Experimental measurements of relative magnetic permeability

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Discussion Overview

The discussion revolves around the experimental measurements of relative magnetic permeability for various materials, specifically ferrite, steel, and wood, using solenoids. Participants explore potential issues with the experimental setup and the calculations involved in determining permeability values.

Discussion Character

  • Experimental/applied
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their experimental setup, including the use of solenoids made from different core materials and the measurement method using a Hall effect sensor.
  • Another participant suggests that the ampere-turns per meter might be excessive, potentially driving the materials into saturation, and recommends using smaller currents.
  • A question is posed regarding the measurements obtained when no core is present in the solenoid.
  • The original poster asserts that they have verified they are in the linear region and that the relationship between magnetic field strength (B) and current (I) remains linear from 0-2A.
  • The original poster also mentions using wood to mimic an air core and notes that both should yield a relative permeability close to 1.
  • The original poster found another equation for non-ideal solenoids that considers length and radius, but states that the results did not improve significantly.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the experimental results and the potential reasons for the unexpected permeability values. There is no consensus on the cause of the discrepancies or the appropriate adjustments to the experimental setup.

Contextual Notes

Participants have not resolved the assumptions regarding saturation limits or the impact of core geometry on the measurements. The discussion includes uncertainties about the linearity of the magnetic response and the adequacy of the equations used for calculations.

brybot
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I am trying to measure the relative permeability of a few materials, but the numbers I'm getting don't quite make sense. Maybe someone here can figure out what might be going wrong.

My setup is as follows. I'm making solenoids using 30 AWG magnet wire wrapped around ferrite, steel and wood cores. I'm doing a single layer of wire and I get about 3600 turns per meter, however the solenoids themselves are about 1"L x 1/4"D. I have a hall effect sensor, the Allegro A1324 which has a sensitivity of 5mv/G. I'm placing the end of the solenoid directly against the sensor and measuring the voltage deviation with respect to current in the solenoid.

With the equation B=kμnI where n=N/L~=3600 I'm solving for the relative permeability k. So k=B/(μnI). At 2 amps I get readings of 201, 170, and 15 G for ferrite, steel, and wood respectively. Then solving for the relative permeability I get 2.2, 1.9 and 0.17. Wood should have something close to 1, so I know something is wrong. And the steel/ferrite should be at least 200.

Any ideas? Thanks
 
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Welcome to Physics Forums. :smile:

Quite possibly your ampere-turns per meter was excessive, and the magnetic materials were driven into saturation. Repeat using much smaller currents.
 
What value do you get if there is no core in the solenoid?
 
NascentOxygen said:
Welcome to Physics Forums. :smile:

Quite possibly your ampere-turns per meter was excessive, and the magnetic materials were driven into saturation. Repeat using much smaller currents.

Thanks for your reply. I'm in the linear region, I've verified experimentally. I'm not sure where saturation will occur, but the relationship between B and I is very linear from 0-2A for these solenoids. Also, 3600 turns per meter is a relatively small value as far as I'm aware.

@ Nasu, I used the wood to mimic an air core. The wire is flimsy and would not hold the shape on its own. Wood and air should both be very close to 1.

I did find another equation for non-ideal solenoids where the length and radius are taken into account. The results did not improve much.
 

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