Magnetic field of running current in a wire

[yrjosmiel]

Why are they circular?
You know, like this:

Why not straight toward the wire?

 

[sophiecentaur]

Why are they circular?
You know, like this:
View attachment 209712
Why not straight toward the wire?

Because that's the way it is. Electric fields can radiate outwards from a point or line but the Magnetic Lines of Force have no beginning or end (even in a bar magnet, the lines can be regarded as running through the metal between the two poles.
You cannot have an isolated magnetic pole but to understand why, you need to get into and be familiar with Maxwell's Equations. I don't know of a simple explanation.

 

[CWatters]

Hint: Why do contour lines on a map go in circles around a hill and not straight up and down the slope?

 

[Dadface]

Due to the symmetry of the arrangement I think the circular shapes are predictable.

 

[pixel]

Due to the symmetry of the arrangement I think the circular shapes are predictable.

Symmetry alone wouldn't rule out radial lines.

 

[sophiecentaur]

Symmetry alone wouldn't rule out radial lines.

Symmetry plus the other factors.

 

[yrjosmiel]

Hint: Why do contour lines on a map go in circles around a hill and not straight up and down the slope?

To represent magnitude?
If that is so, then why the arrows in the field? Why is there a counter-clockwise turning?

 

[CWatters]

Yes. As with contour lines the closer they are together the stronger the field. The arrows indicate the direction of the field.

They point from north to south but north and south poles are just names we allocated years ago. Just as we defined positive current as electricity flowing from positive to negative terminals on a battery.

See also Fleming rules.

 

[yrjosmiel]

They point from north to south but north and south poles are just names we allocated years ago. Just as we defined positive current as electricity flowing from positive to negative terminals on a battery.

Where exactly is the "north" and the "south" in the wire?

 

[Clausen]

Why are they circular?
You know, like this:
View attachment 209712
Why not straight toward the wire?

Good question!

Basically, the magnetic field lines are circular because the wire is circular.

For a typical straight circular wire carrying current, the magnetic field is generated to be perpendicular to the direction of the current.

Around a circular wire, the magnetic field lines will be concentric circles with the current perpendicular to the planes of the circles.

But, it doesn’t always have to be this way! You could have a conductor that has a rectangular cross section, for example, and then determining the shape of the magnetic field lines can get very “interesting”. See this paper for details.

 

[CWatters]

Where exactly is the "north" and the "south" in the wire?

North and South are not in the wire.

Magnetic field lines do not have a "start" or an "end" that you call north or south. The just have a direction from North to South. Even for a bar magnet the magnetic field lines go through the magnet itself.

 

[Dadface]

I would like to add a bit more detail for the case of the case of the straight wire. The circular field lines we see are idealised and take into account the straight section of wire only. They do not take into account the field from other places.

To actually plot the field we have to drive a current through the wire and to do that requires a circuit with a power supply and one or more connecting wires. In other words the actual field we observe in a plane normal to the wire, is not due to the straight section of wire only but is due to the circuit loop.

I like to think of the loop as a mini solenoid with the the poles very close to each other and one at each end of the loop. The attached diagram which is also idealised shows the field due to a circular loop.

 

[Bart Lx]

To represent magnitude?
If that is so, then why the arrows in the field? Why is there a counter-clockwise turning?

My first question would be: why does the image say 'lines clockwise in direction of current', while the magnetic field lines around a
wire are always counterclockwise to the direction of current...? The image is wrong (or at least, the text inside it)

 

[CWatters]

If you look downwards it's counter clockwise but if you look "in the direction of the current" it is clockwise.

 

[Bart Lx]

If you look downwards it's counter clockwise but if you look "in the direction of the current" it is clockwise.

No matter how I look at it, it remains counterclockwise.

 

[CWatters]

Look at it from underneath.

 

[Korak Biswas]

Why are they circular?
You know, like this:
View attachment 209712
Why not straight toward the wire?

This is just because the nature creates magnetic field in this way.

I think you should rather ask about that law of nature which dictates the magnetic field to be so.

Well, there are four equations in Electrodynamics known as Maxwell's equations. Those equations describe the 'behaviour' of electric and magnetic filed. Using those you can easily find out the magnetic filed created by a straight current carrying wire.