Every atom consists of a nucleus surrounded by moving electrons, and each electron possesses a negative charge. A good electrical conductor is a material whose atoms have many electrons able to move freely. A good electrical insulator is a material whose atoms have very few free electrons.
Current (I) has the unit ampere (A) and is the rate of flow of electric charge. Current flow is due to a potential difference (p.d.) between two points in a circuit, the conventional direction of current flow being from positive to negative via the external circuit. Electron flow is in the opposite direction, i.e. from negative to positive. A device capable of producing a p.d. between two points is said to produce an electromotive force (e.m.f.); p.d. and e.m.f. have as their unit the volt (V).
Ohm's Law states that under constant conditions, the current in a conductor is proportional to the p.d. across its terminals
so that 
is a constant. This constant is termed the
resistance (R). Its unit will be volt/ampere, simplified to ohm
(
). By transposition we have
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