# Magnetic field due to moving charges?

1. Dec 20, 2013

### harjot singh

explain me a simple reason why magnetic field is only caused when charges are moving and not when their is no current moving but that is quite tricky even because if we say that their is no potential difference between the wire but in that case also electrons are moving in a random path still,the only thing i want to know is that why cant a magnetic field be produced when current is not there because if electrons are randomly moving than still for the charge outside the wire which will experience the magnetic force in its frame of reference the inside electrons are still moving although randomly but still moving so please explain me everything i know i am wrong in very much parts but please correct me!

2. Dec 20, 2013

### Staff: Mentor

The magnetic fields of electrons moving in opposite directions cancel - if you just have unordered motion without a net current, all contributions cancel.

3. Dec 20, 2013

### harjot singh

thanks mentor mfb could you please explain me the real cause behind magnetism from moving charges because some says its due to relativity in concern but others include maxwell's equations i atlast ask you to tell me what is the real fact of this unusualness of electromagnetism?

4. Dec 20, 2013

Staff Emeritus

What makes you think that Maxwell's Equations and Special Relativity can't both be explanations?

5. Dec 20, 2013

### Staff: Mentor

There is no (known?) reason for physical laws. Our universe just behaves that way.
There is no "unusualness".

You can use the Maxwell equations, or you can use the equations for static fields (in the system of the charge) together with transformations of special relativity. Both methods are fine to describe the process - but they don't answer "why" questions.

6. Dec 21, 2013

### harjot singh

explain to me why magnetic field is not produced when their is no current because still electrons are moving but in random way but still moving then according to length contraction their density should increase action and they should repel a negative charge but nothing happens in absence of current? why?

7. Dec 21, 2013

### Staff: Mentor

That's what I explained in post 2.

8. Dec 21, 2013

### harjot singh

could you please explain me similiar magnetism in bar magnets because their is no current running even out their then how there is a magnetism associated?

9. Dec 21, 2013

### Staff: Mentor

It has to do with the alignment of unpaired electrons and their spins.

10. Dec 30, 2013

### Entanglement

Because when the electrons are randomly arranged there is a very weak field, it is so weak that it can't be taken in consideration, because the produced fields have different directions so they cancel each other, but at a current all, the electrons are directed in the same direction producing a unidirectional field, same thing how ferromagnetic materials turn into magnets when affected by another magnet and then lose their magnetism when the effect of the other magnet vanishes