The National MagLab is funded by the National Science Foundation and the State of Florida.

# Electromotive Force in Inductors

Electromotive Force is an important phenomenon that impacts the way electrons flow through a conductor.

Electromotive Force (EMF) is a measure of the energy transfer between a current and a magnetic field when one or both are changing. EMF is measured in volts.

This tutorial shows a circuit consisting of a battery, lightbulb, knife switch, and an inductor made of a coil of copper wire. The inductor stores energy in the form of a magnetic field.

## Instructions

1. Locate all the elements in the circuit: battery, lightbulb, knife switch and inductor.
2. Note that the inductor and the bulb are connected in parallel.
3. Close the circuit with the knife switch by clicking the “turn on” button.
4. Watch the current travel through the circuit. See the lightbulb flash briefly as the magnetic field grows.
5. Notice how the bulb turns off when the magnetic field reaches and maintains its maximum strength.
6. Click the “turn off” button and watch what happens as energy leaves the magnetic field around the inductor.
7. Observe how the bulb flashes again as the magnetic field shrinks.

Once the knife switch is closed, the battery powers the circuit. The electrons in the current create a magnetic field around the wire. The coiled inductor also creates a magnetic field, one which is much stronger than the field around the straight wire.

When the circuit is energized and the current reaches the junction, it splits and flows to the inductor and to the light bulb. Initially, more current flows to the light bulb because as the magnetic field grows in the coil of wire, it induces its own voltage through Faraday’s law.

The inductor creates a Back EMF, opposing the current that caused it. The Back EMF channels current away from the inductor and towards the light bulb. This causes the bulb to light up – but only for an instant!

As the magnetic field around the inductor reaches its maximum, the Back EMF vanishes and the current now takes the path through the inductor because the bulb presents more resistance to the current.

Similarly, when you open the knife switch and interrupt current flow from the battery to the circuit, the effects of Back EMF are observed again. As the magnetic field around the coil of wire collapses, that change generates back EMF opposing the removal of the current flow. The back EMF attempts to maintain the current, and electrons flow to the light bulb, flashing it briefly before the current is diminished.

EMF and Back EMF are fundamental to how many electrical circuits and devices work. They have major implications in the design of motors, generators, and telecommunication systems.

Check out this See-thru Science Video for a more in-depth look at how EMF impacts electron flow.