Magnetic field through ferrous material and across air gap

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Discussion Overview

The discussion revolves around the behavior of magnetic fields as they pass through ferrous materials and across air gaps, particularly in the context of a mechanical positioning system involving magnets and steel cylinders. Participants explore the implications of material properties on magnetic field strength and distribution.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes a setup involving a steel cylinder and piston with magnets, seeking to determine the necessary strength and material of magnets based on the configuration.
  • Another participant asserts that the magnetic field is continuous across boundaries, emphasizing that the normal component remains the same on both sides of a surface.
  • A different participant raises concerns about varying degrees of magnetic permeability in materials, noting that the magnetic field strength is reduced when passing through a carbon-steel cylinder, as indicated by measurements taken with a Gauss meter.
  • Another participant mentions that while steel has a relative permeability greater than that of vacuum, this does not necessarily imply a stronger field in practical applications.
  • One participant clarifies that only the normal component of the magnetic field is continuous across material interfaces with different permeabilities and suggests that using a steel pipe may hinder accurate position measurements.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of magnetic fields in relation to material properties, with no consensus reached on the implications for the proposed setup. Some participants agree on the continuity of the normal component of the magnetic field, while others challenge the assumptions regarding magnetic permeability and field strength.

Contextual Notes

Participants highlight the complexity of magnetic field behavior in materials with varying permeability, indicating that assumptions about field strength may depend on specific configurations and measurements. There are unresolved questions regarding the practical implications of these factors on the proposed mechanical system.

Stugotz99
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I am attempting to determine how strong a magnetic field will be on the other side of a thick material and then across an air gap.

For instance, I have a closed steel cylinder with a steel piston inside it. I plan to affix a permanent magnet to one side of the piston (inside the cylinder). I am looking to use another cylindrical magnet that can slide inside a separate stainless steel cylinder mounted outside and parallel to the first. My goal is to gage the position of the steel piston within the steel cylinder by viewing the position of the second magnet through windows in the SS cylinder. It will be necessary to have a small air gap between the two cylinders for mounting purposes. Hopefully this description makes sense. A hall effect sensor would work great to determine the magnet's position but I need a mechanical analogue thus not requiring any electric signal. Precision is not terribly critical, with relative position being more important.

So, I am attempting to calculate the required strength/size/material of the two magnets based on the thickness of the first steel cylinder and the air gap between the two cylinders.

Thanks!
 
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The magnetic field B (Tesla) is continuous (because div B = 0) across boundaries. If it is perpendicular to a surface, the normal component is the same on both sides. Stainless steels are either magnetic (martensitic, 400 series) or non magnetic (austenitic, 300 series).

Bob S
 
Well, after doing some more research, and ordering some expensive magnets, my research seems to disagree with what is being said.

Do materials not have different degrees of magnetic permeability? Remember, in my case the magnetic field must first pass through a carbon-steel cylinder.

Quick checks with a Gauss meter show that the magnetic field is being redistributed through the steel cylinder and is no longer as strong as it would be in free space...

This may not work after all...
 
div B = 0 in vacuum right?

The Relative Permeability of steel μ/μ0 = 100

I guess that means steel permits more than vacuum, which should give you a stronger field.
 
Last edited:
Only the normal component is continuous. The tangential components of B are discontinous for a material interface of two different permeabilities.

As a practical matter, a steel pipe will prevent your sensor from making an accurate position measurement. Switch to PVC and you will be in fine shape.
 

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