Induced emf's: Faraday's Law & Lenz's Law

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14 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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Induced
emf’s
: Faraday’s Law


Oersted’s

discovery of
currents producing
magnetic fields begged the
question: can magnetism
produce electric fields?


Michael Faraday conducted
a series of experiments to
determine this


Consisted of two
separate
coils of wire


One influences the other via
magnetic fields




Stationary magnets near a conducting wire = no current


Moving magnet = current


Direction of movement determines direction of current


Faster movement = greater current


Wire loop is moved while magnet stationary


current





Relative motion of loop
& magnet produces
current


Loop parallel to B
-
field


no current


Induced current can be
generated from another
loop close to it


As current “ramps up”,
the B
-
field is changing
(like a moving magnet)
inducing a current in
second loop




Induced current only
when current (and
therefore B
-
field) are
changing


Electromagnetic
induction


Creates and
electromotive force
(
emf
)


energy capable
of moving charges (like
a battery does)


Induced
emf

depends
on the number of B
-
field lines passing
through loop

Magnetic Flux


The number of field lines passing through loop
depends on orientation


Relative measure of field lines passing through
particular loop area = magnetic flux,
Φ



Φ

= B A
cos

θ


Unit: T • m
2

=
weber

(
Wb
)

Faraday’s Law of Induction


Emf

induced in coil of N
loops depends on time
rate of change of
magnetic flux


Faraday’s Law of
Induction




ε

=
-

N
ΔΦ

/
Δ
t


Induced
emf


Minus sign indicates
polarity (direction) of
emf


Applications


A dynamic microphone


a diaphragm vibrates and is
connected to a magnet, the coil surrounding it induces
and
emf

that is amplified


Electric guitar


the “pickup” is a permanent magnet
close enough to the strings to magnetism, the vibrating
string changes magnetic flux in coil, that is amplified




Computer disc drives, credit
card readers & audio tape
players


and electromagnet
produces regions of
magnetic polarity on a
magnetic material


When information is to be
read out, magnetic material
is moved past a gap in an
iron core wrapped in coils


The field increases or
decreases causing a
changing magnetic flux