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In a faraday disc when you spin just the magnet and keep the disc stationary no current is produced...
Actually a voltage is produced in the disk, when you just spin the magnet. It does not matter for voltages along the disk, whether you spin the disk relative to the magnet or vice versa. When you spin the magnet, you will also induce voltages in the wiring from the disk to the volt meter and the wiring from the meter back to the disk. In this wiring the opposite voltage is induced by the spinning magnet, so the meter won't show any signal.
When you spin the disk, but not the magnet, the charge carriers in the disk will feel a Lorentz force, which will create a voltage along the disk. Since the magnet is not spinning, there won't be additional voltages around the rest of the measuring loop. In total you will measure a signal.
A time varying magnet field will, in addition to the effects above, induce voltages in the measurement loop according to Faradays law.
Registered Member #54402
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So in the case of spinnig just the magnet assuming the magnet has a changing field , would that have lorentz forces exerted on te electrons in the stationary disc just as in the situation where the disc was spinning in a static DC field?
1. A charge is moving in a stationary magnetic field. Then it will feel a Lorentz force.
2. The charge is at rest in a field of a moving magnet. A Lorentz (relativistic) transformation of the magnetic field into the rest frame (of the charge) will cause an electric field. Therefore the charge will also feel a force, i.e. the same as in 1.
It is not necessary to assume, that the magnetic field also varies with time in order to generate a force on the charge.
Registered Member #54402
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It is not necessary to assume, that the magnetic field also varies with time in order to generate a force on the charge.
Here you got me confused , if the magnetic field isn't changing with time then it is a static magnetic field or a DC field , and would you then say that spinning a DC field has the same effect on the stationary disc (charge) as spinning a changing manetic field ? Or did I misunderstood something here?
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Don't get confused between a magnet (let's say a cyclindrical magnet with N and S poles on the circular faces) spinning like a cylinder, so on its polar axis, which will generate a stationary field, just the same as if it were stationary, and the same magnet tumbling end over end which will generate a changing field.
Salvador, that's not what I meant. Imagine a charge at rest above a magnet at rest. It won't feel a force.
If you spin the magnet as a cylinder, as Dr. Slack describes, the charge will feel a force, even though the magnetic field is constant (i.e. rotationally invariant and rotating around its axis).
Magnetic fields turn into electric fields if you move relative to them and also electric fields into magnetic ones. The latter effect can easily be understood: Electric fields are caused by charges and moving past them implies currents (i.e. moving charges) from the perspective of the moving observer. Currents then imply magnetic fields. How moving magnetic fields turn into electric fields can be understood by relativistic effects. A bit more involved.
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Yes Dr.Slack I know a cylindrical magnet will not induce time varying changing fields if spinning on its axis , when I said a spinnigmmagnet I was thinking like an electromagnet which has AC current through it.I actually wasn't as much focused on the geometry of the magnet as I was on the very principle of how this could work.
Magnetic fields turn into electric fields if you move relative to them and also electric fields into magnetic ones. The latter effect can easily be understood: Electric fields are caused by charges and moving past them implies currents (i.e. moving charges) from the perspective of the moving observer. Currents then imply magnetic fields. How moving magnetic fields turn into electric fields can be understood by relativistic effects. A bit more involved.
I was aware of this , so applying this to current situation this is also the reason why I am thinking that if we spin just a permanent magnet around it's axis in front of a stationary faraday disc , charge imbalance may happen but there will be no current into the brushes caused by lorentz force which is felt by the electrons when the disc itself spins.
But if we have the same situation only this time the magnet we are spinning has an AC field to begin with will that have the lorentz effect on the electrons in the stationary disc as would be if the disc itself was spinning , or will it just induce some voltage/current like a transformer but no additional current from the rotational energy supplied to the spinning magnet will be convertet to current via the lorentz as ormally happens in a faraday disc when one spins it?
I hope you understood what I meant by this question, please tell me if I need to make something more clear about what i mean. Thank you for yor help so far by the way.
Wrt to the Lorentz force it doesn't matter, whether the magnetic field is AC or DC. For a spinning magnet and a disk at rest you'll have a 0 Volt signal around the measurement loop. There will be forces on the charges (at rest) due to the spinning magnet. I don't know, whether I should call them Lorentz forces, since Lorentz forces require the charges to move. I prefer to call the force to be an electric force coming from a moving magnetic field. An AC field might induce some voltage in the measurement loop due to Faradays law (change of flux through loop)
..that have the lorentz effect on the electrons in the stationary disc as would be if the disc itself was spinning..
As pointed out, whether there are Lorentz forces depends on the point of view, i.e. the reference frame chosen. For the lab rest frame, you'd have for spinning magnet and a disk at rest an electrical field and no Lorentz force along the disk. For magnet and disk both rotating at the same speed, there'd be an electric field and a Lorentz force, which would cancel on the disk, although not in the whole measurement loop.
I don't understand the motives behind your question.
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Ok , one more last question and then I'll be more than glad to tell you my motive behind these questions , if you bother to know at all ofcourse.
In a situation where i have the AC magnetic field from my magnet but the magnet itself is now stationary , the disc spins but has no brushes attached to it , would it then have a magnetic field around it just like whne the brushes are attached and braking occurs due to the lorentz force on the electrons that now form a current through those brushes?
Heres how it is easier for me to visualize , I use magnetohydrodynamics , I imagine a steady non moving stream of particles in a straight line , like ions in a vacuum for example.then I put a magnet beneath them , normally if the magnet is stationary and the stream of particles move they get deflected one way or the other based on the direction of motion or the pole of the magnet facing the particles, this is due to loretz force and is used in MHD generators.
Now I take the same stream of particles and assume they are3 stationary like my stationary conductor , I now take the AC magnet that I was talking about and move the magnet back and forth or just in one direction and the magnet itself has a changing polarity reversing field , Now i think what shoulsd happen is this , as I drag the magnet along the line of particles the particles at every point get deflected say up and down depending at which moment the field reverses polarity + since the magnet is physically moving forward tey should also get dragges along with the magnet ina forward direction so they should both deflect up and down in a sine wave fashion assuming sine wave input into the electromagnet and + they should be dragged forward sine thast the direction in which the magnet itself is traveling,
when I think of a generator I just loop the straight line in a circle and imagine a circular rotor but that sould be the same principle , please correct me if my reasoning is not right but when i think of charged particles and a rotating changing magnetic field I do get lorentz force action on them or deflection as you would say much like whne the disc spins and the brushes are attached and the electrons get deflected into a current through the brushes, what do you think , is there some error here in my reasoning?
A time varying magnetic field, made e.g. by a solenoid driven with AC current will cause circular electric fields around the magnetic field lines. When you have a charge travelling through this field, it'll experience a Lorentz force due to the magnetic field and additionally a force due to the circular electric field. The motion will be somewhat complicated.
When you move a magnet past a charge, the charge won't be dragged with the magnet but instead moved perpendicularly to the direction of the magnets motion.
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self excited homopolar generator (no PMM!!!)
