# Voltage drop across inductor – formula and polarity

There are three basic passive circuit components – Resistor, Capacitor and Inductor. One can find the voltage drop across a resistor simply by using Ohm’s law of current electricity. In another article, we have discussed different ways to find the voltage drop across a capacitor as well. Here I’m going to explain the formula and polarity of the voltage drop across an inductor. This will help to read the characteristics of an AC inductive circuit. For example, the Transformer circuit is an inductive circuit that contains two inductors.

Contents in this article:

• Why is there a voltage drop across inductor in AC circuit?
• Voltage across Inductor formula
• Polarity of voltage drop across an inductor
• How to find the voltage across an inductor?

## Voltage drop across Inductor in AC circuit

Why is there a voltage drop across an inductor in an AC circuit?

If a varying current passes through an inductor, a varying voltage induces across the inductor. Because the inductor is nothing but a coil and the varying current produces a varying magnetic field in that coil which causes the change in magnetic flux linked with the coil. Now, according to Faraday’s law of electromagnetic induction, an alternating voltage induces due to varying magnetic flux through a coil.

In an electrical circuit, the source of a varying current is an external alternating voltage. Thus, the application of an AC voltage across an inductor produces an alternating voltage drop across the inductor. A direct current (DC) cannot produce varying magnetic flux. So, the DC voltage cannot drop a voltage across an ideal inductor that has zero resistance.

## Voltage across Inductor formula

The formula of voltage drop across an inductor of inductance L is, $\color{Blue} V= L \frac{dI}{dt}$………(1)

Where I is the time-varying electric current.

This formula comes from Faraday’s law of electromagnetic induction. Using this equation of voltage across an Inductor, one can find the induced voltage across an inductive coil.

## Polarity of voltage drop across an inductor

Just like a resistor, An inductor does not have polarity. However, the voltage of an inductor has a polarity depending upon the direction of current flow. The polarity of induced voltage across an inductor is determined by Lenz’s law of electromagnetic induction.

According to this law, the direction of induced voltage will be such that it opposes the charge in alternating current passing through it. Since the alternating current always changes its polarity, the induced voltage will also change its polarity with the same frequency as that of the alternating current passing through it.

Whenever a changing current flows through an inductor, an opposite current induces inside it. Hence, an induced voltage arises. The direction of this voltage is always opposite to the direction of the applied voltage.

## How to find the voltage across an inductor?

To find the voltage across an inductor, you need to know two things –

1. The inductance L of the inductor.
2. The expression for time-varying current through the inductor.

After knowing these two parameters you can easily calculate the induced alternating voltage across the inductor by using equation-(1).

Again, you can find it by using the voltage across inductor calculator which is available on the internet.

This is all from this article on formula of voltage drop across the inductor with its Polarity. If you have any doubts on this topic you can ask me in the comment section.

Thank you!

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