Inductance

Induction is caused by a changing magnetic flux, which then induces an electromotive force, emf, in a circuit. This important law of electromagnetism is also known as Faraday's law of induction .

This law came about through the experiments of Michael Faraday of England and Joseph Henry of the U.S. in 1831. Their experiments described how an electric current could be induced in a circuit by a changing magnetic field.

The practical applications of induction deal with the absence of an emf source, an electric cell or batteries. As long as the magnetic field is changing, a current is induced. The induced current is always in such a direction as to keep the magnetic field constant.

The emf induced in a circuit is directly proportional to the time rate of change of magnetic flux through the circuit. Basically this means that the induced emf is directly related to magnetic flux as a function of time.

The induced emf = -(change in magnetic flux) / (change in time). The negative sign is there to signify that the induced emf is opposite in direction of the original emf. This is according to Lenz's Law. (Coming up soon!)

Motional emf, which is emf induced by a conductor when it moves through a magnetic field, can be tied to induction. When the conductor moves through a uniform magnetic field, there is a magnetic force F_m induced which is opposite the motion of the conductor. Thus this example is valid because it follows the principle of energy conservation.

Lenz's Law states that the polarity of an induced emf will produce a current that creates a magnetic flux that opposes the change in magnetic flux through the loop formed by the circuit . The induced current opposes any change in magnetic flux. In case you still don't understand, the induced current must be in a direction so that the flux it produces opposes the change in the external (applied) magnetic flux. For example, if a bar moves to the right, then the induced current would be counterclockwise so that the force produced is directed towards the left.

When the magnetic bar is moved through a conducting loop, a current is induced. This current then produces its own flux which opposes the applied external magnetic flux.

The principle of inductance is very important, as it is the reason why we have:

alternating current generators (AC)= device that converts mechanical energy to electrical energy. A loop is moved by mechanical means. The magnetic flux from the movement changes with time, producing an emf and a current in an external circuit

direct current generator (DC)= generators which have an output voltage always at the same polarity (as opposed to the sinusoidal wave pattern on an AC generator). The reason they are at the same polarity is because the contacts to the split ring reverse their roles every half cycle, causing the polarity of the emf to reverse. This action stabilizes the polarity of the split ring, so that its polarity is constant.

Motors= devices that convert electrical energy into mechanical energy. Motors are generators operating in reverse! Motors require more power when they are started and when they are under heavy loads because of the excess back emf induced which resists the physical motion of the motor.

The equation for the maximum emf induced by a generator is given by: E_max=NABw , where N=number of turns in the loop, A=area, B=magnetic field, and w=a constant angular velocity.

Induced currents and emf are important because the theoretical basis is Lenz's Law and can be applied to our most common power source today, the AC generator. In case you don't know, this power source produces the electricity which we often take for granted. Also important is the DC generator which is more prevalent in portable electronics such as tape players, CD players, electric shavers, etc. The most important thing to know about induced currents or emf is that the induced magnetic flux induced by the current or emf always opposes the motion of the object which is moving. Induced emf or current is produced by a change in magnetic flux.