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

Mechanics

Nov 14, 2013 (5 years and 1 month ago)

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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