Need help with the magnetic field generated by this current distribution

[Magnetosphere]

For north and south pole, I don't see any obvious ones. That would be a good place to start with a qualitative solution if you could demonstrate some kind of direction in the magnetization, but from what I can see, the magnetization $\vec{M}$ might run in a circle, (around the whole disc), in the upper part of the disc, and in an opposite circle on the lower part. In that case, It appears poles might be virtually absent.

To determine magnetic poles of the disk one would assume to follow the right hand rule, when I imagine it it seems north is the top of the disk.

 

[Charles Link]

LOL, this is what i can´t wrap my head around.

An example of this is a toroidal solenoid with an iron core. The magnetization $M$ goes around the toroid in a circle. This generates virtually no magnetic field outside the toroid, but the field is very strong inside. Meanwhile there are no magnetic poles. $\nabla \cdot M=0$ or very close to it.

 

[Magnetosphere]

An example of this is a toroidal solenoid with an iron core. The magnetization $M$ goes around the toroid in a circle. This generates virtually no magnetic field outside the toroid, but the field is very strong inside. Meanwhile there are no magnetic poles. $\nabla \cdot M=0$ or very close to it.

Yes, but a toroidal solenoid does not have a parallel electric field like this disk. Here the electric force is at the same angle through the medium.

 

[Charles Link]

You have a very interesting problem, but I would suggest getting a very good proficiency with some of the more well-known magnetostatic problems, such as cylindrical magnets, and the problem of a magnetized sphere with uniform magnetization before attempting something of this degree of difficulty. See e.g. https://www.physicsforums.com/threa...perature-relationship-in-ferromagnets.923380/ You might find the students' experiment of some interest. See also post 21, where I used a boy scout compass to measure the magnetic field strength of a cylindrical magnet.

 

[Magnetosphere]

You have a very interesting problem, but I would suggest getting a very good proficiency with some of the more well-known magnetostatic problems, such as cylindrical magnets, and the problem of a magnetized sphere with uniform magnetization before attempting something of this degree of difficulty. See e.g. https://www.physicsforums.com/threa...perature-relationship-in-ferromagnets.923380/ You might find the students' experiment of some interest. See also post 21, where I used a boy scout compass to measure the magnetic field strength of a cylindrical magnet.

I need to find out that´s why I found this place. I suspect the answer is easier than I think, I just can´t see it on my own. I strongly suspect the disk has poles, that´s really all I need to know, does it have poles and where are they in relationship to the axis?

 

[Charles Link]

I need to find out that´s why I found this place. I suspect the answer is easier than I think, I just can´t see it on my own. I strongly suspect the disk has poles, that´s really all I need to know, does it have poles and where are they in relationship to the axis?

I have seen a lot of magnetostatic problems, but this one is new to me. Maybe there is a simple answer to it, but it isn't immediately obvious to me what that might be.

 

[Magnetosphere]

 

[kuruman]

I strongly suspect the disk has poles ...

Why? In what direction would the South-North pole direction be? The only possibility would be along the disk axis. Now the wire going into the disk cannot generate a field along that axis and the field generated by a radial current flowing in one direction would be canceled by the radial current flowing in the opposite direction at 180^o from it.

Consider this. Very close to the plane of the disk at height $h<<<R$ above it, the disk looks like an infinite plane. What is the magnetic field due to an infinite plane? Start from there and see if you can extend this picture to distances not as close to the disk.

 

[Magnetosphere]

Why? In what direction would the South-North pole direction be? The only possibility would be along the disk axis. Now the wire going into the disk cannot generate a field along that axis and the field generated by a radial current flowing in one direction would be canceled by the radial current flowing in the opposite direction at 180^o from it.

Consider this. Very close to the plane of the disk at height $h<<<R$ above it, the disk looks like an infinite plane. What is the magnetic field due to an infinite plane? Start from there and see if you can extend this picture to distances not as close to the disk.

I suspect only because the electric field is at a plane, one would conclude magnetism 90 degrees to that plane. You are right that the current is flowing in the opposite direction, that is bothering me. I have a hard time imagining that part with infinite plane, that´s above my capabilities.

 

[kuruman]

I suspect only because the electric field is at a plane, one would conclude magnetism 90 degrees to that plane. You are right that the current is flowing in the opposite direction, that is bothering me. I have a hard time imagining that part with infinite plane, that´s above my capabilities.

https://www.physics.byu.edu/faculty/christensen/Physics%20220/FTI/30%20Sources%20of%20the%20Magnetic%20Field/30.14%20Magnetic%20field%20of%20an%20infinite%20current%20sheet.htm

 

[Magnetosphere]

https://www.physics.byu.edu/faculty/christensen/Physics%20220/FTI/30%20Sources%20of%20the%20Magnetic%20Field/30.14%20Magnetic%20field%20of%20an%20infinite%20current%20sheet.htm

Thanks, an infinite plane would mean there is no in or out, the magnetic field could not complete its loop as I see it. But the disk is not an infinite plane.

 

[kuruman]

Thanks, an infinite plane would mean there is no in or out, the magnetic field could not complete its loop as I see it. But the disk is not an infinite plane.

You missed the point. If you are one micron above a disk that is 10 cm in radius at distance 5 cm the center, the ends and the center of the disk are very very far away relative to your height above the plane. For all intents and purposes the disk looks like an infinite plane and the B-field locally is parallel to the surface and perpendicular to the radial direction. Now if you keep the distance of 5 cm constant and you go around the circle that picture is maintained. So what do you think the closed loop magnetic field lines look like at least very near the surface of the disk?

 

[Tom.G]

Suspiciously similar to a Homopolar generator. Anyone have access to Maxwell simulation software?

 

[Magnetosphere]

You missed the point. If you are one micron above a disk that is 10 cm in radius at distance 5 cm the center, the ends and the center of the disk are very very far away relative to your height above the plane. For all intents and purposes the disk looks like an infinite plane and the B-field locally is parallel to the surface and perpendicular to the radial direction. Now if you keep the distance of 5 cm constant and you go around the circle that picture is maintained. So what do you think the closed loop magnetic field lines look like at least very near the surface of the disk?

When I visualize your description I see magnetic forces going in a circle around the axis and closing the magnetic loop just outside the periphery of the disk (this is most probably wrong). I am not a scientists and not used to this stuff. The shaft being the positive electrode, what direction would the magnetic lines go? I really have no idea. How do you think the closed loop magnetic field lines look like at least very near the surface of the disk? Is this really a tough question or there is clear yes or no to whether the disk has clearly defined poles? I am a complete amateur, that´s why I came here in hopes to get an answer from a scientist.

 

[Magnetosphere]

So much text written and not a single step forward, very typical for forums. I feel like all posts should be removed as they really do not contribute to any forward movement, not that I don't appreciate peoples involvement, it´s just that all those words do not bring anyone looking for a similar problem any closer to the answer, too much text to go through without the award one is looking for.

 

[berkeman]

So much text written and not a single step forward

Actually, you have a pretty good qualitative idea now of what the B-field looks like close to the disc. For a better solution, numerical simulation software has been suggested several times. Do you have an application in mind? Or is this just for general interest?

 

[Magnetosphere]

Actually, you have a pretty good qualitative idea now of what the B-field looks like close to the disc. For a better solution, numerical simulation software has been suggested several times. Do you have an application in mind? Or is this just for general interest?

It´s all wild guesses and I am not convinced my idea how the b field looks like is right at all. Numerical simulation has been suggested but that is an overkill. I can´t spend days learning a new software when all I need to know whether the disk has poles or not. What I want to do is run dc voltage through the disk and align all magnetic forces accordingly to the electric current with the help of a lab magnet.

 

[berkeman]

What I want to do is run dc voltage through the disk and align all magnetic forces accordingly to the electric current with the help of a lab magnet.

What do you mean by "align the magnetic forces"? Do you mean you want to magnetize the iron disc in the radial direction?

 

[Magnetosphere]

No, the radial direction is the direction of the electricity, I need to place a lab magnet in line with the magnetic forces that are produced by the electricity.

 

[berkeman]

No, the radial direction is the direction of the electricity, I need to place a lab magnet in line with the magnetic forces that are produced by the electricity.

When the B-field lines close on themselves (as it appears they do above and below your disc), you could place a small bar magnet lined up parallel to the B-field lines, I guess.

Are you familiar with the B-field lines inside a toroid? The circulating B-field inside looks similar to what it probably looks like above your disc...

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/tor.png

 

[Magnetosphere]

I am not convinced it is the same, if you tell me I am wrong and that you absolutely know this for a fact I will take it for a fact. as I see it this is completely different from my disk. The current in my disk is not turning, it is just going outwards from the axis to the periphery.

 

[gneill]

No, the radial direction is the direction of the electricity, I need to place a lab magnet in line with the magnetic forces that are produced by the electricity.

If you are in possession of the device, why not place a piece of cardboard on top of it and sprinkle some iron filings?Edit: Fixed a silly typo

 

[Magnetosphere]

If you are in possession of the device, why not place a piece of cardboard on top of it and spring some iron filings?

Yes, I thought of that too, I don´t have the means of passing such a current through the disk, I also need to be able to control the amps so the disk doesn't get too hot

 

[Tom.G]

This reminds me of the beginnings of scientific thought. Believe all from past philosophers (aka scientists) as absolute truths. If something does not match, don't run an experiment, just ask someone else.

place a piece of cardboard on top of it and spring some iron filings

I don´t have the means of passing such a current through the disk

For the above suggested experiment use a flashlight battery as a current source. A size 'AA' Alkaline cell has a shortcircuit current of 8 amps so you would dissipate less than 6 Watts in the disk. Prepare the cardboard and filings on the disk and then make battery contact. Even momentary contact will align the filings. With the stated disk size (5cm Dia, 1cm thick) there will be plenty of time before significant disk heating occurs. It's called either Research or Experimentation. And will be a LOT faster than finding someone who has already done the experiment for you.

Let us know what you find.

 

[Magnetosphere]

This reminds me of the beginnings of scientific thought. Believe all from past philosophers (aka scientists) as absolute truths. If something does not match, don't run an experiment, just ask someone else.

For the above suggested experiment use a flashlight battery as a current source. A size 'AA' Alkaline cell has a shortcircuit current of 8 amps so you would dissipate less than 6 Watts in the disk. Prepare the cardboard and filings on the disk and then make battery contact. Even momentary contact will align the filings. With the stated disk size (5cm Dia, 1cm thick) there will be plenty of time before significant disk heating occurs. It's called either Research or Experimentation. And will be a LOT faster than finding someone who has already done the experiment for you.

Let us know what you find.

I really thought this was an easy question, some scientist at some forum will probably be able to answer within a day or so is what I thought. It looks like I will have to do the experiment. I do not have such a disk and will have to make it, that is another reason why I wanted to save time by asking, I really thought it was as easy as asking how does a magnetic field look around a regular magnet. I still think though that this is an easy question, however everything seems difficult until you know it. Will post pictures of the experiment.

 

[Tom.G]

For a 'First Effort' the disk does not have to be custom built. For instance weights that are used on exercise barbells are cast iron. Or you could use the bottom of a pie tin and solder some heavy copper wire around the edge to distribute the current around the periphery. (Check the pie tin with a magnet to make sure it is steel.)

That said, a search returned over 95,000 hits: https://www.google.com/search?&q=magnetic+field+of+homopolar+generators
Here are a few you may find of interest.

https://www.comsol.com/blogs/redesigning-faradays-wheel-creating-efficient-homopolar-generators/

http://www.animations.physics.unsw.edu.au/jw/homopolar.htm

See pg 122 (123 in the .PDF)
https://www.research.manchester.ac.uk/portal/files/54538050/FULL_TEXT.PDF

Cheers,
Tom

 

[Magnetosphere]

Great stuff, what did you put in as search? I searched for a long time and never saw this stuff.

 

[Tom.G]

what did you put in as search?

That said, a search returned over 95,000 hits: https://www.google.com/search?&q=magnetic+field+of+homopolar+generators

The search string is everything following the "&q=" in the Google link above.
Actually I had a space between words and Google changed them to "+". Should work either way. If you put a "+" as the first character of a word, Google treats that word as being required in the search results. If I recall correctly, Google then adds another "+" in the search string! Weird but it works. You can also use a "-" to skip results that do have the word.

Please let us know what you find out.

Cheers,
Tom

 

[Magnetosphere]

Yes, I know how to search google with different search techniques. What search words did you use?

 

[kuruman]

Yes, I know how to search google with different search techniques. What search words did you use?

@Tom.G just told you in #58.

The search string is everything following the "&q=" in the Google link above.