* Referring to your printed notes; either copy these notes or annotate your printed notes

Inductors and Self Inductance

In simple terms an inductor is a coil of wire made of a conductor. As such when a current is passed through it a magnetic field is induced. Or if the coil is moved in a magnetic field a voltage or emf is induced.

The electrical component Inductor comes in two guises, one is air-cored and the other is iron-cored. In the iron filled core version the induced magnetic field is enhanced and as a result the induction of the inductor is increased.

Induced emf

The short video below is an example of electrical generation using a conducting coil and a magnetic field.

You have seen this demonstrated in class.

The magnet is moved in and out of the coil of wire, as a result the coil (inductor) is in the presence of a changing magnetic field.
As such an emf is induced each time the magnetic field changes. This is seen on the voltmeter connected.

Important: The induced voltage always opposes the direction of the change that caused it - this is seen in the video as the induced voltage changes direction as the direction of the changing magnetic field changes.


(If I was you I would read this a few times.)
As explained above, an induced emf in an inductor coil will drive a current through the coil. This current-carrying conductor has itself a magnetic field around it. Now as the current increases or decreases this magnetic field increases or decreases, or changes. As such the wire is in the presence of its own changing magnetic field and as a result an emf is produced. This is referred to as a self-induced emf and it is given a negative sign since it is opposed to the changing emf that caused it.

Experiments on generating voltage show that:

(i)  The greater the rate at which the magnetic field changes (i.e. the faster the magnet moves), the greater

the size of the induced voltage – a statement known as Faraday’s law of electromagnetic induction.

(ii)  The induced voltage (and current) always opposes what causes it a statement called Lenz’s law of

  1. electromagnetic induction

Changing currents and changing magnetic fields will occur:

  • In d.c. circuits when switching the circuit on or off
  • In a.c. circuits all the time