The famous author Isaac Asimov once said, “The most exciting
phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’
(I found it!) but, ‘That’s funny. …’ ” That might have been what Faraday
thought when he noticed the meter deflection upon connecting and disconnecting
the battery.
Even though he didn’t get the result he was looking for —
current flowing steadily through the secondary coil — he did see a hint of
current flow in the form of a slight needle deflection in the galvanometer. But
it was enough to lead him down the right path to the answer.
Eventually, he found that a stationary magnetic field does
not induce current in the secondary coil, but that a changing magnetic field
does.
When a battery is first connected to a circuit, the magnetic
field has to build from zero to its maximum. As the field grows, the lines of
flux of the magnetic field cut the turns of wire in the secondary coil, thereby
inducing a current.
Faraday deduced that a changing magnetic field whose lines
of flux cut through a wire will generate a
voltage. The value of the voltage is proportional to the
rate of change and the intensity of the magnetic flux.
This is known as Faraday’s law of induction.
According to Faraday’s law of induction, it doesn’t matter
whether the lines of flux are cutting through the wire or the wire is moving
through the lines of flux, as long as they are moving relative to each other.
Therefore, a wire can move through a stationary magnetic field or a magnetic
field can move through a stationary wire and it will still generate voltage.
What is important is that the wire is not moving parallel
relative to the lines of flux (0°), otherwise no lines of flux will be cut and
no voltage will be generated. The movement can, however, be somewhere in
between parallel and perpendicular (90°) relative to each other; then some
lines of flux will be cut and a proportional amount of voltage will be
generated.
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