# Magnetic field

1. Oct 18, 2006

### doniv

i have a doubt as to why we get a magnetic field around a current carrying conductor.

2. Oct 18, 2006

### quinn

good for you,.. any specifics

3. Oct 18, 2006

### tim_lou

i do not think there is any proof or description of WHY there is magnetic field. the Maxwell's equation is sort of "axiomic" (at least as far as my limited knowledge goes)... magnetic field exists near a current carrying conductor simply "because" when you put a compass near it, the compass changes direction. So according to experiments, magnetic field exists... (well, many times physics is just non-contradicting theories that correctly approximate experiments. sometimes, there just aren't derivations or explainations)

Last edited: Oct 18, 2006
4. Oct 19, 2006

### Integral

Staff Emeritus
Your "why" is not a question addressed by Physics. We observe a magnetic field surrounding a conductor. Maxwell worked out the mathematics which describe the phenomena. We can use the mathematics to make predictions. That is physics. Why it is so is philosophy.

5. Oct 19, 2006

### quinn

I respond by stating that an electric field has a physical extent and if that field crosses another field they must interact. The interaction of these fields might be such that it is easier for someone to understand if they break the interactions into parellel and perpendicular components that they label with different names such as electric and magnetic fields

6. Oct 19, 2006

### doniv

7. Oct 19, 2006

### ahsaas

hi, i am new to this forum.
well in physics there are some conjugate processes. just as the change in magnetic flux passing through a coil generates electric field as given by FARADAY'S LAW. so the movement of electric charge generates magnetic field. well there is no explanation of the same. but MAXWELL'S equations provide a handsome explanation of the mathematics of relationship between electric and magnetic fields.

8. Oct 19, 2006

### rewebster

i have a doubt as to why we get a magnetic field around a current carrying conductor.

Your "why" is not a question addressed by Physics. We observe a magnetic field surrounding a conductor. Maxwell worked out the mathematics which describe the phenomena. We can use the mathematics to make predictions. That is physics. Why it is so is philosophy.

Do you mean the the question should have been:

"i have a doubt as to how we get a magnetic field around a current carrying conductor."

to be a physics question?

or either question is and should be in philosophy?

9. Oct 20, 2006

### Staff: Mentor

It's because of Maxwell's equations for the electric and magnetic field, in particular the one that relates the curl of the magnetic field to the current density.

This begs the question, "why are Maxwell's equations the way they are?"

It's because the universe apparently obeys a local U(1) gauge symmetry.

This begs the further question, "why does the universe obey a local U(1) gauge symmetry?"

To that, nobody has an answer yet, as far as I know. :uhh:

Last edited: Oct 20, 2006
10. Oct 20, 2006

### rewebster

This begs the question, "why are Maxwell's equations the way they are?"

Are you thinking--'Why did Maxwell write them the way he did?'

or--maybe something else??

------------------

I wonder what Maxwell or Faraday would think about relativity or string theory?

11. Oct 20, 2006

### masudr

Doesn't the gauge symmetry only give rise to the interaction term $j^\mu A_\mu$ in the Lagrangian?

12. Nov 6, 2006

### Staff: Mentor

Isn't that term in the Lagrangian what gives rise to Maxwell's equations, via the "equations of motion" of the $A_\mu$ fields, and the definition of $\vec E$ and $\vec B$ in terms of $A_\mu$? At least that's the way I remember the general idea. It's been a long time since I've seen or worked through the details.

13. Nov 6, 2006

### masudr

I believe the equations of motion arise from varying $F^{\mu\nu}F_{\mu\nu}$ with respect to the $A_\mu$.

Imposing a local gauge symmetry of U(1) then gives rise to the $j^\mu A_\mu$ term, thus showing how the source couples to the field.

I say this all on the basis of if I remember correctly; I'm sure someone could confirm this or tell me that I am wrong.

14. Nov 6, 2006

### StatMechGuy

I think this is a little more sophisticated of an answer than the OP was looking for...

15. Nov 6, 2006

### tehno

And God said :"Let there be a magnetic field around a current carrying conductor."