User:Inconspicuum/Physics (A Level)/Current

Current is the amount of charge (on particles such as electrons) flowing through part of an electric circuit per second. Current is measured in amperes (usually abbreviated A), where 1 ampere is 1 coulomb of charge per second. The formula for current is:

$$I = \frac{\Delta Q}{\Delta t}$$ ([The triangle (Greek letter /delta/) means change in the quantity])



where I is current (in A), Q is charge (in C) and t is the time it took for the charge to flow (in seconds).

In a series circuit, the current is the same everywhere in the circuit, as the rate of flow of charged particles is constant throughout the circuit. In a parallel circuit, however, the current is split between the branches of the circuit, as the number of charged particles flowing cannot change. This is Kirchoff's First Law, stating that:

At any point in an electrical circuit where charge density is not changing in time [ie. there is no buildup of charge, as in a capacitor], the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point.

In mathematical form:

$$\sum I_{in} = \sum I_{out}$$ (The character that resembles a sideways M is the Greek letter /sigma/, meaning 'sum of'.)