3/28/2016
On Inductance, Lenz’s Law, and Faraday’s Law.
Homework: pg 700: 1 & 3
The magnetic flux depends upon the number of
field lines through a given area.
 If you increase the area the flux will increase.
◦ What happens for the opposite?

If you increase the magnetic field strength
you will increase the flux.
◦ What happens for the opposite?

The magnetic flux is the product of the area
perpendicular to the magnetic field.
◦ F = BA cos(q)
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3/28/2016
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F = BA
F = magnetic flux, unit is Weber, Wb = Tm2
B = magnetic field strength, unit is Tesla, T
A = area, unit is m2
Inductance is a measure of the amount of magnetic
flux produced for a given electric current.
 L = F/I
 L = inductance, unit is Henry, H
 F = magnetic flux, unit is Weber, Wb
 I = current, unit is Ampere, A
L = NF/I
N = Number of coils, no unit
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The effect of inductance can be understood
using a single loop of wire as an example. Of a
voltage is suddenly applied between the ends of
the loop of wire, the current must change from
zero to non-zero. A non-zero current induces a
magnetic field by Ampere’s law. This change is
the magnetic field induces an emf that is in the
opposite direction of the change in current. The
strength of this emf is proportional to the change
in current and the inductance. When these
opposing forces are in balance, the result is a
current that increases linearly with time where
the rate of this change is determined by the
applied voltage and the inductance.
Faraday’s law states an electromotive force, Emf or E or
EMF, produced along a closed path is proportional to
the rate of change of the magnetic flux through any
surface bound by the path. An electrical current will
flow in any closed conductor when the magnetic flux
through a surface bounded by the conductor changes.
This applies whether the field itself is changes in
strength of the conductor is moved through it.
EMF = Emf = E = - F/t = - (DBA)/t = - B(l w)/t = - Blv
 E = electromotive force, unit is voltage, V
 F = magnetic flux, unit is Weber, Wb
 t = time, unit is second, s
 l = length of wire, unit is meter, m
 v =velocity (speed), unit is meter/second, m/s
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
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EMF = Emf = - NF/t
N = Number of coils, no unit
An Emf is an induced voltage due to the
changing magnetic flux.
Lenz’s law will give the direction of the
induced Emf.
Lenz’s law states the induced Emf will oppose
the changing magnetic flux.
Since the direction of the Emf is opposing the
change in magnetic flux, the equation has a
negative sign.
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