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

[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.