Pickup theory question

[David Schwab]

The permanent magnetic field does not contribute energy to the process.

So remove the magnet and what do you get? Nothing. Strum as hard as you wish. So then where is the string's energy coming from?

That's a little too pedantic Mike. The current in the coil is from the strings moving in the magnetic field. No magnet, no current.

Sure, the kinetic energy is coming from the strings, but this is a transducer, and the strings alone wont do diddly.

[FunkyKikuchiyo]

No. The energy dissipated in the load on the pickup and in the pickup coil comes from the vibrating string. The permanent magnetic field does not contribute energy to the process.

Exactly! A battery has "potential energy" while magnetism is just an arrangement of poles, a mere physical state. (that is your definition of polarity more or less, by the way) If magnets contained energy, then they'd eventually wear out and need to be charged or replaced. If there were not so, then permanent magnets would in effect be sources of infinite energy, defying the laws of physics, and also making energy companies obsolete. Goodbye oil and coal. Magnets sometimes DO need to be charged, but not because they've been drained by their magnetic load - only because they've been dropped, exposed to a different magnetic field, etc. Do you use a neo to charge alnicos? Has the neo become drained of its energy as it transfers to the alnico?

The magnet does not contribute energy, but it still needs to create the magnetic field in order for the pickup to work. That just isn't an apt analogy david... things can be necessary to a mechanism without contributing energy. Pull out the transmission from a car and the car won't move, but it isn't what is providing energy. It TRANSFERS energy in a certain manner, but it does not CONTRIBUTE energy.

"The coil wire is probably too thin to worry about, but then the current in the coil produces its own magnetic field."

The coil wire is only too thin to not think about eddy currents if you're using litz wire, where the strands are below the relevant skin depth.

My original point is that you can't talk about discrete "output" when you tweak variables as you wish. I don't mind discussing output, but we need to remember how unscientific it is most of the time. Bill Lawrence would get into a tizzy if you asked him if his pickups were "high output" precisely for this reason.

[Steve Conner]

Well, lately we have all these cranks on Youtube who think you can make free energy with magnets.

Magnets do have energy, it's charged into them at the time of manufacture, like a compressed spring or whatever. Thereafter it's stored in the magnetic field. You can stretch or squish the field around, but again like a spring, you can't get any net work out of it. The only way to use up the energy stored in the magnet is by demagnetizing it, and then it's not a magnet any more, so all commercial applications of permanent magnets avoid doing that.

So, in a strict "First law of Thermodynamics" way, you can't say that the magnet contributes energy to the output, any more than you can say that the line cord helps to power your amp.

[Mike Sulzer]

So, in a strict "First law of Thermodynamics" way, you can't say that the magnet contributes energy to the output, any more than you can say that the line cord helps to power your amp.

I like the line cord analogy. Just as the wire brings the electrical energy to the amp, the changing magnetic field of the vibrating string transfers energy from the string to the pickup coil and load.

[David Schwab]

I like the line cord analogy. Just as the wire brings the electrical energy to the amp, the changing magnetic field of the vibrating string transfers energy from the string to the pickup coil and load.

Right, now remove the magnet. The strings still vibrate, and no current flows in the coil. So where is the energy from the string going? Not into current.

It's a transducer, so it converts the strings' motion into current, but needs the permanent magnetic field to do so.

Either way without the magnet you are up that creek.

Lets also remember that magnetism and electricity are not two separate forces.

[Mike Sulzer]

Here is a set of measurements of pickup output voltage as a function of B field strength 3/32" above the pole piece. For this test I used my old aluminum base made in about 1965 when I was in High School. The test used a single humbucker slug coil hay wired into place with one pole under the G string. The magnets used were one through four small neos and an Alnico humbucker magnet. The neos were placed with a pole into the side of the bottom of the slug, approximately evenly distributed around.

The plot of results is here: http://www.naic.edu/~sulzer/voltageVsB.png.

Note that the output continues to increase well above the field level provided by the humbucker magnet. Also note that the extrapolated curve does not pass through zero as one might expect. This might be a result of changes in the permeability of the steel core with changes in magnet strength. The test should be repeated with no core.

[FunkyKikuchiyo]

Right, now remove the magnet. The strings still vibrate, and no current flows in the coil. So where is the energy from the string going? Not into current.

To borrow the earlier analogy, it is the same as removing a line cord. Just because I take the extension cord off of your Christmas tree lights doesn't mean it was the cord that was providing the energy.

The energy of the string vibrates freely until various resisting forces (like air pressure) stop it.

Magnetism and electricity ARE two separate forces. Magnetism is a physical property of the alignment of poles. Electricity is the passing of electrons from atom to atom.

[big_teee]

I don't want to interrupt a good hot debate, but I found this, and it's a pretty good explanation.
Take it, or leave it!
Later,
Terry

Generators work because a coil of wire is rapidly rotated in a fixed magnetic field (or vice versa in some instances). So, why does a guitar pickup work? Neither the magnet nor the coil is moving or vibrating -- why do we still get a signal? Any time a ferous material is placed within a magnetic field, it will "warp" the magnetic flux lines. Therefore, when our steel guitar strings vibrate within the field of the fixed pickup magnet, they cause the magnetic field to "vibrate" as well. This creates motion of the flux lines relative to the coil of copper wire and generates an electrical signal.

[David Schwab]

Magnets do have energy, it's charged into them at the time of manufacture, like a compressed spring or whatever. Thereafter it's stored in the magnetic field. You can stretch or squish the field around, but again like a spring, you can't get any net work out of it. The only way to use up the energy stored in the magnet is by demagnetizing it, and then it's not a magnet any more, so all commercial applications of permanent magnets avoid doing that.

Steve, that's incorrect. Magnets are magnetized, which aligns their domains. Nothing is charged into them. They aren't holding a charge, they are creating the charge.

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field.

Magnet - Wikipedia, the free encyclopedia

When you discharge a magnet, you are changing the orientation of the domains, so the misaligned magnetic moments cancel out.

Magnetic fields and forces

The phenomenon of magnetism is "mediated" by the magnetic field. An electric current or magnetic dipole creates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields.

Maxwell's equations (which simplify to the Biot-Savart law in the case of steady currents) describe the origin and behavior of the fields that govern these forces. Therefore magnetism is seen whenever electrically charged particles are in motion—for example, from movement of electrons in an electric current, or in certain cases from the orbital motion of electrons around an atom's nucleus. They also arise from "intrinsic" magnetic dipoles arising from quantum-mechanical spin.

The same situations that create magnetic fields (charge moving in a current or in an atom, and intrinsic magnetic dipoles) are also the situations in which a magnetic field has an effect, creating a force. Following is the formula for moving charge; for the forces on an intrinsic dipole, see magnetic dipole.

It is true that you must be either having current moving in a wire to create a magnetic field, or a wire moving in a magnetic field to create current. But we can't downplay the role of the magnet.

[David Schwab]

To borrow the earlier analogy, it is the same as removing a line cord. Just because I take the extension cord off of your Christmas tree lights doesn't mean it was the cord that was providing the energy.

That's a weak analogy since it leaves out where the current in the line cord is coming from. Maybe from the coils and magnets in generators? The line cord only supplies current, it doesn't generate it. Pickups generate current. That's like saying your patch cord doesn't produce current. We know that already. Pickups are small AC generators.

The energy of the string vibrates freely until various resisting forces (like air pressure) stop it.

That has nothing to do with the string creating a flowing current in the coil. Without the magnet it wont. It will produce a little heat and some sound waves, but no current.

The string is where the kinetic energy is coming from. I said that earlier. But without the magnetic field you wont get current in the coil. You can also say your arm is contributing to the signal, but it also wont create current flowing in the coil. At some point you have to stop abstracting that any further. I could follow suit and say my brain is creating the current because I decided to pluck a string! I'll stick with the string passing through the lines of flux.

Magnetism and electricity ARE two separate forces. Magnetism is a physical property of the alignment of poles. Electricity is the passing of electrons from atom to atom.

No, they aren't. This has been known since like 1855! Magnetism is caused by the spin of electrons, creating a magnetic moment. When enough of those domains align, you have a magnet. The electron spin is creating the magnetic field. It's not holding a change like a battery (which is a chemical reaction). Electricity is more like the wiggling of electrons while the sea charge is passed via photons. Magnetic fields are also mediated via photons. It's all part of electromagnetism.

Origin of magnetism

The spin of an electron, combined with its electric charge, results in a magnetic dipole moment and creates a small magnetic field. Although an electron can be visualized classically as a spinning ball of charge, spin is actually a quantum mechanical property with differences from the classical picture, such as the fact that it is quantized into discrete up/down states. The spin of the electrons in atoms is the main source of ferromagnetism, although there is also some contribution from the orbital angular momentum of the electron about the nucleus, whose classical analogue is a current loop. When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields add together to create a measurable macroscopic field.

A magnetic field is a field of force produced by moving electric charges, by electric fields that vary in time, and by the 'intrinsic' magnetic field of elementary particles associated with the spin of the particle.

The relationship between the magnetic and electric fields, and the currents and charges that create them, is described by the set of Maxwell's equations. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field tensor; the aspect of the electromagnetic field that is seen as a magnetic field is dependent on the reference frame of the observer. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.

Magnetic field - Wikipedia, the free encyclopedia

An electric field that changes with time (such as due to the motion of charged particles in the field) influences the local magnetic field. That is, the electric and magnetic fields are not completely separate phenomena; what one observer perceives as an electric field, another observer in a different frame of reference perceives as a mixture of electric and magnetic fields. For this reason, one speaks of "electromagnetism" or "electromagnetic fields". In quantum mechanics, disturbances in the electromagnetic fields are called photons, and the energy of photons is quantized.

Electric field - Wikipedia, the free encyclopedia

The electromagnetic force is one of the four fundamental forces. The other fundamental forces are: the strong nuclear force (which holds quarks together, along with its residual strong force effect that holds atomic nuclei together, to form the nucleus), the weak nuclear force (which causes certain forms of radioactive decay), and the gravitational force. All other forces (e.g. friction) are ultimately derived from these fundamental forces.

The electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life, with the exception of gravity. Roughly speaking, all the forces involved in interactions between atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. It also includes all forms of chemical phenomena, which arise from interactions between electron orbitals.

Just Google "magnetism and electricity":

Originally electricity and magnetism were thought of as two separate forces. This view changed, however, with the publication of James Clerk Maxwell's 1873 Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be regulated by one force. There are four main effects resulting from these interactions, all of which have been clearly demonstrated by experiments:

1. Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel.
2. Magnetic poles (or states of polarization at individual points) attract or repel one another in a similar way and always come in pairs: every north pole is yoked to a south pole.
3. An electric current in a wire creates a circular magnetic field around the wire, its direction depending on that of the current.
4. A current is induced in a loop of wire when it is moved towards or away from a magnetic field, or a magnet is moved towards or away from it, the direction of current depending on that of the movement.

Michael Faraday (1791-1867) an Englishman, made one of the most significant discoveries in the history of electricity: Electromagnetic induction. His pioneering work dealt with how electric currents work. Faraday was greatly interested in the invention of the electromagnet, but his brilliant mind took earlier experiments still further. If electricity could produce magnetism, why couldn't magnetism produce electricity. In 1831, Faraday found the solution. Electricity could be produced through magnetism by motion.

Hey, let's pluck that string near that magnet, shall we?

Magnetism, electricity, and special relativity

As a consequence of Einstein's theory of special relativity, electricity and magnetism are understood to be fundamentally interlinked. Both magnetism lacking electricity, and electricity without magnetism, are inconsistent with special relativity, due to such effects as length contraction, time dilation, and the fact that the magnetic force is velocity-dependent. However, when both electricity and magnetism are taken into account, the resulting theory (electromagnetism) is fully consistent with special relativity.

In particular, a phenomenon that appears purely electric to one observer may be purely magnetic to another, or more generally the relative contributions of electricity and magnetism are dependent on the frame of reference. Thus, special relativity "mixes" electricity and magnetism into a single, inseparable phenomenon called electromagnetism (analogous to how relativity "mixes" space and time into spacetime).

OK, I think that sums it up. One way to look at it is, moving current in a wire produces a magnetic field. And moving a wire in a magnetic field produces current. So since magnets do not have energy, where does the magnetic field in a magnet come from, since you need moving currents?

The more physically correct description of magnetism involves atomic sized loops of current distributed throughout the magnet.

[David Schwab]

Peter, I did read, and still read, both David and copperheads saying that different magnets make a qualitative difference in the sound, but do not affect the out level.

No, I said the change in tone is more than the change in level. That has been my experience anyway. An alnico 5 does not seem much louder than an alnico 2, but it is brighter. A ceramic or neo will be quite a bit louder. Maybe I didn't express it well, but I think it's not linear as compared to the tonal change.

[Steve Conner]

Sorry Dave, but magnets do have energy. If you multiply B (the flux density) and H (the MMF) together, the resulting number has units of joules per cubic meter, and it's called the "energy product" of the magnet.

Being energy, this obeys the laws of thermodynamics, so the magnetizer at the magnet factory had to deliver at least that amount of energy, and you would get something like the same amount back on demagnetizing the magnet.

It's of the order of hundreds of kilojoules per cubic meter. Quite a lot, but if it were actually used up, it wouldn't last for long in a 100 megawatt generator at the power station. The magnetic field just acts as a catalyst, if you like, that enables the transfer of energy from mechanical to electrical, without using up its own energy in the process.

Also, there's no need to get all Maxwellian or relativistic to describe the operation of pickups. Sure, there is no such thing as an electric or magnetic field all by itself, but in many situations, one is so dominant that the other can be pretty much ignored. In low-frequency iron-cored devices like transformers and pickups, it's all about the magnetic fields. That's why you need so many turns to get a usable output, because the electric part of the field is so small.

OK, maybe if you wanted to think about eddy current losses in covers, or self-capacitance of the windings, then you might need to think about the E component of the field too.

[Mike Sulzer]

No, I said the change in tone is more than the change in level. That has been my experience anyway. An alnico 5 does not seem much louder than an alnico 2, but it is brighter. A ceramic or neo will be quite a bit louder. Maybe I didn't express it well, but I think it's not linear as compared to the tonal change.

You are responding to this:

Originally Posted by Mike Sulzer Peter, I did read, and still read, both David and copperheads saying that different magnets make a qualitative difference in the sound, but do not affect the out level.

from days ago.

Why? Most of what you have written implies that the strength of the magnetic field has little to do with the output level (A5 same as A2, strong field of strat pickup not giving high level, etc.).

Last night I showed you that it does matter, and now you respond to a days old post, not the one from last night. Thank you for the response at last, but I would have felt better if you had responded to the post from last night.

[Mike Sulzer]

Energy in B field, energy in magnet

Example: Hook a battery across an inductor. It takes time for the current to build up; the higher the inductance, the longer it takes. During that time the magnetic field of the inductor is growing and it is the energy from the battery that "goes into the field", or however you want to look at it.

The energy associated with a magnet in general is a more complicated question. Suppose you "charge" a magnet; that is line up the microscopic currents by applying a magnetic field. It takes some energy to do this, but when you remove the applied field, why do the internal currents stay aligned? This is a complicated matter requiring quantum mechanics to understand.

[David Schwab]

Sorry Dave, but magnets do have energy.

Which is what I said originally.

Being energy, this obeys the laws of thermodynamics, so the magnetizer at the magnet factory had to deliver at least that amount of energy, and you would get something like the same amount back on demagnetizing the magnet.

When you magnetize something you are using that magnetic field to align the domains. If you need a lot of power it's just to saturate the magnet because it's hard to magnetize. You are not storing that amount of power in the magnet. You can demagnetize the magnet by heating it up. With some magnets, like neos, it doesn't take much heat to demagnetize them.

As an example, take an iron rod, align it North and South, and smack it with a hammer. It wont make a great magnet, and it will probably lose its charge, but there you go. And, yeah, it takes energy to swing the hammer...

I was just getting Maxwellian to point out that electrical fields and magnetic fields are not separate.