Download MAGNETIC INDUCTION AND FARADAY`S LAW

MAGNETIC INDUCTION AND FARADAY’S LAW
Joseph Henry and Michael Faraday
“It is possible that a magnetic field can produce an electric current.”
An induced emf is produced by a changing magnetic field.
Electromagnetic Induction
Current Induced by Magnet Moving Near Coil
Magnetic Flux
r r
Φ B = B⊥ A = BACosq = B ⋅ A
r r
Φ B = ∫ B ⋅ dA
( Total Flux )
Faraday’s Law of Induction
e
i=
R
N loops
r r
ΦB = B⋅ A
dΦ B
e =−
dt
Induced emf E is caused by the change in
the magnetic flux.
How about minus sign?
Lenz ‘ s law
Lenz’s Law
An induced emf is always in a direction that “opposes” the
change in the flux.
• flux increasing
• flux decreasing
induced emf will create a current
to reduce it.
induced emf will create a
current to increase it.
Three ways to change the magnetic flux.
1) By changing intensity of the magnetic field B
2) By changing area A of the loop.
3) By changing orientation of the loop with respect
to the field (changing Q).
What is the direction of the induced current?
A square coil of wire with side 5.0 cm contains 100 loops and is perpendicular to a
uniform 0.60 T magnetic field. It is quickly and uniformly pulled from the field to a
region where B drops to zero. At t=0, the right edge of the coil is at the edge of the
field. It takes 0.100 s to move the whole coil into the field free region.
(a) Find the rate of change of the flux. (b) Find the emf and induced current. (c)
How much energy is dissipated in the coil if its resistance is 100 ohms. (d) What
was the average force required?
EMF Induced by a moving conductor
EMF Induced by a moving conductor
The distance the rod travels:
dx
dx = v ⋅ dt
The area swept by the rod:
Change in the flux:
dA = l ⋅ vdt
df B = B ⋅ dA = Blvdt
df B Blvdt
e=
=
= Blv
dt
dt
Magnitude of emf
AC GENERATOR