# Electromagnetic Induction

by Roodles01
Tags: coil, electromagnetic, induction, magnet
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 P: 75 Take apart a speaker.
 Sci Advisor PF Gold P: 2,063 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.
 P: 79 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.
 P: 75 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.
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 Quote by arydberg 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.
 P: 75 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.
 Homework Sci Advisor HW Helper Thanks P: 12,463 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 feild - 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?
 P: 75
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 Quote by arydberg
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?

Were thinking of a magnet constructed as a hollow cylinder, where one pole is inside the cylinder and the other outside?
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 Quote by Simon Bridge 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.
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 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.
 P: 75 Marcusl, Can you make any sense out of this?
 P: 853 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.
 P: 75 Sounds good to me.
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 Quote by arydberg 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.