0

Take a battery, stick a magnet under it and then place a perfectly circular wire on top of the battery, in such a way to form a circuit.

We now have an homopolar motor with a core that has perfect cylindrical symmetry.

Our setup clearly has a preferential vertical direction, that is specify through the orientation of the battery and the magnet, but it has no preferential angular orientation, it's symmetrical under rotation around the vertical axis ($z$ axis).

So why when we place the wire it starts turning? It will start turning either clockwise or counterclockwise, defining a preferential rotational direction. How can a cylindrically symmetrical setup give rise to a rotation, from a symmetry standpoint? (I know of course about the Lorentz Force)

Or if you like to see this question under a different light: can we use this setup to explain to far away aliens what we mean by left and right? (Maybe there would be problems with the magnet direction..)

(One possible way to think about this is that when you observe a rotation there are only really two possibilities: clockwise or counterclockwise, that can be represented with a vector perpendicular to the rotation's plane. So one could argue that there is no symmetry problem here. However I feel that this explanation needs expansion and improvement.)

Noumeno
  • 4,474

1 Answers1

0

From the direction of rotation of the wire, one knows the north-south direction of the magnetic dipole. This is based on the flow of electrons in the wire. As you have noted correctly, this has to do with the phenomenon of the Lorentz force.

At the time of Lorentz, the cause of deflection of a current-carrying wire in a magnetic field was still unknown. Today we know that in a magnetic field electrons and anti-protons moving in a plane perpendicular to the magnetic field are deflected in one direction, while protons and positrons are moved in the opposite direction.

How can a cylindrically symmetrical setup give rise to a rotation, from a symmetry standpoint?

<tldr The Lorentz force is a part of the phenomenon of electromagnetic induction. Besides the causality magnetic field B and movement of a charge v*q results in a deflection of the particle (electric motor), there are also the causalities magnetic field and forced deflection results in a current flow (electric generator) and current flow in a coil results in a magnetic field (electromagnet).

The Lorentz force or more common the electromagnetic induction is based on the nature of the subatomic particles. These particles are not only electric charges but also small magnetic dipoles.

The electric charge is exploited to move the particles in an electric potential field, the magnetic dipole is exploited to align the particles along the dipoles in an external magnetic field and make a magnet out of a non-magnetic body. tldr>

When an electron moves in a magnetic field thanks to an electric potential difference, the electron is deflected laterally. What Lorentz did not see is the emission of electromagnetic radiation by the deflected electron. One can see it also in such a way that the emission of the radiation is the deeper cause of the electron deflection.

<tldr How and why do accelerating charges radiate electromagnetic radiation? tldr>

HolgerFiedler
  • 10,334