Electric Circuits: Current

Electric Current is defined as the rate of flow of charge through a conductor or any other component in a circuit.
Current is measured in amps (A), which equates to 1 coulomb of charge passing a point in the circuit per second (1A = 1C/s).
In terms of electron flow, conventional current flows from the positive (+) to the negative (-) terminal of the power supply. This is opposite to the actual direction of flow of electrons, but it is used for consistency in understanding and analysing circuits.

Current in a conductor results from the movement of free electrons. These ‘drift’ slowly in a direction opposite to the electric field.
The drift velocity of electrons in a metal wire carrying a steady current is typically very small, in the order of millimetres per second.
Despite the slow drift velocity, the effect of a current is transmitted nearly instantly due to the propagation of an electric field throughout the circuit, which occurs at nearly the speed of light.

Direct current (DC) flows in one direction only, and is produced by sources such as cells and batteries.
Alternating current (AC) changes direction periodically, typically many times per second. It is produced by most power stations and is the type of electrical power supplied to homes.
The frequency of AC, in the UK, is 50 Hz, meaning it changes direction 50 times each second.

Understanding the concept of mean drift velocity is key in understanding current.
It’s defined as the average velocity that a free electron inside a conductor (like a metal wire) would achieve under the influence of an applied electric field.
It can be calculated from the equation I = nAvq, where I is the current, n is the number density of the free electrons, A is the cross-sectional area through which the current is flowing, v is the mean drift velocity and q is the charge on an electron.

Ohm’s Law states that current through a conductor between two points is directly proportional to the voltage across the two points. It is represented by the formula I=V/R where I is the current, V is the voltage, and R is the resistance.
The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area, demonstrating a property known as resistivity.
Resistivity can be calculated using the equation R = ρL/A where ρ is the resistivity of the material, L is the length of the conductor, and A is its cross-sectional area.
Remember that certain components (like bulbs or diodes) do not obey Ohm’s law and are referred to as non-ohmic conductors.