# Changing magnetic field

1. Aug 25, 2009

### rajeshmarndi

why does changing magnetic field induced current in a conducting wire?

2. Aug 25, 2009

### TheDestroyer

The change in magnetic field doesn't create a current, but the change in the flux through a loop creates a current, because the loop creates a current that tries to cancel the effect of the changing magnetic field flux, i.e. the magnetic flux in the loop is supposed to be always zero, this is what the loop tries to do, this is called Lenz law.

If you have a more specific question, just ask.

Good luck :)

3. Aug 25, 2009

### rajeshmarndi

why is it that the loop is supposed to be always zero, if u could explain further, thanks

4. Aug 25, 2009

### TheDestroyer

Well, this question is like "why does current flow in a wire when you apply a potential difference on its terminals".

It's just because the system always wants to minimise its energy, therefore it does everything it can to remove the effect of any perturbation you apply.

Hope this answers :), if you want to more clarification, ask again :) you're welcome

5. Aug 25, 2009

### rajeshmarndi

thanks "TheDestroyer"!
if in a uniform magnetic field, a wire perpendicular and perpendicular to the plane of the magnetic field is moved in a line perpendicular and in the plane of the magnetic field. So that there will be no change in the magnetic flux in the wire as it is moved. Does the current is still induced in the wire as it is moved in the mag field.

6. Aug 25, 2009

### TheDestroyer

This is a different situation, where "Lorentz force" happens on the charges in the wire. You can find Lorentz force here

http://en.wikipedia.org/wiki/Lorentz_force

And it's not a current that is inducted, it's that the wire polarises, meaning negative charges (i.e. electrons) accumulate in 1 side, and so there happens to be a static charge in the wire, not a current.

Hope this answers :) if you still have doubts, don't hesitate asking :)

7. Aug 26, 2009

### Bob S

Hi Rajesh-
1) Faraday's Law, in differential form, is
Curl E = -dB/dt
A changing magnetic field creates an azimuthal electric field. This has nothing to do with inducing a current.
2) J = σ E
An electric field in a conductor (conductivity σ) will produce a current. This current is not a requirement of the Faraday Induction Law. The current will be present if an electrical conductor linking the dB/dt s present.

Bob S

Last edited: Aug 26, 2009
8. Aug 28, 2009

### cmos

Well if you're going to be fastidious in saying that it's not the (changing) magnetic field but rather a changing flux, then I might as well point out that it's not a current that's created but rather an emf. The emf in turn drives the current. :tongue2:

Although, by Faraday's law, I have no problem saying it's the changing magnetic field. The flux just makes it easier to solve certain problems.

9. Aug 28, 2009

### bjacoby

Well, you SHOULD have a problem. A changing magnetic field (or flux) does NOT induce an emf! The two are related but not causal. An induced emf is caused by a changing current! The changing current produces an E field parallel or anti-parallel to itself at a distance falling off as 1/R. The changing current ALSO produces a magnetic field traveling away from itself at the speed of light as well. Hence the induced emf and magnetic field are related to each other but the magnetic field does not "cause" the emf. The emf is a real electric field capable of accelerating charges. Hence the emf thence creates a current if such a path is allowed.

Neither the changing magnetic field or changing magnetic flux creates the emf.

10. Aug 28, 2009

### Bob S

So the emf is not produced by the changing magnetic field flux? So Curl E = - dB/dt (Faraday's Law) is not sufficient to produce an emf? Suppose I pulled a strong permanent magnet out of a solenoid with only a voltmeter attached to it. Will I see an induced emf then?

11. Aug 28, 2009

### cabraham

The emf does not "drive" the current. Every time induction comes up, we get a "cause/effect" debate. It is universally known that E & H cannot exist independently under time-varying conditions. The emf & the mmf are functionally related through the resistance per Ohm's law.

I & V are produced in unison. Neither "causes" the other. Is this helpful? BR.

Claude

12. Apr 5, 2011

### bdfischer

why does the flow of electrons through a wire have any effect on bringing the flux back to zero

13. Apr 8, 2013

### MGS.Rajan

The amount of Induced EMF when the magnetic field linked with the coil changes is ...................
(1.Magnetic induction, 2 Current produced, 3. EMF Produced, 4. Changing Current)