# Why there is no EMF induced in this circuit?

#### Podkarpaciarz

Joined Sep 18, 2023
8
Why there is no current flow in this circuit?
On this diagram, the copper wires are long and extend far beyond the magnetic field range and they rotate with multimeter or BT transmitter, magnet is stationary, the blue arrows symbolize hypothetical currents. I don't understand why emf is zero here. I heard that the middle wire is also cut by the magnetic field lines so there is opposite current/voltage induced in the middle wire so everything cancels out, is that true?

The way I see it, the current in the middle wire should be induced perpendicularly to the wire ( - in the middle of the wire and + outside, just like in Faraday disk) so I don't understand how this could block the flow of the other currents coming from the side wires.

#### BobTPH

Joined Jun 5, 2013
8,120
The magnetic field in the middle wire is parallel, which does not induce any current, the other two oppose each other.

#### Podkarpaciarz

Joined Sep 18, 2023
8
The magnetic field in the middle wire is parallel, which does not induce any current, the other two oppose each other.
They do not oppose each other, look at the middle wire and arrows, the currents would flow thanks to the middle wire back to the ground.
The field curves at the end in the middle (this is not shown on the picture but magnetic fields are loops) so tiny voltage should be induced in the middle wire.

#### Alec_t

Joined Sep 17, 2013
14,009
If the magnet's field is circularly symmetrical about that central vertical axis then the horizontal wires move through a uniform magnetic field. With no change in the magnetic flux that they experience there will be no emf generated.

#### Podkarpaciarz

Joined Sep 18, 2023
8
If the magnet's field is circularly symmetrical about that central vertical axis then the horizontal wires move through a uniform magnetic field. With no change in the magnetic flux that they experience there will be no emf generated.
Are you sure? then how faraday homopolar generator works? disk is rotating over magnet and voltage is induced.

#### Alec_t

Joined Sep 17, 2013
14,009
how faraday homopolar generator works?
Different parts of the Faraday disc move in and out of the magnet's field, so experience flux change.
In your system, positioning the magnet to one side of the central vertical should result in a generated emf.

#### BobTPH

Joined Jun 5, 2013
8,120
The way to think about it to look at the flux lines through a loop of wire. A current is induced when they increase or decrease.

#### Podkarpaciarz

Joined Sep 18, 2023
8
Different parts of the Faraday disc move in and out of the magnet's field, so experience flux change.
In your system, positioning the magnet to one side of the central vertical should result in a generated emf.
What parts? magnetic field is all around the disk, disk rotate inside this field. You are saying that the edge of the disk gets less flux because flux is weaker near the end of the magnet? ok but also im my project the wire is like a small section of the faraday disk, the wire part that is near the end of the magnet experience less flux than the center part.

#### Podkarpaciarz

Joined Sep 18, 2023
8
The way to think about it to look at the flux lines through a loop of wire. A current is induced when they increase or decrease.
The section of wire near the end of the magnet experienced less flux.

#### djsfantasi

Joined Apr 11, 2010
9,131
Think of a pie. From the top, it looks like a disk. Cut it into 4 slices, you have 4 different parts moving in and out of the magnetic field. You might have an idea of where I’m going. With smaller and smaller slices, like integration in mathematics, you have more and more sections that approach a solid disk. At least how I first grasped this.

#### Podkarpaciarz

Joined Sep 18, 2023
8
Think of a pie. From the top, it looks like a disk. Cut it into 4 slices, you have 4 different parts moving in and out of the magnetic field. You might have an idea of where I’m going. With smaller and smaller slices, like integration in mathematics, you have more and more sections that approach a solid disk. At least how I first grasped this.
The moving in and out is not something that I can see here... nothing leaves the field. I think that above answers are correct that the magnetic field must change in respect to wires for induction to happened. Faraday homopolar generator works NOT because lines are being cut but due to Lorentz force acting on moving disk: from the perspective of a magnet faraday's rotating disk is already a type of current (copper atoms rotate with electrons) so Lorentz force kicks in and put electrons into a more spiral motion, vortex like, and causes actual charge separation from the center to the rim of the disk, not sure if this is the correct picture?

#### nsaspook

Joined Aug 27, 2009
12,329
The moving in and out is not something that I can see here... nothing leaves the field. I think that above answers are correct that the magnetic field must change in respect to wires for induction to happened. Faraday homopolar generator works NOT because lines are being cut but due to Lorentz force acting on moving disk: from the perspective of a magnet faraday's rotating disk is already a type of current (copper atoms rotate with electrons) so Lorentz force kicks in and put electrons into a more spiral motion, vortex like, and causes actual charge separation from the center to the rim of the disk, not sure if this is the correct picture?
Looks like you are finally accepting the answers you received elsewhere on this question. Good.

#### Podkarpaciarz

Joined Sep 18, 2023
8
Looks like you are finally accepting the answers you received elsewhere on this question. Good.
Well, not exactly, Lorentz force is magnetic force, so it will exert a force on a moving charge because this charge has a magnetic field around it, electrons must be moving between atoms or in free space to have a magnetic field for Lorentz interaction. A spinning copper disk (without a magnet) does not produce ions or magnetic fields, So in Faraday homopolar generators, the disk IS CHARGED FROM LINE CUTTING, and Lorenz moves charges further.

#### nsaspook

Joined Aug 27, 2009
12,329
Lorentz force is a electromagnetic force. It can be mainly magnetic but this is a side-track from the OP question that's been answered here and elsewhere.

#### Podkarpaciarz

Joined Sep 18, 2023
8
Lorentz force is a electromagnetic force. It can be mainly magnetic but this is a side-track from the OP question that's been answered here and elsewhere.
Disc must be charged first before Lorentz force can act on the charges, rotating disc in absence of magnetic field does not create charges or voltage in it self so my previous statement "from the perspective of a magnet faraday's rotating disk is already a type of current" WAS WRONG. Disc must cut the lines of flux and develop voltage across it for the Lorentz force to work.
The answer (as I suspected) was all about geometry and vectors, canceling voltages, but I was unable to see the blocking vectors and no one was kind enough to draw them for me, after heavy discussion elsewhere I finally get it and I drew
this, now I can see the blocking in the disc, one picture is worth a thousand words.

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