# Current, electric

Glossary Definition for 16-19

## Description

The electric current at a point in a circuit is defined as the rate of flow of charge past that point.

Electric current is usually represented by the symbol *I*.

The current does not have to be constant in time. For a time-varying current, the current at any instant, *t*, is

*I* = d*Q*d*t*

where d*Q* is the charge passing the point between times *t* and *t*+d*t*.

## Discussion

In any electrical circuit with a single current path, the current at one point in the circuit is the same as at any other point. Where a circuit branches at a junction, the total current that enters the junction is equal to the total current that leaves it (figure 1).

*Figure 1: If currents I_{1} and I_{2} enter a circuit junction and currents I_{3}, I_{4} and I_{5} leave it then I_{1} + I_{2} = I_{3} + I_{4} + I_{5}*

Although portable devices, such as mobile phones, may use direct current provided by batteries, large items such as washing machines run off the mains supply, which provides an alternating current.

## SI unit

ampere, Amp, A

## Expressed in SI base units

A

## Mathematical expressions

*I*= d*Q*d*t*
where *V*=*I**R*
where *P*=*V**I*=*I*^{ 2}*r*
where

*Q*is the charge passing a point in the circuit.

*V*is the voltage across one or more resistive components, of total resistance

*R*.

*P*is the power dissipated in one or more resistive components, of total resistance

*R*, with a voltage

*V*across them.

## Related entries

- Potential difference, electrical
- Power
- Resistance, electrical
- Voltage

## In context

The current in a typical torch bulb is around 0.5 A. In a high power device, such as a kettle, which runs off the alternating mains supply, the current is around 10 A.