ELECTROMAGNETISM

Electromagnetism

Electromagnetism

Electromagnetic Induction

We have seen previously that when a DC current pass through a long straight conductor a magnetizing force, H and a static magnetic field, B is developed around the wire. If the wire is then wound into a coil, the magnetic field is greatly intensified producing a static magnetic field around itself in the shape of a bar magnet giving a distinct North and South pole.( Kovacs145) The magnetic flux around the coil being proportional to the amount of current flowing in the coils windings as shown.

But what if we reversed this idea by disconnecting the electrical current from the coil and instead of free-air placing a bar magnet inside the open core of the coil of wire. By moving the magnet "in" and "out" of the coil a voltage would be inducted into the coil by the physical movement of the magnetic flux inside it. This then is known as Electromagnetic Induction and is the basic principal of operation of transformers, motors and generators.

Air-core Coil

Electromagnetic Induction was first discovered way back in the 1830's by Michael Faraday. Faraday noticed that when he moved a permanent magnet in and out of a coil or a single loop of wire it induced an ElectroMotive Force or emf, in other words a Voltage, and therefore a current was produced. So what Michael Faraday discovered was a way of producing an electrical current in a circuit by using only the force of a magnetic field and not batteries. This then lead to a very important law linking electricity with magnetism, Faraday's Law of Electromagnetic Induction. So how does this work?.

When the magnet shown below is moved "towards" the coil, the pointer or needle of the Galvanometer, which is basically a very sensitive center zeroed moving-coil ammeter, will deflect away from its center position in one direction only. When the magnet stops moving and is held stationary with regards to the coil the needle of the galvanometer returns back to zero as there is no physical movement of the magnetic field. When the magnet is moved "away" from the coil in the other direction, the needle of the galvanometer deflects in the opposite direction with regards to the first indicating a change in polarity. Then by moving the magnet back and forth towards the coil the needle of the galvanometer will deflect left or right, positive or negative, relative to the directional motion of the magnet.

Electromagnetic Induction

Likewise, if the magnet is now held stationary and ONLY the coil is moved towards or away from the magnet the needle of the galvanometer will also deflect in either direction. Then the action of moving a coil or loop of wire through a magnetic field induces a voltage in the coil with the magnitude of this induced voltage being proportional to the speed or velocity of the movement. In other words the faster the movement of the magnetic field the greater will be the induced emf or voltage in the coil, so for ...