Formulas for Current

In this tutorial we will present the formulas for current .

Current is a flow of electric charge through a medium. This charge is typically carried by moving electrons in a conductor such as wire. It can also be carried by ions in an electrolyte, or by both ions and electrons in a plasma.

Examples are the flow of electrons out of a battery or the current rating described for a power supply. You may know that batteries in a car are usually labeled by ampere capacity and not just by their voltage rating.

In order to calculate the amount of current flow in or out of a device you must be familiar with the correct formulas used to figure out this rating.

Electrons (negative charges) move through a conductor when an electric field is applied. Electron current is defined as the directed flow of electrons from negative to positive.

Current is measured in AMPERES (AMP). One amp of current flows when one coulomb (6.28 × 1018 electrons) passes a point in one second.

This formula may be applied depending on what you are trying to calculate.

The Ohm's law formulas for current are:

Ac current formulas are:

Ohm's law for reactive circuits:

Ohm's law for circuits containing resistance and reactance:

Current across the primary (Ip) of a transformer:

Current across the secondary (Is):

There are several ways to measure current. You can use an ammeter, a multimeter, or a current meter/probe. These devices (used in the right way) can be very accurate in the measure of current.


Human reaction to electrical shock is determined by the amount of current flowing through the body. A 100-milliampere shock for 1 second is usually fatal!

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