# On electromagnetic induction. Does larmor precession cause electron flow in a conductor?

What is happening at the atomic level which makes it necessary for a magnetic field to be changing with time in order to induce an EMF in a coil of wire? Why won't a constant magnetic field induce an electron flow?

I have am assuming that it has to do with the spin-lattice relaxation of Larmor precession. Given that, in a static magnetic field, the precession of nuclei ceases after a time period known as the spin-lattice relaxation time. After this time, most of the nuclei will align their magnetic moments parallel, or anti-parallel, to the (constant) magnetic field...(thus no current flow). To refresh the precession of nuclei, the field has to be brought to zero and then steadily, but quickly enough, increased to the desired value, such that the precession of a large population of the nuclei is maintained. This is achieved in an alternating magnetic field, or in a magnetic field oscillating between zero and some maximum value. Is it accurate to conclude that it's the actual process of precession which causes electron current to flow in a conductor? If this is incorrect, can you please explain what is actually happening?

What is happening at the atomic level which makes it necessary for a magnetic field to be changing with time in order to induce an EMF in a coil of wire? Why won't a constant magnetic field induce an electron flow?

A conductor has many electrons in the conduction band. These are not tied up with the atoms they started with, but with the whole lattice of the solid, and thus act as if a free "gas" of electrons.

If one imposes a fixed electric field on a conductor, the electrons move for a while and then a separation of charges happens negative electrons predominate on one side and positive of the left over ions on the other.

Imposing a variable electric field generates the current one observes in a conductor, the electron distribution following the voltage imposed .

That is the way current is generated in a conductor.

Now if this:

such that the precession of a large population of the nuclei is maintained. This is achieved in an alternating magnetic field, or in a magnetic field oscillating between zero and some maximum value. Is it accurate to conclude that it's the actual process of precession which causes electron current to flow in a conductor? If this is incorrect, can you please explain what is actually happening?

From the above description, you should understand that the main way current is generated in a conductor is through the changes in the behavior of the electrons, which are mobile, not of nuclei which are tied up in the lattice.

The motion of electrons in fields is given by the Lorenz force. There is magnetic induction of currents but it acts on the electrons and their velocity. Interaction of the magnetic dipole moment of the electrons may generate a fine structure but will not be the main current inducing force because they are weaker couplings. These finer effects can be utilized for studying the behavior of materials.

• It is my understanding that current is induced in a conductor by a variable magnetic field. Why do you mention a variable 'electric' field? and then the question becomes: how does a magnetic field have an effect on free electrons within a conductor? Mar 4 '17 at 9:08
• Perhaps with the non-zero spin property of nuclei in a conductor, alignment of their magnetic moments by an external magnetic field causes precession which shakes off and propels free electrons in the direction of their spin. After lattice relaxation time, they no longer precess causing a halt to the shaking off/propelling of free electrons...thus a time varying magnetic field is required to refresh precession and keep current flowing. What do you think? Mar 4 '17 at 9:17
• to your first comment: it is the Lorenz force that will move the electrons in the conductor, both electric and magnetic fields can move the electrons. Not the fixed in the lattice ions. It has to do with the charge if you look at the formula. not the magnetic dipole moment of the electron. Mar 4 '17 at 12:05
• to your second, electrons which are not already in the conduction band, i.e. tied to the whole lattice collectively, are in tightly bound states and will need quantized energy packets to leave the ion . If you are proposing this instead of the Lorenz force it will not work at all. The dipole dipole due to spins interaction is much smaller than the direct moving charge with the magnetic field Mar 4 '17 at 12:12
• I understand your point, but my question still hasn't been answered. I know that Lorenz force moves electrons, but how? Why is it that an external magnetic field moves electrons in a conductor? And more specifically, why is a changing magnetic field rather than a static one necessary to do this? Mar 4 '17 at 20:05

I think you have made it too complicated. I will be boring and start with a standard metal sphere in EM field:

1. Say, you start with a metal sphere in static electric field. You sphere gets polarized along the field. Because the field is static, the charge redisctibution happens once and while the field is there nothing changes.

2. You start varying E field, the charge distribution in the sphere will respond by going back and forth with the field, hence, it will generate current.

3. If you increase the strength of this periodic field, some of the electrons might get enough energy from the field to leave the surface of the sphere and become free electrons. They will continue moving with the field creating current, external to the sphere.

4. Last step is just to add that a metal sphere is a decent approximation for both atom and nucleus. The same processes happen there. You need to supply a photon to kick electron out of the orbit and make it free to move with the field.

5. This photon can be virtual as well, but in the adiabatic limit it generates zero current.

That's the way I think about that and it might be not 100% right, so please correct me if you feel like it=)

• I was asking about varying MAGNETIC fields inducing current flow, not ELECTRIC fields. I don't know why you left that out. Mar 4 '17 at 9:22
• Because it doesn't matter what you vary. Varying magnetic or electric field you get one and the same result - real photon. In this sense electric and magnetic fields are equivalent. Mar 4 '17 at 21:48