with the net transport of electric charge their motion causes. The electric current in a wire is the amount of charge passing a fixed mark on the wire in unit time. In CGS units current will be expressed in esu/sec. The SI unit is coulombs/sec, or amperes (amps). A current of 1 ampere is the same as a current of 2.998 × 109 esu/sec, which is equivalent to 6.24 × 1018 elementary electronic charges per second.

It is the net charge transport that counts, with due regard to sign. Negative charge moving east is equivalent to positive charge moving west. Water flowing through a hose could be said to involve the transport of an immense amount of charge—about 3 × 1023 electrons per gram of water! But since an equal number of protons move along with the electrons (every water molecule contains 10 of each), the electric current is zero. On the other hand, if you were to charge negatively a nylon thread and pull it steadily through a nonconducting tube, that would constitute an electric current, in the direction opposite the motion of the thread.