## Field strength vs. gap between poles

## Field strength vs. gap between poles

(OP)

I have a question about how the distance between a north and south pole effects the field strength at a point equidistant from each pole face. My college physics book seems to treat it as if it will not change with distance if the faces are infinitely large planes. Visualizing this, as long as the imaginary field lines are parallel there is no change in flux, which I believe is in line with Gauss' law(?) Is this true, and if so how does it work out in the real world? is there a ratio between the distance poles are apart and a distance from the edge of a pole that we can consider the flux uniform?

## RE: Field strength vs. gap between poles

## RE: Field strength vs. gap between poles

In formula B=B

_{r}/(1+g/h), g being the gap length (total in series in the circuit) and h the length (total in series) of the the active permanent magnets with residual induction B_{r}.Now h, at least for modern RE magnets, is of the same order of magnitude as the gap g, say between 1 and 2 times g. From the formula above, if the gap doubles, going from 0.5h to h, the induction reduces by 75%, which is not really constant.

prex

http://www.xcalcs.com

Online tools for structural design

## RE: Field strength vs. gap between poles

I'm still learning about magnetic circuits, it's totally new to me.

This question pertains to the hobby cyclotron that I posted a question on this forum a few weeks ago. So what I'm really trying to do is calculate the radius of a particle of known mass, at a known speed in a given B field. This I can do if I know the B field, but I'm trying to figure out what the field will be at a given distance. I'm sure the formula given by prex is applicable, but I don't really understand where the h comes from or if it's applicable to my situation. I also don't understand the concept of the B sub r, although I'll try to research it on my own, maybe it's in the freq. asked question area.

Most magnet manufacturers rate their magnet strength and are around 1 Tesla for the ones I'm looking at, but I assume that's at zero distance, which is making it difficult because I can't, for example, "double" the distance in my calcs.

If anyone can clarify it would be much appreciated.

thanks again

-Todd

## RE: Field strength vs. gap between poles

_{r}.And you are correct (good starting point for deepening your knowledge): this will be the induction in a circuit with zero gap (a totally useless circuit by the way).

prex

http://www.xcalcs.com

Online tools for structural design

## RE: Field strength vs. gap between poles

= = = = = = = = = = = = = = = = = = = =

Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

## RE: Field strength vs. gap between poles

PC=(Lm/Lg)(Ag/Am)

Bm=(Br)(PC/[PC+1])

Bg=Bm(Am/Ag)

PC = Permeance Coefficient

Lm = Length of the Magnet

Am = Area of the Magnet

Lg = Length of the Gap

Ag = Area of the Gap

Br = Magnet flux density remenance

Bm = Flux density in the magnet

Bg = Flux density in the gap

Assumes recoil permeance is one.

## RE: Field strength vs. gap between poles

I had no idea that the magnet length had such an effect.

Am I interpreting this correctly?

## RE: Field strength vs. gap between poles

## RE: Field strength vs. gap between poles

At least with steady field you don't have to add the compication of laminating everything.

= = = = = = = = = = = = = = = = = = = =

Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

## RE: Field strength vs. gap between poles

for example:

If I have just (2) 1.2 Tesla Br RE magnet disk assemblies (probably 15cm Dia) which are each 2cm thick, and are held apart 1cm

using B=Br/(1+g/h) and SI units

B=1.2/(1+.01/.04)= .96T

or is it

B=1.2/(1+.01/.02)= .08T ?

Also to Ed Stainless, two things, first I am trying to look at the demagnetization curves as you suggested, but the one I found was basically illegible.

Second I'm assuming you know something about stainless steel, what grades of S.S. are basically "non-magnetic"?

Thanks for helping out a "newbie" I've learned a lot from you guys.

-Todd

## RE: Field strength vs. gap between poles

## RE: Field strength vs. gap between poles

http://w

To get a feel for the geometry effects on field strength on the gap.

Gareth P. Hatch, Ph.D.

Director of Technology

Dexter Magnetic Technologies

http://www.dextermag.com