# Magnetic induction

#### boris16

hello

*We are able to explain why current is induced when piece of wire inside MF is pulled with force F[pull] ( wire is thus moving perpendicular to MF or at some angle ):

Example A1:

If we put a wire inside MF and we start moving a part of wire called L ( drawn in green ) with length b to the right with force F[pull], then MF will push each free electron inside L with force F[M] = e * v * B in downward direction ( v is velocity, B is density of MF and e is charge ). That creates current I. BTW, those little X marks represent magnetic field lines going into the screen:
http://img221.imageshack.us/img221/3564/10rf0.png [Broken]

Magnetic field will ( due to induced current I ) start pulling L in opposite direction of F[pull] --> F[opposite] = -F[pull]

*We can also explain why even if piece of wire is at rest, but instead MF is moving, a current is created inside that wire:

Example A2
Voltage difference in a wire L gets induced even if wire is at rest and we bring a magnet near this wire with velocity v:

http://img75.imageshack.us/img75/4909/14uv9.png [Broken]

Since magnetic field lines are in a way moving in downward direction, it is as if charges are moving in upward direction and thus charges in a wire will be separated like in the above picture ( it’s not apparent in the picture, but magnetic field lines are almost perpendicular to wire).

Even my book says that if wire is moved parallel with magnetic field lines then there isn't any induced voltage and no current runs and thus no force opposes F[pull].
But then pages later my book suddenly tells us that induced current runs whenever there is change in magnetic flux. And it provides us with an example:

If we have two coils, C1 and C2 and if we put a ferromagnet inside C1 and run a current through C1, then magnetic field lines will go through the second coil and due to change in magnetic flux, a second coil will have an induced current:
http://img486.imageshack.us/img486/6325/feromagnetrc4.png [Broken]

a)

Now in the examples A1 and A2 one was able to explain why current was induced without the need to resort to magnetic flux lines. But how can we explain why magnetic field exerts force on charges inside coil C2? Unlike with example A2, here even if we pretend that MF is at rest and instead charges are moving in downward direction, the fact remains that charges are moving parallel to magnetic field lines and thus MF shouldn’t exert any force on charges inside coil C2, no matter how much magnetic flux is changing!

b)

IMPORTANT - Even if one accepts the fact that if strength of MF is changing that will cause induced current, but why can both cases ( one where charge is moving and other where strength of MF itself is changing ) be explained by the same formula --> U[induced] = Flux / t

Flux … change in magnetic flux
t … time

?

With wire moving through the magnetic field we can rationalize why MF created induced current. But if strength of MF itself is changing ( while charge in a wire is at rest ) there is no apparent reason, or even if there is some apparent reason, it's not due to charge weakening MF on one side and strengthening it on the other side.
So why does same formula work for both cases?

bye

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#### ranger

Gold Member
Now in the examples A1 and A2 one was able to explain why current was induced without the need to resort to magnetic flux lines. But how can we explain why magnetic field exerts force on charges inside coil C2?
There need not be any "charge" in C2 for the magnetic flux to influence. C2 can be without power, which it is. Current flowing in a wire will induce a magnetic field that is concentric around the wire. Consider this diagram:
http://sol.sci.uop.edu/~jfalward/physics17/chapter9/fieldnearwire.jpg [Broken]
When the power is turned on to the first coil, a magnetic will "spring into being", filling the space around it. And since C2 is in its vicinity, there will be induced current as the changing magnetic flux cuts across C2

IMPORTANT - Even if one accepts the fact that if strength of MF is changing that will cause induced current, but why can both cases ( one where charge is moving and other where strength of MF itself is changing ) be explained by the same formula --> U[induced] = Flux / t

Flux … change in magnetic flux
t … time

With wire moving through the magnetic field we can rationalize why MF created induced current. But if strength of MF itself is changing ( while charge in a wire is at rest ) there is no apparent reason, or even if there is some apparent reason, it's not due to charge weakening MF on one side and strengthening it on the other side.
So why does same formula work for both cases?
?
All of your cases have one thing in common, the magnetic flux being cut by the object is changing per unit time. Whether its being cut becuase of a changing magnetic field or moving object. This is all thats needed to induce a current using a magnetic field.. You dont need to complicate things.

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