Basic Units

Current

The total amount or charge is important but so is the rate at which it is moving. This rate is not related to the speed at which the charge is moving but the amount of charge that is moving.

Current is defined as the rate of change of charge. If we draw a line across the conductor and monitor the amount of charge that crosses the line per second then this is equal to the current (I). The units of current are amps (or Coulombs/second) and this is a fundamental unit.

Now, for historical reasons, current is defined to be positive when it is formed from positive charge that flows from a positive to negative voltage. Unfortunately, the charge that flows in most materials is composed of electrons, which are negatively charged, and so conventional current flows in the opposite direction to electrons. This is inconvenient but does not really cause a problem.