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

Mécanique

14 nov. 2013 (il y a 4 années et 11 mois)

99 vue(s)

Induced
emf’s

Oersted’s

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

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
)

Emf

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

Induction

ε

=
-

N
ΔΦ

/
Δ
t

Induced
emf

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
players

and electromagnet
produces regions of
magnetic polarity on a
magnetic material

When information is to be