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!).

[expiracy]

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

[BradleyPeacock-orlr]

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.

[alecgoodchild]

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

[dremaro]

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 !

[cameronchambers]

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

[alanx]

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

[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.

[xsirfr]

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?

[Geri_crs]

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

[zisisgkitsis]

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) ???

[aphysicist]

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.

[aphysicist]

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.

[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.

[MichaelK.-xlqk]

So then why does the copper disc still generate current when the magnet is not stationary, but is attached to it and rotates at the same speed as it does?

[ylstorage]

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.