Protons and neutrons have almost equal masses, which are nearly 2000 times greater than the electron mass. Every atom has a small, central nucleus of protons and neutrons with its electrons moving about the nucleus some distance away. The number of protons and electrons is equal in a normal atom, which is therefore electrically neutral. An atom that has lost one of more electrons is a positive ion, and an atom that has picked up one or more electrons in excess of its usual number is a negative ion. A flow of charge from one place to another is an electric current .
Charge flows easily through a conductor, only with difficulty though an insulator.
According to Ohm's law, the current in metal conductor is proportional to the potential difference between its ends and inversely proportional to its resistance. The power of an electric current is the rate at which it does work. All atoms contain moving electrons, and permanent magnets are made from substances, notably iron, whose atomic magnetic fields can be lined up instead of being randomly oriented. Electromagnetic induction refers to the production of a current in a wire when there is relative motion between the wire and a magnetic field. The direction of an alternating current reverses itself at regular intervals.