Faradays discovery

The discoveries of the 1820s prompted speculation that since electric currents created magnetic fields, could magnets in turn be used to make electric fields? If so, the electric field created by a magnet could exert a force on electric charges, causing them to move and creating an electric current.

The simplest experiment was to place a magnet beside a piece of wire — perhaps it would produce a current in the wire? Experiments using the strongest available magnets failed to produce the slightest hint of a current.

In subsequent attempts the magnet was replaced by a wire car- strongest possible rying the strongest possible current. _steaefy^cui-rert

no charges 'dragged along'

somehow 'drag' even the smallest in neighbouring conductor current parallel to it in the second wire? Again, no matter how large the current in the first wire was, and no matter how close they were together, nobody could detect the slightest movement of charge in the wire.

In 1840 Michael Faraday (1791-1867) realised that what had been missed was an essential feature. A magnetic field does not, simply by its presence, create an accompanying electric field. Electric effects appear only if a magnetic field is changing. When, rather than using a steady current, he decided to change the current in the first wire, a surprising thing happened — a current

suddenly appeared in the second wire! We now call this phenomenon electromagnetic induction, where the current in the second wire is an induced current. By stopping and starting an electric current Faraday produced a current in the neighbouring wire, which was also stopping and starting.

To make a quantitative statement about electromagnetic induction it is convenient to define magnetic flux (pB in a similar manner to the way we defined electric flux (pE. Again the name 'flux' does not imply actual physical movement. Magnetic flux is a measure of the number of magnetic field lines crossing an area.

There are many ways of inducing current. We could change the magnetic field by moving a magnet towards a loop of wire (Figures 10.15 and 10.16). As we move the magnet, there is an increase in magnetic flux through the loop, and a current appears in the wire. This current is present as the flux is increasing, but stops when the flux reaches its maximum value. If the magnet is then moved away from the loop, the current flows in the opposite direction. The direction of the current depends on whether the flux through the loop is increasing or decreasing. The magnitude of the current depends on the speed at which the magnet is moved.

Rotating a coil in a magnetic field is another, more practical, way to produce induced current (Figure 10.17). The current alternates at the rate of two cycles for every complete revolution of the loop. When the coil stops rotating the current stops.

(0is the angle between the normal n to the area A and the magnetic field B)

f/

S

N

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Figure 10.15 Inducing a current by changing the magnetic flux through a loop.

n turns YVYVYVYVW

AMAAMAMJ ni

Figure 10.16 The current is increased n times.

Figure 10.17 The principle of the dynamo.

To create larger currents a rotating coil can be used instead of a single loop. Such a system is known as a dynamo.

It is interesting to note that in the dynamo the role of magnetism is passive. Its presence, however, makes it possible to change the mechanical energy of the rotating coil into the energy of an electric current. Work has to be done to keep the coil rotating — the principle of conservation of energy ensures that we cannot get something for nothing!

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