How Does the Source of EMF Change with Movement in a Magnet and Coil System?

[Roodles01]

EDIT! Hmmm, sorry.
What is the difference between the origin of the emf in a small current loop when;
a) a magnet is stationary & the coil moves,
b) the coil is stationary & the magnet moves


This was the original message, now realized to be incorrect.
A current is generated when a magnet & coil come close to one another, but is there any difference between;
a) when a magnet is stationary & the coil moves,
b) when the coil is stationary & the magnet moves.

Thank you.

 

[Simon Bridge]

What is the difference between the origin of the emf in a small current loop when;
a) a magnet is stationary & the coil moves,
b) the coil is stationary & the magnet moves

Um... nothing.
I think the only way to tell the difference is if one or the other is accelerating.

 

[CompuChip]

That's exactly why it's called EMF. In the old days, electricity and magnetism were seen as two completely different things. It is due to a bunch of 19th century physicists - notably Farady and Maxwell - that we found out that they are really the same thing.

As Simon remarks, the idea that the laws of physics should look the same to observers at constant speed relative to each other (because each of them can claim that he is at rest and the other is not) imposes quite heavy (mathematical) restrictions, which eventually lead to Maxwell's laws.

 

[dydxforsn]

According to the beginning of this episode of the Mechanical Universe, this situation was a starting point for Einstein's special theory of relativity (fast forward to 1 minute 22 seconds): http://www.learner.org/vod/vod_window.html?pid=613

 

[Roodles01]

Is it different mathematically, though?
I'm under the impression that it can be shown to be different in terms of line integrals, but not qite sure how.

 

[arydberg]

Maybe this is what you need:

http://www.physics.ucla.edu/demoweb...cs/special_relativity/special_relativity.htmlThe bottom line is that this can be thought of as a relativity experiment. In one frame there is only an electric field. In the other an magnetic field and a electric field exist.

 

[MS La Moreaux]

I believe that when the coil moves, motional emf is induced in it. When the magnet moves, because its field is not uniform, its magnitude changes with time at the position of the coil. This is accompanied by an electric field which results in an emf in the coil, as given by the Maxwell-Faraday Law, which is one of Maxwell's equations. If the magnet had a uniform field, its motion would not result in a change in magnitude at the position of the coil and there would be no emf. But, if instead, the coil moved in this uniform magnetic field, an emf would be induced in it.

 

[marcusl]

There is no difference, as Simon says.

I believe that when the coil moves, motional emf is induced in it. When the magnet moves, because its field is not uniform, its magnitude changes with time at the position of the coil. This is accompanied by an electric field which results in an emf in the coil, as given by the Maxwell-Faraday Law, which is one of Maxwell's equations. If the magnet had a uniform field, its motion would not result in a change in magnitude at the position of the coil and there would be no emf. But, if instead, the coil moved in this uniform magnetic field, an emf would be induced in it.

This is incorrect.

 

[Simon Bridge]

If the magnet had a uniform field, its motion would not result in a change in magnitude at the position of the coil and there would be no emf.

You can still induce a current in a uniform B field by changing the area of the loop perpendicular to the magnet ... you can do this with a stationary loop and a moving magnet by changing the angle of the magnet and the other way by changing the angle of the loop. It is how modern electricity generators typically work. The magnitude of B everywhere on, around, or within, the loop need not change.

 

[marcusl]

True, coil rotation in a uniform field will induce an emf. But it still doesn't matter whether it is the magnet or the coil that moves, contrary to what MS La Moreaux stated.

 

[MS La Moreaux]

A stationary coil in a constant magnetic field experiences no emf. The motion of the magnet is irrelevant. It only becomes relevant if the motion of the magnet results in a changing magnetic field.

 

[Simon Bridge]

True, coil rotation in a uniform field will induce an emf. But it still doesn't matter whether it is the magnet or the coil that moves, contrary to what MS La Moreaux stated.

That's what I said ;)

The statement of belief in post #7 was that if the magnitude of the B field does not change then there is no induced emf. You asserted that this was incorrect and I sought to support your assertion by providing an example. Sorry for the confusion.

 

[arydberg]

All that is required for an emf is that the wires of the coil "cut" the lines of the magnetic field.

The field can be uniform as it is ( almost) in a d'arsendal meter movement.

 

[marcusl]

A stationary coil in a constant magnetic field experiences no emf. The motion of the magnet is irrelevant. It only becomes relevant if the motion of the magnet results in a changing magnetic field.

This again is slightly off the mark. It is not the field at the loop that counts, but the integrated flux across the loop area. An emf is produced in response to time-varying flux.

That's what I said ;)

The statement of belief in post #7 was that if the magnitude of the B field does not change then there is no induced emf. You asserted that this was incorrect and I sought to support your assertion by providing an example. Sorry for the confusion.

Likewise sorry if I misunderstood. You and I are on the same page.

 

[Roodles01]

Thank you all for the input.

As with many things which seem trivial in the definition there are always different perspectives & appreciate the constructive arguments here.

 

[arydberg]

So could someone explain how a radio speaker works where the field is radial. The coil is perpendicular to the field and a current causes a force perpendicular to both Where is the chance in flux per unit time?

 

[Simon Bridge]

As with many things which seem trivial in the definition there are always different perspectives & appreciate the constructive arguments here.

Reality is not a matter of opinion, and perspective is a matter of geometry. To see someone elses POV, you just need ruler and compass.

So could someone explain how a radio speaker works where the field is radial. The coil is perpendicular to the field and a current causes a force perpendicular to both Where is the chance in flux per unit time?

I don't think that description is correct for the speakers I know about - but I may be misunderstanding you - do you have a reference?

Anyway:
The change in flux comes from the signal - as you know.
Under the speaker cone is an electromagnet in which the current varies with the signal, varying the magnetic field-strength, and thus the flux.
http://electronics.howstuffworks.com/speaker6.htm

 

[arydberg]

Your source is overly simplistic. The magnet is at the back of the speaker but the lines of magnetic flux are conducted through soft iron to a Cylindrical gap. A voice coil exists within this gap. The voice coil moves perpendicular to both the radial field and the direction of the coil.

These same voice coil movements were used in the early 8 inch floppy disks.

 

[Simon Bridge]

Your source is overly simplistic.

So show me your source.

 

[arydberg]

Take apart a speaker.

 

[marcusl]

We have been discussing induced EMF's, which are described by Faraday's law. This is a new topic and belongs in a new thread, because loudspeaker operation depends on the Lorentz force $$\vec{F}=q\vec{v}\times\vec{B}$$
It describes how the force is orthogonal to both current I=qv and field B.

 

[MS La Moreaux]

Let us shed a little light on the subject by considering a case simpler than a coil, namely the Faraday Paradox. This employs two disks, say of the same size. One is made of copper and the other is a magnet with its faces the poles. These disks are arranged face to face, close but not touching. Each is mounted on an axle like a wheel and the axles are colinear. If the copper disk is spun while the magnet is stationary, a non-electrostatic emf appears between the copper disk's center and rim. If the magnet is spun and the copper disk remains stationary, there is no emf in the copper disk.

 

[arydberg]

There seems to be two effects here. One where a coil is put into or out of a magnetic field and a EMF is produced.

But the loudspeaker has a radial magnetic field in the shape of a cylinder and a EMF is produced when the coil is moved up or down . The field inside the coil is the same before and after the movement.

 

[Simon Bridge]

Take apart a speaker.

Found one - it looks just like the link I gave you above.
There is a cylindrical magnet, with a coil on a diaphragm. Input wires go to the coil. The field near the coil is not radial.
Note - the speaker back sticks nicely to my fridge.

Please provide a clear reference of what you are trying to describe.

But the loudspeaker has a radial magnetic field in the shape of a cylinder and a EMF is produced when the coil is moved up or down .

Sounds like you are thinking of a microphone - you speak into a diaphragm, which vibrates, moving a coil back and forth next to a magnet, which induces an emf, producing the signal.

A speaker goes the other way - a varying current is supplied directly to the coil. No need to induce an emf to make it work.
This changes the magnetic field of the coil, changing the amount that it is attracted to the magnet. The field inside the coil changes with the input signal.

 

[arydberg]

You have to take it apart. Cut the voice coil wires. Use a razor blade and cut the cone loose, Cut around the spider and lift out the cone & voice coil assembly and you will see what i mean.

 

[Simon Bridge]

Nope - done it - don't see what you mean. The speaker I got off an old computer chassis looks like the diagram in the link after cutting it open.

I can investigate the fields with a small compass and I can trace the wires - even put an ammeter on the wires to confirm that there is a current flowing in the coil in response to the input signal. Using a low frequency input I can confirm that the current causes the movement in the speaker, and the compass confirms that the current changes the magnetic field of the coil.

The cylindrical magnet does not have a strictly cylindrical field - the field lines wrap around it like I expected.

Probably I have misunderstood your description.
That is why I am asking for a reference, or perhaps you can rephrase your question to highlight the bit that puzzles you.

Or maybe you can answer for me:
Why would you need to induce an emf as part of the normal operation of a speaker?
Surely the current from the output of the sound-system changes the flux from the coil?

 

[arydberg]

see here
http://www.h2wtech.com/Pages/Voice-Coil-Linear-Actuators.aspx?gclid=CIupsrOlvLUCFUiRPAodlAcAPw

 

[Simon Bridge]

see here
http://www.h2wtech.com/Pages/Voice-Coil-Linear-Actuators.aspx?gclid=CIupsrOlvLUCFUiRPAodlAcAPw

That site says:

Voice Coils operate on the principal of the Lorentz Force Equation ... Simply stated, a current carrying conductor placed in a magnetic field will have a force exerted upon it. This force is proportional to the direction and magnitude of the current and the flux density field. Since the permanent magnet flux density field is fixed, the direction of the linear displacement depends on the polarity of input current. The amount of force that is produced is directly proportional to the magnitude of the input current.​

... Isn't that what I've been saying... the changing flux is created by the current in the coil?
If they talk about inducing an emf, I missed it.

How do these things support:

So could someone explain how a radio speaker works where the field is radial. The coil is perpendicular to the field and a current causes a force perpendicular to both Where is the chance in flux per unit time?

? Answer: in the coil.

Were thinking of a magnet constructed as a hollow cylinder, where one pole is inside the cylinder and the other outside?

 

[marcusl]

Were thinking of a magnet constructed as a hollow cylinder, where one pole is inside the cylinder and the other outside?

More like two concentric cylinders, with the voice coil in between.

The concept of flux change is not central to loudspeaker operation, because the motion results from the Lorentz force instead.

 

[Simon Bridge]

The concept of flux change is not central to loudspeaker operation, because the motion results from the Lorentz force instead.

Well yes ...
likewise induced emf.

 

[arydberg]

Marcusl, Can you make any sense out of this?

 

[Crazymechanic]

So interesting to see that people who talk about particles and forces on daily basis which are hidden from the naked eye then come back and have a hard discussion about a thing we have been using pretty successfully for the past 100+ years.

Well basically every loudspeaker behaves also like a microphone a little bit.I just tested that as I have plenty of them where i write from a simple everyday multimeter shows reading of 1-2 volts when moving the voice coil by hand.Every speaker is also a tiny microphone.And vice versa. But this is not of high importance as the speaker is designed to move according to the input voltage/amperage.And as every electromagnetic apparatus creates a back emf that tries to resist the change through the magnetic field lines.
The magnetic field of the magnet of the speaker has a south and north.One pole is where the coil enters from the face of the speaker the other pole is from the back where the sticker of brand and parameters usually are located.
The metal ends are just to end the magnet lines and the middle metal cylinder is to make the other pole of the magnet the one in the back to meet the one in the front and in between is the air gap and the coil is moving through that place of field lines going from N to S and pushes against those lines in a direction according to the applied polarity so the polarity changes in the exact frequency at which the music is playing so you hear sound.
So it's pretty similar to a linear motor that's moving back and forth at a certain frequency and in fact they do that all the time in assembly lines and other places.only the speaker is round because of it's properties and physics.

The speaker could still operate without the central metal cylinder but it would be very weak as the strength comes from bringing the opposite poles of the magnet together in a gap that is as tightly spaced as possible hence the magnetic force falls of with distance.And hence the metal is a good conductor of magnetic field it is used to bring the back pole of the magnet up front so that the could would have "two arms" with which to push itself instead of one long and skinny.Sorry for the somekind simple and maybe sometimes stupid explanation but I hope I did my best.

 

[arydberg]

Sounds good to me.

 

[marcusl]

Marcusl, Can you make any sense out of this?

Your description of loudspeaker operation was accurate--the cylindrical voice coil sits within a radial magnetic field created by the magnet and its iron pole pieces. Current is driven through the coil by an audio amplifier, producing a Lorentz force parallel to its axis. Since the coil is attached to the speaker cone, this moves the cone in or out.

The question of induced emf is of little concern in this application except, perhaps, to the amplifier designer. Any coil has inductance and will produce a back emf in response to a changing current, and this appears across the amplifier terminals. In effect, the amplifier must be capable of driving a reactive load. I think the reactance is quite small, however, so the speaker looks largely like a 4 or 8 ohm resistive load.

This is a different situation from the original question in this thread, which involved an open-circuit coil moving in a magnetic field. In that case, the induced emf is of interest and, conversely, there is no current and no Lorentz force. That's why I think these two questions should have been asked in two separate threads.

If this is still unclear, please specify the point of confusion so we can clear it up.

 

[berkeman]

Thread closed for Moderation...

Thread re-opened. Let's be very clear and accurate in our posts in this thread please.

And, thread is closed again. Thanks folks.