How shape of ferrite core in coil influences change of inductance?

Hy,

I wish to know how shape of ferrite core in coil influence on change of inductance. Suppose we have two ferrite cores (rod and tube) but except that all other technical characteristics are the same (permeability, lenght, diameter, etc). I wish to know what are the difference in case we use this two ferrite cores in coil. I assume that in case we use tube ferrite core we can expect smaller change of inductance than in case we use rod ferrite tube. Is that correct?

I am looking also for low density ferrite cores (usually density is around 4800kg/m3). One solution is ferrofluid, but I do not want to use it. So if you maybe know for any other low density ferrite cores (suppose that permeability and shape are not important ), i will be very grateful for any info.

Thank you very much in advance!

Re: Ferrite cores

On your density, you are up against the laws of physics. The magnetic properties are caused by metals. Ferrite is a mixture of the metals and ceramic materials on a molecular basis. If you use the hollow tube, the equivalent mu goes down. (This lower value of mu may work properly in your circuit.)

Another form of material is metal particles in a binder. Arnold products are an example. You can grind it up and add more filler and cast it in a new shape.

Re: Ferrite cores

On the first point you are correct: the inductance is inversely proportional to the reluctance. Since the reluctance is inversely proportional with the cross sectional are of the core, it will be higher for less ferrite. Higher reluctance then results in lower inductance.
Note that with rods and tube cores, a lot of the reluctance is due to the rest of the magnetic circuit, which is the air, so the relationship will not be strictly linear.

Re: Ferrite cores

thank you for help. I have got another question. For example I have a tube and rod ferrite core, same lenght and cross-section. Is change of inductance in coil than the same?

Re: Ferrite cores

Yes, I believe it will be the same.