Electric Current
Electricity and Magnetism

Current, electric

Glossary Definition for 16-19 IOP Glossary Project

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 = dQdt

where dQ is the charge passing the point between times t and t+dt.

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 I1 and I2 enter a circuit junction and currents I3, I4 and I5 leave it then I1 + I2 = I3 + I4 + I5

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 = dQdt
  • where Q is the charge passing a point in the circuit.
  • V = IR
  • where V is the voltage across one or more resistive components, of total resistance R.
  • P = VI = I 2r
  • where 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.

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