Why is the magnetic field of a wire circular

In summary: intensity...of the current ?their product is directly proportional to the...intensity...of the current ?
  • #1
Entanglement
439
13
The title is enough
 
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  • #2
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
jtbell said:
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!

Could you please copy and paste these links in a reply because the mobile version doesn't allow make these links appear
Thanks very much
 
  • #4
unfortunately, I couldn't get much from related discussions, could you provide a thorough explanation,
thanks in advance
 
  • #6
It's simply due to the cylindrical symmetry of the system.
 
  • #7
UltrafastPED said:
It's an experimental observation, first made in 1820. See http://inventors.about.com/od/lessonplans/ht/magnetic_fields.htm

The Biot-Savart law was inspired by this and other observations.
See http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/biosav.html

So you should be able to start with the Biot-Savart law, and get back to Oersted's observation.
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.
 
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  • #8
WannabeNewton said:
It's simply due to the cylindrical symmetry of the system.
How isn't it a physics question if I want a deeper answer??
 
  • #9
ElmorshedyDr said:
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.

Mathematics _is_ logic; and if you have a good intuition for geometry, you will do better with physical intuition as well.
 
  • #10
Isn't there any other explanation, because bio-Savarts law is a way beyond my level b
 
  • #11
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
Why isn't there any poles
Why are the lines rotating around the wire and not ending or beginning at certain poles ?
 
  • #13
"Why" questions in physics are always so ill-posed.

Why not?
Can you answer that?
 
  • #14
ElmorshedyDr said:
Why isn't there any poles
Why are the lines rotating around the wire and not ending or beginning at certain poles ?

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?
 
  • #17
Ampere Law states that

2[itex]\pi[/itex]r[itex]\Huge[/itex] [itex]\propto[/itex] [itex]I[/itex]

[itex]\beta[/itex] [itex]\propto[/itex] [itex]I[/itex]

2[itex]\pi[/itex]r[itex]\LARGE[/itex] is the maximum circumference for the field??
 
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  • #18
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
mfb said:
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.
What does it mean that circumference is directly proportional to the intensity ? Since there are infinite concentric circles
 
  • #20
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.
 
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  • #21
mfb said:
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.

I think Br [itex]\propto[/itex] [itex]I[/itex] seems more meaningful
their product is directly proportional to the intensity
 
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  • #22
how is B at the center of a circular loop and a solenoid derived from :
Ampere's circular law
B = [itex]\mu[/itex][itex]I[/itex] [itex]/[/itex] 2[itex]\pi[/itex]r
 
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  • #23
ElmorshedyDr said:
I think Br [itex]\propto[/itex] [itex]I[/itex] seems more meaningful
their product is directly proportional to the intensity
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
ElmorshedyDr said:
how is B at the center of a circular loop and a solenoid derived from :
Ampere's circular law
B = [itex]\mu[/itex][itex]I[/itex] [itex]/[/itex] 2[itex]\pi[/itex]r
?
 
  • #26
My Math Abd physics level doesn't allow me to understand bio-Savart's law isn't there any other way to understand via amperes law
 
  • #27
I don't think you can use Ampere's Law for this (magnetic field at the center of a circular loop). There isn't enough symmetry. I don't remember ever seeing this done using Ampere's Law.

However, you can use Ampere's Law to find B inside a long straight cylindrical solenoid, because of its symmetry:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html#c2
 
  • #28
You can use it. Take a surface integral of amperes law on a circle. Stokes theorem tells us the surface integral of the curl of B is equal to the line integral of the tangential magnetic field around the loop bounding the circle. From symmetry, the magnetic field cannot have any on the azimuth angle, so the local field can be derived using the radius of the circle.
 

1. Why is the magnetic field of a wire circular?

The magnetic field of a wire is circular because of the right-hand rule. When current flows through a wire, it creates a magnetic field that wraps around the wire in a circular shape. The direction of the magnetic field is determined by the direction of the current flow, following the right-hand rule.

2. What causes the circular shape of the magnetic field?

The circular shape of the magnetic field is caused by the flow of current through the wire. When current flows through a wire, it creates a magnetic field that wraps around the wire in a circular shape. This circular shape is a result of the interaction between the electric current and the magnetic field.

3. Does the strength of the magnetic field affect its circular shape?

Yes, the strength of the magnetic field does affect its circular shape. The strength of the magnetic field is directly proportional to the current flowing through the wire. Therefore, the stronger the current, the stronger and more pronounced the circular shape of the magnetic field will be.

4. Can the circular shape of the magnetic field be changed?

Yes, the circular shape of the magnetic field can be changed by altering the current flow through the wire. Changing the direction or strength of the current will result in a change in the direction or shape of the magnetic field. Additionally, external magnetic fields or the presence of other conductors can also affect the shape of the magnetic field.

5. Is the circular shape of the magnetic field present in all wires?

Yes, the circular shape of the magnetic field is present in all wires when current flows through them. This is a fundamental principle of electromagnetism and is applicable to all wires, regardless of their size or material. However, the strength and shape of the magnetic field may vary depending on the properties of the wire and the current flowing through it.

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