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enter image description hereWhen we move a wire upwards through a uniform magnetic field, the magnetic flux does not change. If we consider the equation for magnetic flux which is:

ΦB = B ⋅ A = BAcosθ

The magnetic field is uniform so B is the same and A certainly did not change, same goes for the angles involved so why is emf still induced?

Is this not an example of electromagnetic induction?

Also why doesn't the Lorentz force act on the protons? If we take that into account then there should be no separation of charges in the wire whatsoever.

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  • $\begingroup$ If the field is uniform, and flux does not change, then induced EMF is zero. But if you try this experiment, you will measure non-zero EMF. This is because it is not easy to prepare uniform magnetic field; any magnetic field you can set up will always be non-uniform. In the picture, magnetic field strength depends on position wrt magnets. $\endgroup$ Commented Nov 9 at 23:18

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Is this not an example of electromagnetic induction?

Faraday, who described electromagnetic induction, would have said, yes, this is an example of electromagnetic induction.

For Faraday, it was the relative motion of a magnet and conductor that caused an induced emf in the conductor. A magnet moving inside a ("stationary") coil, or a coil moving over a ("stationary") magnet were equivalent relative motions, and both produced an EMF in the coil/conductor.

This makes perfect sense from a relativistic point of view. However, Faraday's observations failed to give precise quantitive answers to physical problems involving electromagnetics.

Maxwell solved the quantitative issue by using fields. Unfortunately, with Maxwell's fields, the symmetry between the stationary magnet / moving coil and the stationary coil / moving magnet scenarios was broken. For Maxwell, if the magnet was stationary, the B field did not change, and no E field was induced. Whereas, if the magnet moved, the B field changed with time, and an E field was induced.

In the case of the moving magnetic / induced E field, it was the E field which caused electrons to move within the conductor. In the case of the fixed magnetic field, moving conductor, it was the magnetic field (B) that caused the electrons to move within the conductor, via what we now call the Lorentz force.

Einstein restored the relativity of motion in Maxwell's theory by positing that the geometric properties of the fields depend upon the inertial frame of the observer. There is one electromagnetic field, that is seen in any frame to have an electric field component, and a magnetic field component. In different inertial frames, the division into electric components and the magnetic components appear differently. What is primarily a magnetic phenomenon in one frame of reference is primarily and electric phenomenon in another, and vice versa.

Also why doesn't the Lorentz force act on the protons?

The Lorentz force does act on protons. Protons have charge, and thus the Lorentz force acts upon them. But in a solid metallic conductor, the protons are bound inside the nuclei of atoms, and the atoms are bound inside a solid structure. Atoms may move slightly relative to their neighbors, but they do not slip past one another as in a liquid or gas. Some of the electrons in a metallic solid, however, are free to move about hither and thither in within that metallic solid. Those electrons which are free to move about are said to be in a conduction band. But not all the electrons in the metallic solid are so free. Others are in orbitals bound to individual atoms. Electromagnetic fields will cause the orbitals bound to individual atoms to distort, but the electrons in those orbitals will stay with that atom (unless they are given sufficient thermal energy to move from that orbital into the conduction band). Thus, the different effects an electromagnetic field has upon conduction band electrons vs protons and non-conduction band electrons is not due to any difference between how electro-magnetic fields act upon electrons vs protons, but to the different mobility of these two kinds of charged particles.

If we take that into account then there should be no separation of charges in the wire whatsoever.

As explained above, some of the electrons are free to move throughout a metallic solid, while there are no positive charges that are free to do so. This permits charges to be separated by an electromagnetic field.

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    $\begingroup$ Nice. I've seen it claimed that it was Maxwell's quite different treatments of the moving magnet and moving circuit cases when there was just relative motion between them that set Einstein on the train of thought leading to Special Relativity. $\endgroup$ Commented Nov 9 at 17:48
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You still have the Lorentz force $F=q*v*B$ if v is perpendicular to B. If you measure the voltage at the end of the wire you have an enclosed area with the wire and the voltmeter, otherwise there is just an electric field in the wire opposing the Lorentz force but no emf.

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  • $\begingroup$ So... Is this electromagnetic induction or not? $\endgroup$
    – Sher
    Commented Aug 26 at 12:59
  • $\begingroup$ Which scenario with or without a closed loop. Without its not usually called induction, but it's not forbidden. Why ask just for names for something you know happens? $\endgroup$
    – trula
    Commented Aug 26 at 17:12
  • $\begingroup$ I was just asking because my textbook claims it's EMI. Also, how does attaching a closed loop make it EMI? $\endgroup$
    – Sher
    Commented Aug 31 at 4:46
  • $\begingroup$ if you have the closed loop with the Voltmeter, moving the wire changes A. $\endgroup$
    – trula
    Commented Aug 31 at 14:25
  • $\begingroup$ Does the A change even when the wires are completely regid? $\endgroup$
    – Sher
    Commented Oct 4 at 8:25

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