Current and charge

In metals, current is the movement of negative charge, i.e. electrons; on gases and electrolytes both positive and negative charges may be involved. Under the action of a battery, charges of opposite move in opposite directions and so a convention for current direction has to be chosen. As far as most external effects are concerned, positive charge moving in one direction is the same as negative charge moving in the opposite direction. By agreement all current is assumed to be due to the motion of positive charges and so when current arrows are marked on circuits they are directed from the positive to the negative of the supply. If the charge carriers are negative they move in the opposite direction to that of the arrow.

The basic electrical unit is the unit of current- the ampere (abbreviated to A); it is defined in terms of the magnetic effect of a current. The unit of electric charge, the coulomb (C), is defined in terms of the ampere.

One coulomb is the quantity of the electric charge carried past a given point in a circuit when a steady current of 1 ampere flows for 1 second.

If 2 amperes flow for 1 second, 2*1 coulombs (ampere-seconds) pass; if 2 amperes flow for 3 seconds then 2*3 coulombs pass. In general if a steady current I (in amperes) flows for time t (in seconds) then the quantity Q (in coulombs) of charge that passes is given by: Q=It

The flow of charge in a conductor is often compared with the flow of water in a pipe. The flow of water in litres per second, say, corresponds to the flow of charge in coulombs per seconf, i.e. amperes.

The charge of an electron, i.e. the electronic charge, is 1.60*10^-19 C and is much too small as a practical unit. In 1 C there are therefore 1/(1.60*10^-19), i.e. 6.24*10^18, electronic charges. A current of 1 A is thus equivalent to a drift of 6.24*10^18 electrons pass each point in a conductor every second.

Smaller units of current are the milliampere (10^-3 A), abbreviated to mA, and the microampere (10^-6 A), abbreviated to uA.