# Why does charge produce magnetic field

1. Apr 19, 2012

### sheshank

Q : I always wondered why a moving charge is always associated with a magnetic field around it. I read that, if a charge moves with respect to something, then it produces magnetic field. During the spin of charges, they rotate with respect to themselves (Or perhaps space. Justify me). So, how do they produce, and what causes them to produce magnetic field? What's happening in and around the moving charge?

2. Apr 19, 2012

### Staff: Mentor

Why do charges produce an electric field? ;)
It is unknown why electromagnetism (or, more general, the electroweak force) exists. But it has been observed that it exists and that it satisfies the Maxwell equations. These equations state that a moving charge produces a magnetic field.

This is independent of spin. You would get magnetic fields from charged particles without spin, too.

3. Apr 19, 2012

### sheshank

Yeah... a Charge moving in a space (not spinning), and moving along in the same direction on and on also produces magnetic field. Its right. But, my question is, why is it so?

I think there needs to be some theory proposed for this. Experiments should be done. That would help us more greater than the present experiments.

4. Apr 19, 2012

### Staff: Mentor

There is some theory. At the deepest level, this is the quantum field theory of the electroweak force. You can use this, add a symmetry breaking (Higgs mechanism), and you get the weak and the electromagnetic force. You can then proceed and derive an effective theory for the electromagnetic force. This produces the Maxwell equations. You can then use these equations and get as result that moving charges produce a magnetic field. This requires a lot of work, but it can be done and it has been done.

The remaining question is "why do we have the electroweak interaction with its broken symmetry"? And nobody knows the answer to this. It is just an experimental result. Maybe its existence follows from some deeper theory, which is not known at the moment. But this just adds another step of derivation.

5. Apr 19, 2012

### sheshank

Excellent bro. I think soon we are going to find out this.

Kudos for your framework about this.[/COLOR] Any way thank you all for this. I only get to know that its an undeveloped field yet. I think it has left some scope for me.

6. Apr 19, 2012

### Mindscrape

Classically, we observe that a magnetic field is generated according to the Biot-Savart law for both a single charge or a current density. This is something that's observed, a law of nature. So if you want to go deeper you eventually end up hitting a spot where you just have to concede that this is the way nature works and will continue to work. Why laws of nature are constant, though I hear some physicists argue they are not, is a whole new question.

7. Apr 19, 2012

### Bob S

There are two ways a magnetic field can be generated by a charge or current, both of which are contained in Maxwell's equation:
$$\nabla x \overrightarrow{H}=\sigma \overrightarrow{E}+\epsilon\frac{\partial \overrightarrow{E}}{\partial t}$$
where σ is conductivity and ε is permittivity.

8. Apr 19, 2012

### Whovian

I'm surprised no one's mentioned the concept of a photon/W/Z "cloud" which is the cause of the forces between particles affected by the electroweak force yet; i.e., force carriers.

9. Apr 19, 2012

### sophiecentaur

There is never a complete answer to this question. However deep you care to go in the 'why' direction, there is always another level so you will never get a satisfactory answer if that's what you want.
The other posts have shown you various levels of info about the fact that moving charges produce a magnetic field - take your pick as to the one that you feel is good enough for you as an explanation.

10. Apr 19, 2012

### QuantumPion

11. Apr 19, 2012

### tiny-tim

hi sheshank!
it's an inevitable consequence of Coulomb's law and newtonian relativity …

a uniformly moving charge is (obviously) a stationary charge in its own reference frame, and has a purely radial field that obeys Coulomb's law, F ~ 1/r2

if you graph how a particle moves in that frame, and then convert to the original frame, you'll see that in the original frame there appears to be (and therefore is) a velocity-dependent force