# Do moving charges produce magnetic fields?

As I understand it, a moving charge produces a magnetic field. Now it may be that I have have a misunderstanding here and if so I need to have the proper understanding of this. So if I do have a misunderstanding here could someone correct me on this. Based on my current understanding though my question concerns moving charges and why they're not producing measurable magnetic fields. Let me give two examples.

Example 1 - The Mir space station is moving through space relative to earth at a speed of roughly 7660 m/s. That's quite fast compared to most moving things here on earth. It has a mass of 450,000Kg. That's a ton of charges all moving together as one. Mass obviously is composed of things like protons and electrons which are charged particles. Doing a rough estimate that is 2.56 x 10^32 charged particles moving through space quite fast. With that amount of charge moving that fast why don't wee see a magnetic field coming off the space station?

Example 2 - The moon doesn't have a magnetic field based on current measurements. Obviously there's quite a bit more charges comprising the moon than even the Mir space station. It moves through space relative to earth at roughly 1000 m/s. That's a bunch of charges moving at a decent clip. Why don't we observe a magnetic field associated with the moon?

The Mir Space station is composed of a lot of protons and a lot of electrons, but their charges are opposite, while their velocity isn't, this means that any magnetic field produced will cancel itself out. (Unless there is an unbalanced number of protons/electrons, and the station has a net charge)

This is the same thing with the moon.

it turns out that the magnetic interaction is just a manifestation of the static electric interaction, then also taking into account the effects on observation of special relativity. you can set up a thought experiment of two identical, parallel, and infinite lines of charge. if they sit stationary next to an observer, that observer will observe the two lines repelling each other at some rate.

but of the two lines of charge are whizzing past the observer at high speed (remaining parallel to each other), that observer will observe the rate of repulsion to be reduced in comparison to the stationary case. that rate of reduction can be accounted for by special relativity and no separate magnetic interaction or with no relativistic effects and an interaction we call "magnetism" that is distinct from electro-static interaction.

Did you ask yourself, how charged particles produce a common magnetic field? Every charged particle has a magnetic dipole moment and a corresponding intrinsic spin. Suppose you have a flow of electrons. Naturally nor the dipole magnetic moments nor the intrinsic spins are aligned.

There are two possibilities to get a common magnetic field of this electrons. First, it is possible, that the current goes through an external (non parallel to the flow) magnetic field. Then the magnetic dipole moments of the electrons will be aligned. We have not to forget, that the intrinsic spins will be aligned too. So the electrons get deflected. This we call the Lorentz force.

Second, the flow of electrons happens in a coiled wire. Then the intrinsic spins get aligned due to the gyroscopic effect. As a result the magnetic dipole moments get aligned too. This we call magnetic induction.

Last step we have to take in attention is, that the moon and a satellite too are moving without acceleration and this revolution around the earth is not the same situation as the electrons revolution inside a coil. No acceleration, no gyroscopic effect.

• Even electric charges without intrinsic magnetic dipoles moments produce magnetic fields when they move. Additionally, objects that are orbiting the earth are accelerating. – d_b Jul 5 '15 at 19:15
• User37496 All electric charges (electron, proton, positron, ...) have magnetic dipole moments. And neutron has such a moment too. – HolgerFiedler Jul 6 '15 at 5:15
• That's true, but we can predict the magnetic field of a moving charge classically without invoking the intrinsic dipole moment whatsoever. – d_b Jul 6 '15 at 5:30
• Note that (a) the alignment effect is very weak at the average velocities on day-to-day currents in wires and (b) without a great deal of effort the alignment of spins in a current is randomly parallel or anti-parallel so the expectation is for zero net effect. Finally, if the effect you suggest were present we be able to measure a difference in the strength of the induced field divided by current between room temperature and cryogenic currents. – dmckee Jul 6 '15 at 19:33
• @dmckee What parallel and antiparallel intrinsic spin means? If you look on two particles with different spin (but both axis in the observation plane), their axis could be oriented in all 360°. If rotate their axis to 0° there are exact two possible of spin - left fist and right fist. The point is, that the related magnetic dipole moments are antiparallel to. If influent this particles by an external magnetic field, all particles get aligned identical. – HolgerFiedler Jul 7 '15 at 5:22

It's already known that electric charges produces magnetic field known as electromagnet. Yes it's true moving charges produce magnetic fields but in most cases the magnetic capacity are low. The reason the magnetic fields of the moon don't exist is bc it is given off

• It's unclear what "magnetic capacity" is supposed to be, or how the moon "gives off" magnetic fields. – ACuriousMind Jul 15 '15 at 6:06