# Moving charged particle in a magnetic field

I know when a charged particle moves across a magnetic field, a force (according to Lorentz force law) will be applied to this particle.

There are however a three things I do not understand very well.

1. Every force is a par action/reaction, so where will the reaction force be applied to? Is it to the source of the magnetic field?

2. I suppose that a moving charge across a magnetic field will experience a perpendicular force to it's velocity and to the field because the moving charge will create a magnetic field and this magnetic field created by the charge will interact with the "main" magnetic field, right? So how do exactly two different magnetic fields interact? I mean if the field created by the moving charge has no poles how can it be attracted/repulsed by the main field?

3. A moving charge creates a magnetic field. What if I have a stationary charge on my hand and someone walks around me? I guess that I won't see any magnetic field but the person walking will see?

Thanks!

• Daniel, it's typically considered good form here to postpone accepting the first answer, even if it perfectly answers your question, in order to give others the opportunity to read your question and decide to take some time to compose a quality answer. Jun 9, 2018 at 22:28
• @AlfredCentauri thanks for the advice. I will wait 1-2 more days for others to give their answer and then I will accept the best one in my point of view. Jun 10, 2018 at 0:39

1) Yes that is correct. The source of the magnetic fields itself consists of moving charge or electron spins. These will react to the magnetic field created by the first moving charge.*

2) Correct until "because the moving charge...".**

3) That is correct.

*As Feynman discusses in his famous lectures, the magnetic force does not obey Newton's third law. This is quite casually "resolved" at the end of the next chapter.

**The standard way is to use the Lorentz force and not worry too much about the fields.

Every force is a par action/reaction, so where will the reaction force be applied to? Is it to the source of the magnetic field?

If this would be so, the magnetic field has to get exhausted, but this isn’t the case. With a permanent instead of an electromagnet one is able to deflect moving charged particles as long as one need without any weakening of the magnet.

I suppose that a moving charge across a magnetic field will experience a perpendicular force to it's velocity and to the field because the moving charge will create a magnetic field and this magnetic field created by the charge will interact with the "main" magnetic field, right?

The charge has a magnetic dipole moment, it’s an intrinsic (existing independent from any circumstances) property. This magnetic moment - this you are right - gets aligned by the external field, leading to the deflection of the charge. But any deflection is an acceleration and probably you’ve learned that any acceleration is accompanied by electromagnetic radiation, this the emission of photons. This emission decreases the charges velocity and dissaligned the magnetic dipole moment again. Now the process repeats until the charge comes to rest in the centre of its spiral path.

So how do exactly two different magnetic fields interact? I mean if the field created by the moving charge has no poles how can it be attracted/repulsed by the main field?

Firstly the charge is a dipole. Secondly the external magnetic field acts like a spring during the alignment and dissalignment of the charge.