Current & Potential Difference

Electric current

A simple electrical circuit needs a source of potential difference and a conducting path through which charge can move. A continuous current flows only when the conducting path forms a complete circuit between the source terminals.

Electric current is the rate of flow of electric charge.

Conventional current and electrons

In a metal conductor, the mobile charge carriers are electrons. Because electrons are negatively charged, they move through the external circuit from the negative terminal towards the positive terminal.

Conventional current is defined as the direction in which positive charge would move. It therefore runs from the positive terminal towards the negative terminal, opposite to the direction of electron flow.

The direction marked as current on a circuit diagram is normally the direction of conventional current.

Worked example: A charge of $4.0\,\mathrm{C}$ passes a point in a circuit in $500\,\mathrm{s}$. Calculate the current.

Use:

$$I=\frac{\Delta Q}{\Delta t}$$

Substitute the values:

$$I=\frac{4.0}{500}=8.0\times10^{-3}\,\mathrm{A}$$

Therefore, $I=8.0\,\mathrm{mA}$.

Exam Tip: Unless electron motion is specifically requested, arrows labelled as current should show conventional current from positive to negative. In calculations, convert quantities such as mA into amperes before substituting into SI equations.