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Entanglement
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I know when the magnetic flux falling on a wire changes, emf is induced and a potential difference is created in the wire, I don't understand how can a change in the flux give electrons potential to create a current
So induction actually happens due to Lorentz's force, doesn't it ?rude man said:For a closed loop like a coil of wire,with time-varying magnetic field B inside the loop,
del x E = - ∂B/∂t, found by experiment.
where E is the electric field everywhere inside the loop.
That plus Stokes' theorem gives you the emf induced around the loop.
If an isolated wire of length L moves thru a B field with velocity v such that the B field and the wire direction are orthogonal (or partly orthogonal), then think of free electrons sitting along the wire. The force on an electron is F = q v x B so the electrons will be pushed to one end of the wire where they bunch up. That leaves the other end positively charged (lack of electrons). Equilibrilum is reached when the internal electric field = 0.
So there is an emf generated by the moving wire = (B x L) * v.
ElmorshedyDr said:So induction actually happens due to Lorentz's force, doesn't it ?
ElmorshedyDr said:When a wire moves perpendicularly to a magnetic field B with velocity v
" ... in the case of inducing an emf the electrons are moving WITH THE WIRE not along it but along the v vector, so it's affected by a force along the wire which is perpendicular to v and B.
ElmorshedyDr said:But what causes Lorentz force and the force that makes the electrons accumulate at one side is almost the same.
ElmorshedyDr said:Thanks a lot I got that part, but I don't understand how a changing magnetic flux can force the electrons to do the same thing (accumulate at one side) inducing an emf
rude man said:It's not always flux, as I said. I can give you an example where emf = -dø/dt does not work.
ElmorshedyDr said:What we've been taking about is motional emf, what about if we move a wire towards a magnet so there's a time varying flux, according faraday' law we say that an emf is induced,
How is that ??
But when moving a wire towards a magnet the velocity vector is no longer perpendicular to the field how does Bev and consequently Blv still work?rude man said:emf = BLv. In this case B is not a uniform field but that makes no difference. Moving a wire in a constant field also generates an emf.
rude man said:No. emf is induced around any closed path, per Stokes as mentioned. Even in thin air! Otherwise there'd be no e-m waves.
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ElmorshedyDr said:But when moving a wire towards a magnet the velocity vector is no longer perpendicular to the field how does Bev and consequently Blv still work?
Thanks a lot, but that's not what I meant.jtbell said:You take the angle into account via a factor of cos θ.
http://farside.ph.utexas.edu/teaching/302l/lectures/node88.html
(see equation 205 and the text immediately before and after it)
cabraham said:"In thin air" the "induced emf" is pretty ambiguous. Unless a specific path is defined, the emf across 2 points in thin air is undefined unless a path is specified.
Claude
rude man said:I don't get your point.
I said the emf is about a closed path. Why are you bringing up an open path?
The emf around a closed path in 'thin air' is not at all ambiguous. That's why we have radio and TV.
ElmorshedyDr said:Please guys don't change the topic
ElmorshedyDr said:So when a wire (has no current) cutting a magnetic field, the wire carries electrons that are moving with the wire along the v vector so these moving charged electrons induce a magnetic field which interacts with the external magnetic field producing a force that makes the electron to move perpendicularly to the v vector and to B, so it moves along the wire creating a potential difference and creating a current if it is connected to a closed circuit.
But when a magnet is moving toward a still wire creating a time varying flux an emf is also induced, I can't understand why does this happen I can't relate it to what I understand.
There things I'm missing causing misunderstanding, so I need some clarification, thanks in advance.!
ElmorshedyDr said:But when a wire moves towards a magnet or when a magnet moves towards a wire the velocity vector of electrons within the wire are no longer perpendicular to the field
cabraham said:Maybe I'm being too strict, but emf in thin air is ambiguous. But I agree that if we define emf as what would be the work per charge if conductor and charge were present, it makes sense.
Claude
ElmorshedyDr said:Hey rude man you were doing great where are you?? :sad:
Read my last quoterude man said:Here I is!![]()
rude man said:By Stokes and Maxwell, emf in air is perfectly well defined, no conductor is needed, and without it we would not be communicating via my wi-fi:
IN AIR or anywhere else,
emf = ∫E.ds = ∫∫{∇xE}.dA
where the first integral is around any contour anywhere and is the definiton of emf around a contour, and the second is over the surface defined by the contour.
ElmorshedyDr said:But when a wire moves towards a magnet or when a magnet moves towards a wire the velocity vector of electrons within the wire are no longer perpendicular to the field
The wire is perpendicular to the flux but the v vector is parallelrude man said:Which way does the wire point? Towards the magnet or at right angles?
ElmorshedyDr said:The wire is perpendicular to the flux but the v vector is parallel
If a coil is perpendicular to the flux and its velocity vector is parallel I suppose there is an emf, isn't that right ??rude man said:If you magnet has a radially symmetric B field about its extended axis and you move your wire with its middle along the extended axis, no part of any flux line is ever perpendicular to the direction of the wire and the velocity. So you'd get zero emf.
But if you deviated from the axis (say you moved parallel to the axis but 2cm radially away from it) you would cut flux lines and therefore generate an emf.
ElmorshedyDr said:If a coil is perpendicular to the flux and its velocity vector is parallel I suppose there is an emf, isn't that right ??
That's my problem, I understand the way the emf is induced when a wire cuts the flux such that the electrons accumulate at a region creating a potential difference, but the example where the the axis of the coil is parallel to the field moving towards a magnet, how faraday discovered induction, since the electrons velocity ISN'T PERPENDICULAR to the field I'm unable to apply the same idea of the fist example when the wire cuts the field where the electrons' velocity is PERPENDICULAR to the fluxrude man said:I assume you mean the coil's axis is parallel to the B field?
If the flux is not changing in time or space there is no emf generated.
If B is caused by a permanent magnet then there is an induced emf since the velocity generates dB/dt inside the coil. That's how Faraday first found induction. The magnet's B field gets stronger as you approach it.