# Faraday’s Law – Electromagnetic Induction

Statement of

Faraday’s lawsummarizes the results of his experiments:A voltage (electromotive force) is induced in a circuit when there is a changing magnetic flux passing through the circuit.

The induced voltage is equal to the rate of change of the magnetic flux.

In symbols, induced voltageƐ=Δф/t

The rate of change of flux is found by dividing the change in flux **Δф** by the time t required to produce this change.

The process of inducing a voltage in a circuit by changing magnetic flux passing through the circuit (as described in Faraday’s law) is called

electromagnetic induction.

- The results of all of Faraday’s experiments indicate that an
*electric current*is induced in a coil or circuit when the magnetic field passing through that circuit is changing. Because the amount of current flowing in the secondary circuit depends on the resistance of that circuit, we generally express these results in terms of the induced voltage rather than the current. - The more rapidly the magnetic flux through the circuit changes, the larger the induced voltage, which can be readily observed in the experiment involving the moving magnet.

If we move the magnet in and out of the coil quickly, we get larger deflections of the galvanometer needle than if we move it more slowly. - The magnetic flux passing through a coil of wire runs through each loop in the coil, so the total flux through a coil is equal to the number of turns of wire in the coil times the flux through each turn,

or**ф**= NBA. The more turns of wire in the coil, the larger the induced voltage. - A coil of wire can be used to assess the strength of a magnetic field. The magnetic flux through the coil can be quickly reduced to zero either by removing the coil from the field or by giving it a quarter turn so the field lines lie parallel to the plane of the coil. If we know the time required to make this change, we can find the strength of the magnetic field by measuring the induced voltage.
- The magnetic flux is related to the number of magnetic field lines passing through the area bounded by the loop of wire. For a simple loop of wire lying in a plane perpendicular to the magnetic field, the magnetic flux is the product of the magnetic field B and the area A bounded by the loop. In symbols, the flux definition takes the form
**ф**= BA. The Greek letter**ф**(phi) is the standard symbol used for flux.