Magnetic Force and Torque Question

Click For Summary

Discussion Overview

The discussion revolves around calculating the torque required to rotate a cylindrical magnet that is diametrically magnetized, with a focus on the magnetic interactions between two such magnets. The scope includes theoretical considerations and numerical approaches to modeling magnetic fields and torques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Kevin introduces the problem of determining the torque needed to rotate magnet (B) so that like poles face each other.
  • Some participants clarify that each magnet has one pole-pair.
  • One participant suggests that calculating the torque is complex and depends on factors such as the airgap width and the homogeneity of the magnetic field.
  • Another participant mentions the need for additional information, such as the relative permeability and mechanical dimensions of the magnets, to perform accurate calculations.
  • There is a suggestion that maximum torque occurs when the magnetic fields are perpendicular to each other.
  • Participants discuss the possibility of using numerical methods and programming to analyze the magnetic field and torque, with one participant expressing intent to write code for this purpose.
  • A drawing is shared to illustrate the magnetic field, although participants note the difficulty in accurately representing the circulation field.
  • One participant emphasizes the importance of verifying that the calculated magnetic field curves are correct and suggests considering programming efficiency.

Areas of Agreement / Disagreement

Participants generally agree on the complexity of the problem and the need for numerical methods, but there is no consensus on the specific approach or the exact parameters required for calculation.

Contextual Notes

The discussion highlights limitations such as missing assumptions regarding the magnetic properties and dimensions of the magnets, as well as the challenges in modeling the magnetic field accurately.

KevinBlueBox
Messages
5
Reaction score
0
Hello,

I'm new to the forums but I had a problem I was trying to solve that I could use some help. Hoping someone here could point me in the right direction.

Imagine you have two (2) cylindrical magnets that are diametrically magnetized. Both magnets are of equal size and magnetic strength. Magnet (A) is stationary and held place by an external force. Magnet (B) is stationary and held in place by an external force. However magnet (B) can rotate along it's axis. The magnets are located near each with their cylindrical axis in parallel. At their starting state the magnets have their opposite poles facing each other (North to South).

The question is how much torque would be needed to rotate magnet (B) so that the poles the same poles are facing each other (North to North... or South to South).

Greatly appreciate any help or assistance.

- Kevin
 
Physics news on Phys.org
Is there only one pole-pair as per magnet ?
 
Hesch said:
Is there only one pole-pair as per magnet ?
Yes, there is one pair per magnet.
 
I think that will be very hard to calculate. The torque is due to magnetic energy in the airgap, and you can approximately calculate such a torque when the width of the airgap is small, compared to the size of the magnet, when the magnetic field is homogenous inside the airgap and so on.

It could be calculated numerically ( by computer program ).

Some information is missing, for example the relative permeability as for the magnets, how exactly are the magnets magnetized and of course all mechanical dimensions.

But I will for sure follow your thread. :smile:
 
Hesch said:
I think that will be very hard to calculate. The torque is due to magnetic energy in the airgap, and you can approximately calculate such a torque when the width of the airgap is small, compared to the size of the magnet, when the magnetic field is homogenous inside the airgap and so on.

It could be calculated numerically ( by computer program ).

Some information is missing, for example the relative permeability as for the magnets, how exactly are the magnets magnetized and of course all mechanical dimensions.

But I will for sure follow your thread. :smile:

Thank you Hesch!

Currently I'm trying to determine the equations I were to use and not a numerical value for the torque. Depending how much torque is required (relative to magnetic size) will determine what size magnets, air gap, and field strength I decide to use.

I wouldn't mind writing a code to determine the value. I'm decent enough of a programmer that I could write something and let it run overnight. Probably wouldn't very accurate but would work for purpose.

Do you know the general concept you would use to approach this situation?
 
Well, actually I tried to sketch the magnetic field on a drawing, but it's rather difficult to sketch a field, which must be a circulation field and must have the correct direction as to the north/south poles.

I must try again later.

Anyway you will get a maximum torque when the fields from the magnets are perpendicular to each other ( like in a synchronous motor ).
 
Hesch said:
Well, actually I tried to sketch the magnetic field on a drawing, but it's rather difficult to sketch a field, which must be a circulation field and must have the correct direction as to the north/south poles.

I must try again later.

Anyway you will get a maximum torque when the fields from the magnets are perpendicular to each other ( like in a synchronous motor ).

That helps a lot. I didn't think about doing a finite analysis of this problem with a program. To get a good answer I think that's the only way.

The magnetic poles are in the same orientation as this magnet.

http://www.kjmagnetics.com/proddetail.asp?prod=R424DIA

I'm turning the magnet with a servo motor and not sure how powerful of servo would be needed.
 
Well, here it is attached ( the drawing ).
The "diameters" are some coils that I have wound to be able to calculate with Biot-Savart. But you see that the magnetic fields will be completely unhomogenious.
The drawing just shows a slice of the magnets with the thickness dL. Having found the torque from a slice, you can just integrate/sum the torques along the cylinder.
 

Attachments

Hesch said:
Well, here it is attached ( the drawing ).
The "diameters" are some coils that I have wound to be able to calculate with Biot-Savart. But you see that the magnetic fields will be completely unhomogenious.
The drawing just shows a slice of the magnets with the thickness dL. Having found the torque from a slice, you can just integrate/sum the torques along the cylinder.

Thank you Hesch! I understand what you are showing and the technique. I'll wrote a code later this week to figure out what I need.
 
  • #10
When you have made a program that calculates the B-field, you must test that the curves are (nearly) correct closed.
If not, your program is calculating wrong.
And yes, as you said: Calculate overnight. Maybe you should use assembler programming? :smile:
 

Similar threads

Undergrad Magnetic field strength of a stack of magnets
  • · Replies 7 ·
Replies
7
Views
4K
High School Magnetic fields attracting two disc-shaped magnets
  • · Replies 11 ·
Replies
11
Views
3K
Undergrad Magnetic dipole and a torque
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad How Do Magnetic Forces Differ Between Magnets and Wires?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Questions related to magnetic shielding separating two like poles
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad Homopolar generator question: Magnetic “wake”
  • · Replies 7 ·
Replies
7
Views
3K
High School Magnetic attraction force Question
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Motional EMF in Faraday disc, co-rotating magnet axial mean flux
  • · Replies 5 ·
Replies
5
Views
977
Undergrad Magnetism: Same Pole Repulsion & Heteropolar Attraction
  • · Replies 32 ·
2
Replies
32
Views
3K
Undergrad How induction works within a coil (nuances of it)
  • · Replies 6 ·
Replies
6
Views
2K