Download Faraday`s Law of Induction

Faraday’s Law of Induction
 Faraday discovered that a moving conductor (perpendicularly) in a
magnetic field has a current induced to flow in it. (Faraday’s Law of
Induction relates to this, but in more detail).
 The conductor disturbs the magnetic field lines which forces the
electrons to move in the wire.
 Lenz applied the energy of conservation concept that you can’t get
something for nothing: the induced current flows in such a way as
to oppose the changing magnetic field that produced the current.
(Lenz’s Law)
 This allows for built in resistance/friction that must exist as the
electrons can’t flow freely by themselves. (No such thing as
perpetual motion).
 The induced current only exists when the magnetic field changes.
 A good example is to have a fixed coil, not connected to any
external battery source, with a changing magnetic field nearby:
Ex1) A North pole of a bar magnet is physically pushed by a keen
physics student into a coil. Determine the direction of the induced
current in the coil, using the laws of induction.
N
Step 1: Use Lenz’s law to predict the induced poles in the coil that would
oppose the changing magnetic field: the end closest to the magnet
becomes ______.
Step 2: Use the hand rule for coils (left hand for electron current) to
determine the current direction on the front of the coil as _______.
Ex 2)
S
Ex 3) The coil is rotated (by wind in a windmill) around a fixed axis.
S
N
Ex 4) A conducting rod is moved West through a magnetic field directed
down.
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Lenz’s law predicts that the induced current will flow to generate a
magnetic field that will oppose the motion of the rod. Therefore, a force
on the rod is experienced ________. Use the 3D hand rule to predict that
the current in the rod is ________.
Induced Voltage: the current flow that is induced implies that energy is
given to the charges and therefore a voltage. (Induced voltage is referred
to as a EMF (electro-motive force).)
V = LvB
V is the induced voltage, L is the length of the conductor, v is the velocity
of the conductor and B is the magnetic field strength.
In example 4, if v = 12 cm/s, B = 3.0 mT and L = 5.0 cm, what is the
induced EMF?
V = LvB
= (0.05 m)(0.12 m/s)(0.003 T)
= 1.8 x 10-5 V