The translational force on a piece of iron in a magnetic field

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Discussion Overview

The discussion centers on the forces acting on a piece of iron placed in a magnetic field, particularly focusing on the magnetic force that causes the iron to move towards a magnet. Participants explore various factors influencing this force, including the geometry of the iron, the nature of the magnetic field, and the developed pole strength of the iron.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant queries the force on iron in a uniform magnetic field and seeks to express it in terms of variables like cross-sectional area and length.
  • Another participant asserts that the net force is zero in a uniform field but acknowledges the possibility of torque depending on the orientation of the iron and the magnetic field.
  • A later reply clarifies that the discussion pertains to a bar magnet, which creates a non-uniform field, and proposes finding the pole strength on the iron's faces.
  • Participants discuss the relationship between the magnetic field inside the iron and external factors, questioning how to derive the developed pole strength.
  • One participant suggests that the developed pole strength may vary with the position of the iron bar within the magnetic field.
  • Another participant presents a formula for pole strength involving susceptibility, magnetic field, cross-sectional area, and permeability, while questioning if length should also be included.
  • There is a discussion about the cancellation of length in certain equations related to magnetizing fields and moments.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the forces acting on the iron and the role of various parameters in determining the magnetic force. There is no consensus on the correct approach to calculating the magnetic force or the developed pole strength.

Contextual Notes

Participants mention various assumptions and dependencies, such as the uniformity of the magnetic field and the properties of the iron, which may affect the calculations and conclusions drawn.

deep838
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Ok so if there be a piece of iron in a uniform magnetc field B, what will be the force on the iron?
You can assume any variable you want, but i want to find the magnetic force that causes the iron pieces to move towards the magnet in terms of any variables, like it corss-sectional area, length, position etc.
 
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The net force is 0. There might be a torque, depending on the orientation of the iron and its magnetic field (if present) and the external magnetic field.
You need a field which is not uniform to move magnets.
 
mfb said:
The net force is 0. There might be a torque, depending on the orientation of the iron and its magnetic field (if present) and the external magnetic field.
You need a field which is not uniform to move magnets.

Oops... sorry! i forgot that! what i really had in mind was the force exerted on the iron by a bar magnet... in that case the field is not unifrom... sorry for that bit of wrong info! Anyway, i plan to first find the pole strength that develops on either faces of the iron and then use F=sB, s=pole strength... So how do i get my s?
 
mfb said:
The iron bar will get a magnetic field similar to the external field[/url]
Isn't the field inside the iron bar B=μ(I+H) where H= magnetizing filed and I= intensity of magnetization?
But then what to do? I need to find the developed pole strengths first, right?
How do I do that?
 
A magnet does not consist of two separate poles. If you know the magnetic field (and assume it is the same everywhere in the magnet), it is fine.
 
mfb said:
A magnet does not consist of two separate poles. If you know the magnetic field (and assume it is the same everywhere in the magnet), it is fine.

Tell me one thing... to obtain the force on a magnetic material why should'nt I multiply the inensity B with the developed pole strength in the material (s)?
Have I got a misconception about these things? Please help me. I am absolutely at a loss now!
 
I think it is a detour. It is possible, but you just multiply with things (like the volume) and divide by them again afterwards.
 
Ok... One more thing, does the developed pole strength at each end of the iron bar change with its position? I think the pole strength will keep varying as a function of the field the iron bar is in. Am I right?

By using the properties of the material, I got an equation for the pole strength. Its given by,
s=χBA/μ , where χ is the susceptibility of iron, B is the field it is kept in, A is its cross-sectional area and μ is the permeability of the surrounding medium. Is this correct?
 
  • #10
One more thing, does the developed pole strength at each end of the iron bar change with its position? I think the pole strength will keep varying as a function of the field the iron bar is in. Am I right?
Right.

s=χBA/μ , where χ is the susceptibility of iron, B is the field it is kept in, A is its cross-sectional area and μ is the permeability of the surrounding medium. Is this correct?
I would expect the length somewhere, too, but it could be true.
 
  • #11
mfb said:
I would expect the length somewhere, too, but it could be true.

Well surprisingly enough, the length canceled when i equated the magnetizing field in terms of the moment and the volume. Here it is:

I→=M→/V=M→/A.l=s.l.M^/Al=M^s/A
Then again, χ=I→/H→=I→μ/B→... and then the above expression of the pole strength
 

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