| We've expounded on the fact that an Emf
is generated by changing magnetic flux. If you change
the magnetic field through a loop, or you change the area
of a loop, or you rotate the loop in a magnetic field,
you get an Emf. But how do you explain that these changes
cause electrons or charge carriers to move and create
current? |
 |
Remember that question from the activity where you
were asked how an Emf can be generated if a loop is
moved back and forth along the axis of a solenoid?
|
The answer was that outside the solenoid,
the magnetic field is diverging, so it has a radial component
with respect to the loop. If the loop moves along the
solenoid axis, then by F = qv x B
the force is tangent to the loop, and a force pushes the
charge carriers around the loop. |
| But what about when the loop is still and
the magnetic field changes?
F = qv x B doesn't work because v = Zero. Yet,
there is a current induced in the loop, so there must
be something pushing the charge carriers around the loop. |
 |
|
If not a magnetic force, then it must be an electric
force. Remember that an electric field exerts a force
on a charge. F = qE It turns out that when you have
a changing magnetic field (actually a changing magnetic
Flux), there is an electric field induced around the
magnetic field.
∫ E • ds = εo d/dt ΦB
That electric field encircles the magnetic field,
so the electric force on a charge carrier is always
tangent to a circular path. That's why charge carriers
in the loop get pushed along and create current.
If a changing magnetic flux produces an electric field,
then if the universe is a nice place, by symmetry it
should follow that a changing electric flux produces
a magnetic field. And it does. If it feels good, it
is good. So we have......
∫ B • Ds = εo
μo
d/DTΦE
You remember our "old" Ampere's Law which
said:
∫ B • Ds = μo i
That's still true, so this new version above is the
second right side term which gives the full and complete
form of Ampere's Law.
∫ B • Ds = μo i +
μo εo
d/DTΦE
You can think about this in terms of currents --- a
conduction or "real" current (the physical
motion of "things"), and a displacement or
"fictitious" current created by a changing
electric field flux.
∫ B • Ds = μo
iconduction + μo
idisplacement
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