Faraday`s Law
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Transcript Faraday`s Law
Faraday’s Law
Area Change
The sliding bar creates an emf
by changing the area in the
magnetic field.
• Constant magnetic field
The potential was due to the
time rate of change of area.
BA
V vBL
t
Field Change
An emf can also be generated
by changing the magnetic field.
The time rate of change of the
field through a fixed loop
provides the potential.
A B
V
t
Field Orientation
The emf depends on the
change in field or the change
in area.
• Area perpendicular to the field
This suggests that the product
of the field and area
perpendicular matters.
A B AB cos
V
t
t
Magnetic Flux
The product of the field and area perpendicular to the
field is the magnetic flux.
M AB cos
The magnetic flux is measured in webers.
• 1 Wb = 1 T m2
The magnetic field can be thought of as a flux density.
M
B
A
Faraday’s Law
The flux can be used to get
the induced emf.
• Sign indicates polarity
M
t
This is Faraday’s Law of
induction.
For multiple turns the emf is
multiplied.
• N turns of wire
• N is the flux linkage
M
N
t
Coil Flux
A circular flat coil has 200
turns of wire with a total
resistance of 25 W and an
enclosed area of 100 cm2.
There is a perpendicular
magnetic field of 0.50 T that is
turned off in 200 ms.
Find the current induced in the
coil.
This problem has three parts.
To get the current from the
resistance the voltage is
needed.
To get the voltage the flux is
needed.
• Flux linkage works, too
Find the flux first.
Flux to Current
The magnetic flux is = BA.
• = (0.50 T)(100 cm2)
• = (0.50 T)(0.010 m2)
• = 0.0050 T m2
The induced current comes
from Ohm’s Law.
• I = V/R
• I = (5.0 V) / (25 W)
• I = 0.20 A
The change in flux is
negative since it is turned off.
The induced emf is
E = N /t
E = -(200)(-0.0050 Tm2) /
(0.20 s)
• E = V = 5.0 V
•
•
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