Electromagnetic Induction | The Faraday Disk Dynamo Explained | A-level Physics

We explain the Faraday Disk (Disc) Dynamo, a famous example of electromagnetic (EM) induction. This is an example of a DC (direct current) generator, and often turns up in exam questions on EM (regenerative) braking in vehicles. We discuss the physics of the dynamo, touching on Faraday's law, Lenz's law and Fleming's left hand rule. Throughout, we pose some questions for you to try, to keep the video interactive (and to keep you on your toes!).
/ @forestlearn

[@expiracy9458]

hi there! Great video but im just a bit puzzled why the right hand generator rule gives me the wrong direction for the current.

[@ForestLearn]

Hey, thanks for the kind words! Throughout the video (and playlist), I only discuss the left hand rule, not the right hand one (I'll say more on this later...). Can you tell me which way you are pointing your fingers?

[@expiracy9458]

@@ForestLearn thanks for the quick reply, much appreciated! I am aware that the right hand rule is a sort of "cheesy method", however I was just using it to verify my answer since I was assuming it would give the same :( I did get the same answer as you using the left hand rule however. As for directions, I put the thumb as pointing towards me due to to the discs rotation, the field as upwards and the current as pointing to the left.

[@ForestLearn]

@@expiracy9458 That's exactly right! Look at the diagram at say 5:39 - the induced (conventional) current would be flowing to the left along the sector. Hope that clears it up? Let me know if it doesn't.

[@expiracy9458]

@@ForestLearn Oh sorry I am stupid for missing that arrow 😂 I got a bit confused by the + and - on this disc and forgot completely to even consider the circuit loop, thinking that the induced current was an eddy current. Also if you don't mind mind me asking, when is your next video on EM induction of type 3 coming out, and would it be possible for you to cover questions involving eddy currents (such as Arago's disc)? If not that's fine, I am just curious. :) Thanks very much for the help though, it is much appreciated and has helped greatly in my understanding of this topic.

[@ForestLearn]

@@expiracy9458 No Problem! I was hoping to do a vid on eddy currents next, followed by discussing type 3 (along with the kinetic theory derivation of the ideal gas equation). Unfortunately I can't give any estimates of when they'll be done - wish I could make them quicker! Thanks for the feedback, I'm glad you've found the vids useful :)

[@BradleyPeacock-or8lr]

Heres a derivation of the formula at 8:00 : Velocity varies with radius (v=ωr) so we consider de and dr instead of e and l in the original formula (e =Blv). de= vBdr, de = ωrBdr, de/dr = ωrB. Integrate this with limits 0 and R to get e = (ωBR^2)/2.

[@ForestLearn]

Exactly, thanks for sharing this!

[@dremaro2967]

thank you very much ! I have been looking for a clear video explaining this effect. I personnaly prefered to use the Lorentz force on electrons inside the conductor to understand the current flows. I also think the reactive force we have to apply in order to spin the disk can be understood with the Laplace force, but it end up in the same to think with Lenz's principale. Anyway, thank you for this good video !

[@ForestLearn]

Thanks :)

[@alecgoodchild1572]

Great video series, your explanations are top-notch and the visuals are really well done.

[@ForestLearn]

Thanks for the kind words, they mean a lot to me :) Glad you've found the vids useful!

[@cameronchambers2907]

The magnetic force on the sector that is resisting the change in direction , is this where and why there is back emf on motors ?

[@ForestLearn]

Hi, great observation! The cutting of field lines gives rise to a magnetic force which is responsible for the induced emf + current in the Faraday dynamo. In a motor, again, the cutting of field lines gives rise to a magnetic force. But this force now opposes the motion of the electrons i.e. reduces the current; remember the current is present in the first place due to an applied/external emf across the motor. Thus, the overall emf is reduced by an amount known as the 'back emf' (the word 'back' implying opposition/reduction). [Note: the back emf is usually referred to as an induced emf, but I'm not sure that's helpful terminology.] Anyway, hope the above made some sense, hope to do a vid on back emf one of these days. In the meantime let me know if you have any further questions!

[@lakipalaniappan]

At around 9:08 you say that the current in the sector is going to the left, but previously you said that since the electrons are moving towards the current is moving away from us. I'm a bit confused.

[@ForestLearn]

The current in the sector is an induced current (which arises due to the magnetic force). The motion of the electrons toward us (and the associated current) is simply due to the rotation of the disk - it should not be thought of as an induced current. This is perhaps the trickiest aspect to grasp about 'cutting field lines' induction; I recommend you check out or rewatch the discussion in that video first before returning to this, hopefully it should then make sense. Feel free to ask if you have any further questions.

[@lakipalaniappan]

@@ForestLearn Ohhhhh for some reason I thought of the associated current and the induced current as one of the same. Thanks again!

[@ForestLearn]

@@lakipalaniappan You're welcome :)

[@alanx4121]

is there a current density distribution considering that the speed of the charge from inside to outside radius increases?

[@ForestLearn]

Hi, thanks for your question and apologies for the late reply! Yes - you're correct :)

[@zisisgkitsis6414]

Essentially, the force exerted to the electrons, pushing them towards the rim of disk, is produced due to the circular motion of the Disk (F_centripetal) ???

[@ForestLearn]

Hi, thanks for your question. No, the force exerted on the electrons is a magnetic force. The origin of this force has nothing to do with circular motion - if the magnetic field were removed and the disk continued to rotate, this magnetic force would no longer exist and we would no longer have any EM induction going on. Remember also that a centripetal force is a resultant force toward the centre of a circular path. You may find it helpful to listen back to the discussion at 4:38, and might find the second video in the playlist helpful to better understand what's going on conceptually: kzbin.info/www/bejne/kJyykJl4icenm8U Let me know if you have any further questions.

[@zisisgkitsis6414]

@@ForestLearn thank you so much for pointing out to me. I did a common mistake, using the right hand rule instead of the left hand one. Also, considering Lens’ Law I could easier deduce the direction of the current, therefore the direction the electrons are moving. Although not quite sure that’s a correct way to view it, it helps me simplify the problem. So thank you for your educational video and your response.

[@ForestLearn]

@@zisisgkitsis6414 My pleasure! You are correct that Lenz's law (LL) can also be used to figure out the direction of the induced current - nothing wrong with that, and if it's easier for you to figure out what's going on using LL then great!

[@xsirfr1958]

If there are disc magnets on both sides, which rotate at the same speed as the conductive disc, then there are no B field lines being cut. In such a case, will a current be generated? Can Bruce DePalma's N-generator be explained with known physics?

[@ForestLearn]

I'm unaware of this so can't comment on it.

[@magnetomancer]

This is the clearest explanation of the Faraday disk I've seen to date. But the cutting of field lines is not valid because, surprise, no field lines exist. Instead, only a linearization of magnetised Space occurs caused by the alignment of the atom's magnetic domains. Moving across this linearized Space causes atoms in the conductive material of the spinning disk to develope a charge potential, a Voltage at maximum at the fastest area - the rim of the disc, at minimum at the slowest area -the axil of the disc.

[@Geri_crs]

What would happen if you use a YBCO disc at 65°K ?

[@Geri_crs]

*with-rotation axile inline crystal structure

[@aphysicist9558]

When I try to use the d(phi)/dt = d(BA)/dt formula here it appears the flux is not changing because there is no change in B or A. Maybe from the perspective of the charges in the disc the number of flux lines they 'count' as they cut through them as the disc rotates varies. Hence emf is induced.

[@ForestLearn]

See my reply to your other comment.

[@aphysicist9558]

When I try to use the formula d(phi)/dt = d(BA)/dt it appears the flux is not changing as there is no change in A or B. Maybe the charges in the wire experience a change in the magnetic force they feel as they cut through the flux lines while the disc rotates. Hence an emf is induced.

[@ForestLearn]

The easiest/most transparent way to understand the induced emf is due to the cutting of field lines and the resulting magnetic force, as explained in the video. It is possible to derive the emf from rate of change of magnetic flux, but the 'loop' through which the flux changes isn't obvious and (way) beyond what's expected at A-level.

[@Daniel-vu7pi]

You're correct, I don't think this is really a case of magnetic induction either; it's better understood as the Hall effect. Recall that the Hall effect is that a potential difference is set up across a conductor if it is transporting electric current and subjected to a magnetic field perpendicular to that current. To analyze this example, consider a charge carrier on any point at the disc, the disk is rotating so any free charge in the metal disk can be considered to be a moving charge with velocity perpendicular to the radial vector from the centre of the disk. Then simply use F = qv x B and you will see that negative charges will get a magnetic force moving them to the outside perimeter of the disk and thus a positive charge will accumulate on the centre of the disk, giving you the desired potential difference.

[@LalitaandMonika]

Nice

[@ForestLearn]

Thanks! And thanks for watching!

[@ylstorage7085]

I have major issues with this. you are viewing the disk as an integral of tiny slices of pizza what if you are viewing the disk as closely packed concentric circles of wires? no more flux cutting, no voltage would be induced. how we mathematically describe the object should not have affected how the real world has behaved. your view also fails when viewing the disk as a whole. total flux has not changed going thru the disk, no voltage should have been generated.

[@ForestLearn]

Thanks for your questions and sorry for the hugely delayed response. The discussion from about 4:38 outlines that a free electron in the disk experiences a magnetic force which fundamentally explains the induced current. You are right that it shouldn't matter whether I draw a sector ('tiny slice of pizza') or concentric circles - the electrons in the disk don't care about this choice either and experience just one magnetic force. Regarding the flux through the disk not changing, please see the video at 9:40 onwards - this exemplifies my whole approach to these induction videos in stressing the different types of induction. Perhaps you did not reach the 9:54 mark where I make this point again. This experiment was done by Michael Faraday and conclusively demonstrated the presence of an induced emf (not to mention the countless times it has been done since then).

[@mrpicky1868]

wrong. forget the magnetic lines as they dont exist. it's all about dynamic induction. so you can get currrent from this even if both disc and magnet stationary and outside contact stays at same distance from center. kzbin.info/www/bejne/nZXYiqKKbtiAl6csi=0WdL6ZkCtSz2XE2X