Electromagnetic Induction

Following Oersted's discovery that an electric current produces a magnetic field around a wire, the English scientist Michael Faraday showed in 1831 that a magnetic field could in effect produce an electric current. In particular, he showed that a changing magnetic field could induce current in a coil. The creation of such a current is called Electromagnetic Induction.

Faraday's Induction Experiment

Figure1 demonstrates what happens when a magnet is moved in relation to a coil. The North Pole of the magnet provides a changing magnetic field at the face of the coil - but only so long as the magnet is moving. When the magnet is stationary, no current flows in the coil. Note, the movement of the magnet in different directions (towards and away) results in current flowing in corresponding opposite directions. If the experiment is turned around so that this time the magnet is stationary but the coil is moved, again current is observed to flow as before.

What is important to create an induced current is a changing magnetic field due to the relative motion between the magnet and the coil. Moreover, it is observed that the larger and the faster the change, the greater will be the size of the current.

Magnetic Flux and Magnetic Field Density

To explain electromagnetic induction, it is helpful to introduce a new physical quantity, the magnetic flux, phi, which is the product of the magnetic field times the area perpendicular to the field:

Magnetic flux: phi = BA cos (theta),

where B is the magnetic field strength, A is the area at right angles to the field, and theta is the angle the area makes with B. Magnetic flux maybe thought of as a the number of magnetic lines crossing the area. Figure 2 provides an illustration of this principle:

• In Fig. 2a, a magnetic field passes through a small area, but in Fig 2b the area is larger.
• In each case, the number of magnetic lines is the same. That is, the magnetic flux is the same.
• The smaller area has a stronger magnetic field because lines of force (flux) are closer together than in the larger area. For this reason, the magnetic field strength, B, is also known as the magnetic flux density.

Unit of Magnetic Flux

Magnetic flux is given by the magnetic field strength (Tesla) times area (m2) - i.e., the unit becomes the Tesla metre squared, which is also known as the Weber (1 Wb = 1 Tm2)

Induced Current and EMF

Faraday showed that the amount of induced current in a coil was proportional to the rate at which the number of lines of force cutting through a coil was changing. That is, it was proportional to the rate of change of flux (phi). If one considers electrical resistance, induction can be expressed in terms of a voltage source created, or electromotive force, EMF:

EMF = - N d(phi)/dt

where N is the number of turns in a coil, d(phi) is the change in flux and dt, the change in time. Notice, a negative sign appears in the expression. This is normally ignored in simple calculations but the sign has important significance as it implies opposition to an applied, external change in flux.

Conclusion

Michael Faraday showed that a changing magnetic field could produce a current in a coil that is proportional to the rate of change of magnetic flux in the coil. Magnetic flux can change when there is a changing field strength through a coil, or when the perpendicular area of the coil is changing (such as a rotating loop). The induced voltage source is known as the electromotive force, which has a direction such as to oppose the change in flux that produced it.

The reader may be interested in more details on this topic, or learn about the workings of the electronic transformer, a ubiquitous device based on the principle of magnetic induction.

Fig.1 Induced Current in a coil

Fig. 2 Magnetic Flux

Bar Magnet

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