Exploring the EMF of Guitar Pickup Coils

[yungman]

I have a discussion on how guitar pickup work, pickup is a coil with thousands of turns and magnet poles in the middle of the coil under each strings. My idea is the magnet poles magnetize a short portion of the string and when the string vibrates, it create a varying magnetic field. When the field cross the cross section area of the coil, a voltage is induced in the coil according to Faraday's law of magnetic induction. My theory is there is no difference whether the coil has magnet or only air regarding to sensing the magnetic field from the string. The magnet and the coil do two separate function. The magnet magnetize the short section of the string, the coil produce EMF from the magnetic field from the string. So as long as the string is being magnetized the same way, the coil can be air or magnet and it work the same.

I want to find out what is the difference between induced EMF of a coil with core of different materials in the presence of external magnetic field. I know

$$EMF= -\frac {\partial \Phi}{\partial t} \;\hbox { where } \Phi = \int_S \vec B \cdot d \vec s$$

From the equation, B is external magnetic field, the EMF is the rate of change in flux that pass through the cross section area of the coil.

Assume the coil has certain height and have many turns, what is the difference between the same coil with air core, a ferro magnetic material core and a magnetic core( core make of magnet).

 

[Mreilander]

In respect to the above post, what effect will an oscillating ferromagentic material above the coil have on said coils.(ie. guitar string) And what would the effect of the oscillating ferromagnetic material have on the said coils if magnetized from an outside source. (if magnets were placed above guitar string, outside of coil as opposed to within the coil)

 

[technician]

Guitar pickups must contain a magnet, a coil surrounds the magnet and any voltage induced in the coil is the voltage that is amplified.
Electric guitar strings contain steel and the vibrating steel in the magnetic field produces changes in the magnetic field through the coil... an emf is induced and amplified.
If there are no magnets there will be no magnetic field.

 

[yungman]

Guitar pickups must contain a magnet, a coil surrounds the magnet and any voltage induced in the coil is the voltage that is amplified.
Electric guitar strings contain steel and the vibrating steel in the magnetic field produces changes in the magnetic field through the coil... an emf is induced and amplified.
If there are no magnets there will be no magnetic field.

Yes we know that, question is what is the mechanism. My theory is the magnet magnetize a short section of the string and when the string vibrates, the induced magnet start moving and create a varying magnetic field and the coil sense the varying magnetic field.

My point is it is a two step mechanism not relate to each other. The coil can sense the vibration of the string as long as it is magnetized. The coil don't need to have magnet to sense the strings. In another word, you can use an external magnet to magnetize the string on top of a coil WITHOUT magnet and it works the same. Is this true?

 

[Mreilander]

http://blogs.howstuffworks.com/2010/09/02/how-does-an-electric-guitar-pickup-work/

Watch the vid, starting at 2:38, has a great graphic of it.

http://www.youtube.com/watch?v=lBAZepM5F_0&feature=player_embedded

 

[yungman]

Anyone have an opinion. The debate is about how the pickup produce the signal. I believe that the magnet pole magnetize the section of the string and when the string move, the varying magnetic field inducing an EMF in the coil. As long as the strings are magnetized, the coil will pick up the signal with or without the magnet. I do believe a ferro magnetic pole will help gather the flux to go down the coil. but whether it is magnetized or not don't make no difference.

The other theory is different, it claims the magnet establish a field pattern and the ferro magnetic string movement change the field lines and cause the signal in the coil. To me, changing the field lines don't change the amount of flux going through, the effect would have to be secondary, unlike the idea that the magnetized string moving and produce a varying magnetic field and the field is actually changing with time.