# Magnetic Field Outside of Finite Solenoid Greater for Air-Core or Ferrite-Core?

## Main Question or Discussion Point

These are finite solenoids/inductors and I only care about the field outside of the solenoids.

Assuming identical coils and the distance on axis from the coil and the current through the coil is the same, will an air-core or ferrite-core inductor/solenoid create a greater magnetic flux density? What about a point in the middle of the solenoid and perpendicular to the axis of the inductor?

I'm messing around with building a near-field magnetic communication system and am curious to know if I should use air-cored or ferrite-cored inductors. I don't know if a ferrite core will keep the magnetic field from propagating out as far on the transmitter side. I also don't know if it will affect the ability of the inductor to induce a current from a magnetic field on the receiver side.

Related Other Physics Topics News on Phys.org
These are finite solenoids/inductors and I only care about the field outside of the solenoids.

Assuming identical coils and the distance on axis from the coil and the current through the coil is the same, will an air-core or ferrite-core inductor/solenoid create a greater magnetic flux density? What about a point in the middle of the solenoid and perpendicular to the axis of the inductor?
Easily answerable with a minor amount of research:
From Wikipedia:
" Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance. Low frequency inductors are constructed like transformers, with cores of electrical steel laminated to prevent eddy currents. 'Soft' ferrites are widely used for cores above audio frequencies, since they do not cause the large energy losses at high frequencies that ordinary iron alloys do..."

http://powerelectronics.com/mag/power_inductor_core_material/

Creator

Thanks for lookin around for me.

Ha. That's funny that I didn't see that any of the many times that I looked at that page.

Ok, so how do I calculate the field outside of a ferrite-cored inductor? Would I just use Biot Savart and multiple the permeability of free space by the relative permeability of the core material? I don't see how that would confine the field close to the coil.

I'm actually trying to build something. Those links have a ton of information but don't help me with my questions.

What are some of the other things I need to consider?

From http://en.wikipedia.org/wiki/Magnetic_core#Straight_cylindrical_rod" wikipedia page. "The presence of the high permeability core increases the inductance but the field must still spread into the air at the ends of the rod. The path through the air ensures that the inductor remains linear. In this type of inductor radiation occurs at the end of the rod and electromagnetic interference may be a problem in some circumstances."

That makes me think that I would just multiply the free space permeability by the core's relative permeability in the Biot Savart equation. I don't really get how the core would hold the field in close.

Last edited by a moderator:
Ok. I changed my question a bit:
Do magnetic fields always have the same fall off rate?

From what I read, I believe they usually do. I came across the near-field and far-field and that was a bit confusing. I didn't get any clear answers on the difference for a magnetic field. All I found was info on electromagnetic waves.

Anyway, if a magnetic field of a solenoid falls off with distance at the same rate whether the core material is air or ferrite, then the ferrite core will provide a stronger magnetic field assuming all else is the same. Right?

I read http://ieeexplore.ieee.org/Xplore/login.jsp?url=http://ieeexplore.ieee.org/iel5/5720497/5723469/05723604.pdf?arnumber=5723604&authDecision=-203" paper and it said that a ferrite core significantly increases the range of the magnetic field. The equations just multiplied the magnetic constant by the relative permeability of the core material, everything else was the same. The paper seemed a bit poorly written to me, but it did have some good information.

Last edited by a moderator: