6.1 FARADAY’S LAW
One day in 1837 Michael Faraday was
working in his laboratory when by accident he dropped a magnet into a coil of
wire which happened to be connected to a galvanometer. He noticed, to his surprise, that the
galvanometer needle gave a kick when this happened. He was even more surprised to see, when he
took the magnet out, that the needle kicked the other way.
This started a train of thought
which finally led to a monumental discovery which was to become the whole basis
of modern electrical engineering: it was the theory of ‘Electromagnetic
Induction’.
FIGURE
6.1
FARADAY’S
LAW OF ELECTROMAGNETIC INDUCTION
These are heavy words, but in short they mean that, if a
conductor is moved in a magnetic field, then an ‘electromotive force’ (emf) -
that is, a voltage - is induced in that conductor. This is shown in Figure 6.1. It follows that, if the ends of the conductor
are connected to a load, then an electric current, driven by that voltage, will
flow from the conductor, through the load and back again.
Whereas Oersted had shown that an electric current
moving in a wire gives rise to an artificial magnetic field, Faraday showed the
opposite - that if a wire moves in a magnetic field an artificial charge, or
voltage, will be created in that wire. Electricity
and magnetism were now firmly tied together by these two great discoveries.
Here then is the basis of electrical power
generation. We start with a magnetic
field, either a natural magnet or an artificial electromagnet of Oersted’s
type, and cause a conductor or a number of conductors to move past it, from
which the current can be extracted as they are moving. How this is done in practice is shown in the
manual ‘Fundamentals of Electricity 2’, but first look at one other rule which
determines how the directions of field, voltage and movement are related.
Figure 6.1 shows ‘Fleming’s Right-hand Rule for
Generators’. If the right hand is held
with the thumb, forefinger and centre finger extended mutually at right angles
as shown in the figure, then, with the magnetic field in the direction (North
to South) pointed by the forefinger and the motion of the conductor in the
direction indicated by the thumb, the centre finger will point in the direction
in which the emf (i.e. voltage) is induced in that conductor (and in which
current will flow when connected to a load).
The magnitude of the voltage induced in the moving
conductor depends on the strength of the magnetic field and the speed of
movement, and on nothing else.
Use is made of these laws and rules when considering the
Principles of Generation described in the manual ‘Fundamentals of Electricity
2’.
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