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Transcript
Electricity & Magnetism
Michael Faraday discovered this idea in 1831 and it has been the basis for all
generators & dynamos. Whenever a magnet moves into a current carrying coil, or a
current carrying wire moves through a magnetic field, it induces a voltage, causing a
current to flow.
The strength of the induced current depends upon:
 The speed of movement
 The magnetic field strength
 The number of turns on the coil
Suppose a magnet is moved at a uniform speed into a current carrying coil of N turns.
Fleming’s RIGHT HAND RULE tells us the direction of the induced current.
“The induced e.m.f. in a circuit is equal to the rate of change of flux linkage (i.e.
change of total magnetic flux cut through, d= d(BA) )”.
In symbols:
 = -Nd/dt
measured in Volts

the induced voltage acts so as to oppose the
change which produces it.
This is known as LENZ’S LAW.
V
V
This idea is used in ELECTROMAGNETIC BRAKES on trains:

As the disc rotates a current flows to/from
the edge of the disc, inducing an e.m.f.
This is fed through the cable, producing
a magnetic field that opposes the original
one, slowing the disc down.
If we have a straight conductor, such as a metal rod, moving through a magnetic field,
at a speed, v, then the area it cuts through in a time, dt, is equal to the length of the rod
(l) multiplied by the distance it travels (d), so: A = lvdt.
Movement
V

 = -d/dt
= -BA/dt
= -B(lvdt)/dt
 = -Blv
for a moving, straight conductor.
When an aeroplane flies across the Earth’s magnetic
field it actually generates a small e.m.f. across its wingtips.
Magnetic Field
V
V V V V V V

Induc ed
Current
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