# Why is the magnetic field of a wire circular

1. Apr 17, 2014

### Entanglement

The title is enough

2. Apr 17, 2014

### Staff: Mentor

Look at the list of "Related Discussions" at the bottom of this thread, and you'll find two threads with the same question as yours. One of them has 24 responses, which looks promising!

3. Apr 17, 2014

### Entanglement

Thanks very much

4. Apr 17, 2014

### Entanglement

unfortunately, I couldn't get much from related discussions, could you provide a thorough explanation,

5. Apr 18, 2014

### UltrafastPED

6. Apr 18, 2014

### WannabeNewton

It's simply due to the cylindrical symmetry of the system.

7. Apr 18, 2014

### Entanglement

I know the observation, but I can't figure out its reason. Bio-Savarts law will only guide me to the direction of the field mathematically not logically nor intuitively.

Last edited: Apr 18, 2014
8. Apr 18, 2014

### Entanglement

How isn't it a physics question if I want a deeper answer??

9. Apr 18, 2014

### UltrafastPED

Mathematics _is_ logic; and if you have a good intuition for geometry, you will do better with physical intuition as well.

10. Apr 18, 2014

### Entanglement

Isn't there any other explanation, because bio-Savarts law is a way beyond my level b

11. Apr 18, 2014

### Staff: Mentor

What else than circular could it be? Do you expect rectangles? Why would you expect one direction from a straight wire look any different from other directions?

12. Apr 18, 2014

### Entanglement

Why isn't there any poles
Why are the lines rotating around the wire and not ending or beginning at certain poles ?

13. Apr 18, 2014

### mikeph

"Why" questions in physics are always so ill-posed.

Why not?

14. Apr 18, 2014

### UltrafastPED

Because magnetic fields don't originate from "magnetic charges". Thus they always go in a loop.

You see this if you have some iron filings (in a sealed plastic box) - as you bring any magnet close the filings will form an alignment which connects the poles.

If you stick two magnets together, N-S or N-N, you will see changes in the alignment, but always in a closed loop.

If you "break" a magnet it will always have two poles.

One of Maxwell's equations describes this behavior, but you will need to study vector calculus to understand the math; at this point simply try to understand what you are seeing, and how it changes with the experiment.

For example, what do you expect the magnetic field to look like if you bend the current carrying wire into a loop? Into a long coil?

15. Apr 18, 2014

### Staff: Mentor

16. Apr 18, 2014

### Entanglement

17. Apr 18, 2014

### Entanglement

Ampere Law states that

2$\pi$r$\Huge$ $\propto$ $I$

$\beta$ $\propto$ $I$

2$\pi$r$\LARGE$ is the maximum circumference for the field??

Last edited: Apr 18, 2014
18. Apr 18, 2014

### Staff: Mentor

2πr is just the exact circumference of a circle with radius r.
There is nothing "maximal". The (theoretical, ideal) field extends to infinity.

That is a weird way to reduce Ampere's law.

19. Apr 18, 2014

### Entanglement

What does it mean that circumference is directly proportional to the intensity ? Since there are infinite concentric circles

20. Apr 18, 2014

### Staff: Mentor

As I said, that is a weird way to reduce Ampere's law. You could read it "with larger current, you get the same field strength (which you omitted) at a larger radius".
And constant factors like 2π are irrelevant anyway if you look at proportional quantities.

21. Apr 18, 2014

### Entanglement

I think Br $\propto$ $I$ seems more meaningful
their product is directly proportional to the intensity

Last edited: Apr 18, 2014
22. Apr 18, 2014

### Entanglement

how is B at the center of a circular loop and a solenoid derived from :
Ampere's circular law
B = $\mu$$I$ $/$ 2$\pi$r

Last edited: Apr 18, 2014
23. Apr 19, 2014

### Staff: Mentor

Yes.

B at the center of a circular loop is similar to a circular line around a straight wire. Both can be calculated from the more general Biot-Savart law.

24. Apr 21, 2014

### Entanglement

?????????

25. Apr 21, 2014