Current the unit used to measure the flow of electrical charges through a conductor. It flows whenever there is a source, which is usually a voltage and, a complete circuit. The electrons which act as the charge carriers move from the point of high potential to a point of lower potential. The rate of flow, or the amount of charges passing through a point in a conductor per unit time, is what determines the amount of current. (Current = charge passing a point/time).

Current is measured in amperes, where an ampere of current is equivalent to one coulomb of charge passing a point in one second.
1 Amp = 1coulomb/second. (1 A = 1C/s).
1 Coulomb = 6.2 × 1018 electrons.

There are two types of current:

Direct current (D.C) in which the flow of the electric charges is in one direction. D.C sources include:

  • D.C generators
  • Batteries
  • Solar Cells
  • AC to D.C converters

Alternating current (A.C) in which the charge flows back and forth or reverses direction several times per second. The cycles per second is referred as frequency. Common frequencies are 50 and 60Hz (50 and 60 cycles per second)

  • A.C generators
  • Inverters

The amount of current in a circuit is determined by the voltage, and the resistance of the conducting material (I= V/R). Almost all materials offer resistance to the flow of electrons and hence the current. This depends on the material structure, type of atoms and the spaces between the atoms. Conductors, semiconductors and superconductors allow the easy flow of current. While the insulator materials offer the highest resistance and do not allow any current to flow.

Resistance arises from the tiny flaws and other imperfections in the material which cause the moving electrons to collide with the stationary atoms. Since the electrons have some mass, the collisions results in loss of some of their energy in the form of heat.

Some circuits are required to have very little resistance to minimize heat losses, increase efficiency and minimize the source current. However, intentional resistance may be introduced at various stages of the circuit to limit the amount of current flowing through to the next level.

High resistance materials or insulators are used where direct contact of the current currying conductor is not desired. These are used as insulation in power cables, electronic component packaging and others.

The relationship between the voltage, resistance and current in a resistive load circuit is given by the ohm’s law:

V=IR, or I=V/R

V – Voltage in Volts
I – Current in Amperes
R – Resistance in Ohms

So to work out the Voltage simply times the Current by Resistance, or to work out the current divide the Voltage by Resistance.