# Curious question regarding Faraday disc / induction, etc.

• I
before I ask anything I know this has been probably talked about alot in the past from various different angles, yet when I am searching for answers I cannot find a satisfactory answer that would look at it from the perspective I wish so here is my try.

I've talked with a mentor of mine about the Faraday disc and homopolar generators in general, so I kind of have always had this idea that every ordinary generator/motor works simply by induction as the magnetic pole pairs move past the stator or as in older versions rotor windings there is a time varying (increasing/decreasing) EM field. B field lines are increasing/decreasing through a loop which induces current in it. so far so good.
when it comes to the Fardaday disc it cuts exactly the same amount of B field lines as it moves and they never increase/decrease so my natural reaction is to assume that the current is generated due to the Lorentz force law , as a charged particle is moving through a B field experiences a force.
So i assume that there should be charge separation created on the disc even when no current collecting contacts are connected as the disc spins the charges still should experience a force due to the b field.

Here is where the hard part comes in. I have read academic studies that say that a Faraday/homopolar generator is not possible without brush contacts.
I've made myself various little experiments which are crude and imprecise yet still, where I try to put a disc or a wire in a homogeneous b field and then attach a load that is out of the b field but co-rotating with the rotor, I never get any current in my load, to be honest i've also tried a loop routed such that both sliding contacts are close to rotor and got no current.

So I wonder and please help me out with the theoretical side here, is it possible to have a Faraday type generator assuming we arrange the homogeneous B field so that it cuts the conductor at correct angles (so as not to oppose the generated current at other parts of the circuit) so that the load can rotate at the same speed as the conductor generating the current, because all the classical examples show us a single loop of wire of which one part is rotating (the disc usually) while the other part usually making the load is stationary with respect to the disc. But if the Lorentz force acts on the charged electrons inside the conducting spinning disc then why can't we have current running through a load attached to the disc if we make the B field such that it only cuts the disc and not the load as to not oppose the generated current?

P.S. I've read explanations that even though the Faraday disc uses a static homogeneous B field it still operates under induction laws similarly to an ordinary generator/motor with the example being the rolling conductor between two rails which forms a sort of a rectangular circuit and as the rolling conductor rolls across the rails it makes the loop to either increase it's area or decrease it's area so increase the B field lines it cuts or decreasing them with respect to time which is another way at looking at induction. So in the disc example the connection (at the outer rim where most field lines are cut) is essential because only when that connection rotates with a different speed to the disc itself or is stationary with respect to the disc , only then the imaginary wire connecting the outer brush and disc center cuts B field lines but with the load physically connected to the disc and rotating with it no field lines are being cut by the loop even if the load is magnetically sealed from the disc in order not to generate a countercurrent in the same direction.
please tell me which is the truth here and what is wrong?

thank you.

I would be very thankful if someone participated in answering this question as i think i now almost understand it, thank you in advance

Paul Colby
Gold Member
I think I understood this for about 5 minutes at some time passed. Here is a good demo

I have seen the video, but it has a few problems, essentially it only correctly shows the fact that current is generated both when the magnet spins with the disc and also when the disc spins alone which is already known to me and to many as not being a paradox because the magnet's axial field strength does not change by rotating it around it's axis so the disc cuts the same amount of field lines no matter the magnets rotation.

the third part where he only moves the brushes back and forth cannot be considered as a real result because the B field of the magnet is open as can be seen it has no flux shaping metals to contain the field so he is essentially moving the brushes and the wires that are connected to them through a B field so it is really unclear whether the current generated is from the disc or the moving wires and I tend to think it is from the moving wires themselves not the disc because in order to generate a current in a static homogeneous B field the conductor must be moving in order for the electrons in the conductor to experience Lorentz force.

The thing I am looking for is a bit different, I am curious to know whether a load attached to the same axis as the disc and co-rotating with the disc can have current through it if the B field is shielded from the load wires and only applied to the current generating part of the setup in this case the disc or some other shape of conductor.
Normally if one solders a small light bulb on top of a disc like in the video there would be no current because current would run in the same direction both in the disc and in the load wires, so I wonder whether such a light bulb can glow if its load wires are separated from the homogeneous B field of the disc?

Paul Colby
Gold Member
the third part where he only moves the brushes back and forth cannot be considered as a real result because the B field of the magnet is open as can be seen it has no flux shaping metals to contain the field so he is essentially moving the brushes and the wires that are connected to them through a B field so it is really unclear whether the current generated is from the disc or the moving wires and I tend to think it is from the moving wires themselves not the disc because in order to generate a current in a static homogeneous B field the conductor must be moving in order for the electrons in the conductor to experience Lorentz force.

I can't know of course, but given the magnet shown the field is rotationally symmetric to what? 5% maybe 10% Given the response when moved by hand is roughly commensurate with the other tests my bet is the conclusion holds and further refinement would concur. Have at it.

The thing I am looking for is a bit different, I am curious to know whether a load attached to the same axis as the disc and co-rotating with the disc can have current through it if the B field is shielded from the load wires and only applied to the current generating part of the setup in this case the disc or some other shape of conductor.

Isn't the answer known to be yes in this case? The ##v\times B## applied to the charges in the disk would give rise to a voltage drop from the disk edge to the axis in the case you describe.

Merlin3189
Homework Helper
Gold Member
I... The thing I am looking for is a bit different, I am curious to know whether a load attached to the same axis as the disc and co-rotating with the disc can have current through it if the B field is shielded from the load wires and only applied to the current generating part of the setup in this case the disc or some other shape of conductor. ?
I thought this was what you were after,but I'm still waiting for someone to show that this idea is realisable.

Paul Colby
Gold Member
I thought this was what you were after,but I'm still waiting for someone to show that this idea is realisable.

Why not use a linear pole gap and use translation rather than rotation.

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Merlin3189
Homework Helper
Gold Member
So you have a conductor looped round one arm of a long horseshoe magnet?

And if you could get a uniform field, would there be any change of flux linked to the coil? I think so.
Certainly you should get an emf here.with the conductor moving through the field, uniform or not.
So how do we predict no emf here?

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Paul Colby
Paul Colby
Gold Member
So how do we predict no emf here?

The magnet shown (ideally) applies 0 B field to the loop containing the load while applying a uniform B field to the plate in the gap. The emf in the plate arises from the ##v\times B## forces on the charges in the plate. Why would anyone expect 0 emf in this case? My reading of the OP is that this is a solution to the topological and physical problem posed. The load loop is fixed to the plate and the assembly is moving in a field applied to just the plate. Just from personal experience playing with linear motors there will be ample currents induced in plate. Makes an effective magnetic break.

thanks for stopping by this thread, well maybe if I could only ask for the animations to be clearer because at this point it is really hard for me to understand them.

the whole idea for this is simple, most if not all sources always say that the Faraday disc cannot be done without brushes so I always wondered whether this is due to some physical law or whether it is simply due to geometry and materials limitations, and now after I know some deal about EM and electronics and magnetism I think it is simply because it is hard to make a geometry in which a B field can be routed in such way in order to make it not only homogeneous but also to make the circuit such that the b field lines only interfere with one specific part of it in order not to generate countercurrents, so I assume this is the reason why all such devices use brush contacts, since current in this type of device is generated due to the Lorentz force, a stationary circuit can be in the same B field as the rotating one but only the rotational part gets current generation.

But thinking more about it I thought but maybe it is indeed possible to make it a brushless device, at least that would be very interesting because the homopolar generator is the only true DC motor/generator as it's output is not rectified AC but real linear DC which is not surprising since we know that the current is due to Lorentz force acting on charged particles, much like what happens in a tokamak plasma and elsewhere.

I think I need to setup a physical experiment because "seeing is believing" , I will try to understand your proposed setup which I can't visualize at the moment but what I was thinking would be something like this.
Imagine taking a cylinder of conductive material like an aluminum beer can, then cutting out a slice of it both at the top and bottom and also along the side so that it is one big piece, then taking one or two permanent magnets and simply putting them along the axial slice and then using some soft metal to route the B field lines around the sides so that they don't interfere with the ends, attach this to a shaft of some sort which I have a few laying around and solder the ends to some brush contacts if I will get voltage then I can try to attach a permanent load rotating on the shaft.
I guess using multiple turns of enamel wire is better than using a single sheet type conductor in order to get higher voltage and make an easier reading but it also complicates the setup as in how to make sure that no B field lines interfere with the non current generating part of the loop in order not to create counter-currents.