EMF Induction: How Flux Change Creates Potential

[rude man]

The wire is perpendicular to the flux but the v vector is parallel

If you magnet has a radially symmetric B field about its extended axis and you move your wire with its middle along the extended axis, no part of any flux line is ever perpendicular to the direction of the wire and the velocity. So you'd get zero emf.

But if you deviated from the axis (say you moved parallel to the axis but 2cm radially away from it) you would cut flux lines and therefore generate an emf.

 

[Entanglement]

If you magnet has a radially symmetric B field about its extended axis and you move your wire with its middle along the extended axis, no part of any flux line is ever perpendicular to the direction of the wire and the velocity. So you'd get zero emf.
But if you deviated from the axis (say you moved parallel to the axis but 2cm radially away from it) you would cut flux lines and therefore generate an emf.

If a coil is perpendicular to the flux and its velocity vector is parallel I suppose there is an emf, isn't that right ??

 

[rude man]

If a coil is perpendicular to the flux and its velocity vector is parallel I suppose there is an emf, isn't that right ??

I assume you mean the coil's axis is parallel to the B field?
If the flux is not changing in time or space there is no emf generated.
If B is caused by a permanent magnet then there is an induced emf since the velocity generates dB/dt inside the coil. That's how Faraday first found induction. The magnet's B field gets stronger as you approach it.

 

[Entanglement]

I assume you mean the coil's axis is parallel to the B field?

If the flux is not changing in time or space there is no emf generated.

If B is caused by a permanent magnet then there is an induced emf since the velocity generates dB/dt inside the coil. That's how Faraday first found induction. The magnet's B field gets stronger as you approach it.

That's my problem, I understand the way the emf is induced when a wire cuts the flux such that the electrons accumulate at a region creating a potential difference, but the example where the the axis of the coil is parallel to the field moving towards a magnet, how faraday discovered induction, since the electrons velocity ISN'T PERPENDICULAR to the field I'm unable to apply the same idea of the fist example when the wire cuts the field where the electrons' velocity is PERPENDICULAR to the flux

 

[Drakkith]

The field lines generated by the magnet are not parallel to the coil's axis. They emerge from the magnet in loops like the following examples.

http://figures.boundless.com/17182/full/figure-24-02-01.jpe
http://oceans582.files.wordpress.com/2013/05/lenz.jpg

 

[Entanglement]

The field lines generated by the magnet are not parallel to the coil's axis. They emerge from the magnet in loops like the following examples.
http://figures.boundless.com/17182/full/figure-24-02-01.jpe

http://oceans582.files.wordpress.com/2013/05/lenz.jpg

Thanks a lot, you have corrected my misunderstanding,

But my final question is that in case of plunging the magnet in the coil or pulling it out, the velocity of the electrons is zero, how is there a "Bev" force acting on the electrons,
Does this have something to do with relativity ??

 

[Drakkith]

It's not about the velocity, it's about the changing magnetic field. Relative motion between the magnet and the coil just causes the magnetic field to change and an EMF is produced. The same effect can be produced using an electromagnet and an alternating current even though there's no relative motion between the electromagnet and the coil. This is how a transformer works.

 

[Entanglement]

It's not about the velocity, it's about the changing magnetic field. Relative motion between the magnet and the coil just causes the magnetic field to change and an EMF is produced. The same effect can be produced using an electromagnet and an alternating current even though there's no relative motion between the electromagnet and the coil. This is how a transformer works.

Isn't emf induced due to Bvq force acting on the electrons creating a potential difference ?

 

[sathwik]

If you magnet has a radially symmetric B field about its extended axis and you move your wire with its middle along the extended axis, no part of any flux line is ever perpendicular to the direction of the wire and the velocity. So you'd get zero emf.

But if you deviated from the axis (say you moved parallel to the axis but 2cm radially away from it) you would cut flux lines and therefore generate an emf.

As the wire moving along the direction of the magnetic field there will be no change in the flux ,hence change in flux is zero . Therefore there is no e.m.f is induced in it

 

[sathwik]

Isn't emf induced due to Bvq force acting on the electrons creating a potential difference ?

Your case is possible in the rod or wire if and only if the velocity vector is perpendicular to the magnetic field

 

[sathwik]

If a coil is perpendicular to the flux and its velocity vector is parallel I suppose there is an emf, isn't that right ??

As the coil is perpendicular to the magnetic field its surface vector is parallel to the magnetic field so there is no change in flux hence change flux is zero therefore induced emf zero

 

[Entanglement]

Your case is possible in the rod or wire if and only if the velocity vector is perpendicular to the magnetic field

But isn't Bvq the main reason for induction ??

 

[Drakkith]

Isn't emf induced due to Bvq force acting on the electrons creating a potential difference ?

Yes, but that's not the only way to find the EMF. You can also find it by finding the rate of change of the magnetic flux.

 

[Drakkith]

But isn't Bvq the main reason for induction ??

It is my understanding that the "reason" for induction is that the magnetic field is changing, either due to motion between the magnet and the conductor, or because of an actual increase or decrease in the magnetic field by something like an electromagnet. I could be incorrect, so if so, someone say something.

 

[Entanglement]

I think Changing in magnetic flux is a fact not an explanation

 

[Drakkith]

I think Changing in magnetic flux is a fact not an explanation

Perhaps. But then how does a transformer use induction if there is no motion?

 

[Entanglement]

Perhaps. But then how does a transformer use induction if there is no motion?

That's my question, I know the part that a time varying flux induces emf, but I still can't get why that's true

 

[Drakkith]

That's my question, I know the part that a time varying flux induces emf, but I still can't get why that's true

Ah, I see the dilemma now.

 

[nsaspook]

That's my question, I know the part that a time varying flux induces emf, but I still can't get why that's true

What at a basic level does a 'time varying flux' mean? Is it 'energy' of some type and how is that force expressed as potential electrical energy in a conductor with free electrons as charge carriers.

 

[Drakkith]

I can only refer you to the maxwell-faraday equation which states that a time-varying magnetic field is always accompanied by a spatially-varying electric field.

http://en.wikipedia.org/wiki/Electromagnetic_induction#Maxwell.E2.80.93Faraday_equation

Perhaps someone else has a more fundamental answer.

 

[mikeph]

No. Faraday's law covers this whole thread. a dB/dt is accompanied by a curl(E) of the same magnitude and opposite direction. That's it.

 

[Entanglement]

No. Faraday's law covers this whole thread. a dB/dt is accompanied by a -curl(E) of the same magnitude.

How does that varying flux create an electric field ?

 

[jtbell]

How does that varying [magnetic] flux create an electric field ?

How does a charge produce an electric field?

How does a current produce a magnetic field?

All these come out of Maxwell's equations, which are a starting point for classical electromagnetism as Newton's laws of motion are a starting point for classical mechanics.

And there's more! Maxwell realized that the equations as he originally formulated them were mathematically inconsistent, and to resolve that, he proposed adding a new term to one of them, which predicted that:

A varying electric flux creates a magnetic field.

This led him to predict that self-propagating waves of electric and magnetic fields should exist, and that they should travel at a speed which turned out to be equal to the speed of light! (within experimental uncertainties in electromagnetic experiments and in the speed of light)

 

[Drakkith]

I don't think there is an easy answer for this. I believe a full understanding would involve getting into the basic relationships of electric and magnetic fields and how they depend upon frames of references.

 

[Entanglement]

How does a charge produce an electric field?
How does a current produce a magnetic field?
All these come out of Maxwell's equations, which are a starting point for classical electromagnetism as Newton's laws of motion are a starting point for classical mechanics.

Maxwell's equations are way more difficult than my level, I was just seeking for a basic explanation.

 

[jtbell]

Maxwell's equations are the basic explanation. Sorry.

 

[Entanglement]

Maxwell's equations are the basic explanation. Sorry.

I understand how an emf is induced when a wire cuts magnetic flux lines.
The motion of the electrons with the wire induces a magnetic field which interferes with external magnetic field so the electrons are forced to move along the wire, leading to the accumulation of the electrons at one end of the wire, creating a potential difference. I thought that induction in coils should be somehow similar to induction in a wire cutting flux lines.

 

[nsaspook]

I thought that induction in coils should be somehow similar to induction in a wire cutting flux lines.

Two phenomena

http://en.wikipedia.org/wiki/Faraday's_law_of_induction#Faraday.27s_law_and_relativity
http://en.wikipedia.org/wiki/Relativistic_electromagnetism

 

[sathwik]

But isn't Bvq the main reason for induction ??

Bvq is also a reason for induction
There is also other case where the emf is induced due to rate of change of area

 

[rude man]

Maxwell's equations apply to all stationary e-m phenomena, including how a change in B generates an emf. They are based on EXPERIMENTAL EVIDENCE and must be accepted. There is no other way to explain induction in non-moving media.

Bvq is the magnetic force on a charge in motion. The emf = Blv law stems directly from that. (Simplified, E = emf/l = F/q = qvB/q so emf = Blv).

Maxwell's equation on which stationary-media emf generation is based is ∇xE = -∂B/∂t.
But for moving media this has to be expanded to ∇xE = ∇x(vxB) - ∂B/∂t. The Blv term comes from the first term on the right.

I know you don't have the math background to understand these equations but you should at least be able to appreciate the fact that there are two separate induction mechanisms for generating an emf.