# Magnetic field along loooong pole pieces...

#### Externet

Joined Nov 29, 2005
1,529
Hi all. Unsure if asked before here. Trying again.

A magnet fitted with long, straight, parallel pole pieces; will it yield a constant magnetic field strength between them, near or distant from the magnet ?

======================================================================== <--- long iron bar polepiece
N
magnet
S
======================================================================== <--- long iron bar polepiece

A piece of iron across the polepieces, placed near the magnet; at the middle of length or placed at the far right a few metres away will attach with the same force ? In other words, will the magnetism be attenuated by lenght of polepieces ?

#### nsaspook

Joined Aug 27, 2009
7,478
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#### Externet

Joined Nov 29, 2005
1,529
Thanks.
The poles reside near the ends of a 'horseshoe' or bar magnet; but for a bar magnet being at one (left) far end with attached polepieces as sketched, I have doubts the pattern at the right end would be the same as in your image.

If the poles show up by the far (right) end of the non-permanently-magnetized iron polepieces, does it mean that the poles migrate instantly changing their location to the right when the iron bars attach to the (left) bar magnet ?

If the strongest field area shows up by the very far (right) end, that could be perhaps and why not a mile away ... I have no problem with that, but would it mean there is no magnetic attenuation along the iron polepieces ?
Would be interesting to measure the magnetism pole-shift propagation speed too.

#### Glenn Holland

Joined Dec 26, 2014
705
The magnetic field is a continuous loop and the total amount of "flux lines" (the mathematical representation of the field) is constant anywhere along the path.

This is commonly stated as "Gauses Law" or "conservatory of field". The concept is to draw a plane through a cross section anywhere along the path and the total flux will always be the same. In simple words, you cannot take a slice out of a field and have the rest of it remain the same.

However, the actual attractive force on a ferrous object will vary with the flux density (such as the number of lines per square centimeter) which is partially a function of the distance and permeability of the space around the poles of the magnet.

#### Alec_t

Joined Sep 17, 2013
11,526
Why not mock up a loooong coil and use a compass to plot the field distribution?
If the coil extends in the x direction I would expect field strength to be substantially independent of x except near eithe end of the coil.

#### MrAl

Joined Jun 17, 2014
7,758
Hi all. Unsure if asked before here. Trying again.

A magnet fitted with long, straight, parallel pole pieces; will it yield a constant magnetic field strength between them, near or distant from the magnet ?

======================================================================== <--- long iron bar polepiece
N
magnet
S
======================================================================== <--- long iron bar polepiece

A piece of iron across the polepieces, placed near the magnet; at the middle of length or placed at the far right a few metres away will attach with the same force ? In other words, will the magnetism be attenuated by lenght of polepieces ?
Hi,

I am not clear on what you are asking really. Maybe draw a picture and describe exactly what you are looking for. Text does not work well for drawing purposes unless a lot of care goes into the text rendering.

#### Externet

Joined Nov 29, 2005
1,529
Hi Al. Thanks.
Sketch without the upper iron bar -for clarity?-

A piece of iron between the upper (not drawn above) and lower very long iron bars; (20 feet, 200 feet long, whatever you want) will attach with more force by the left, centre or right ?
Will the distance from the magnet change the pull strenght by "magnetic field attenuation" of the iron bars ?

#### MrAl

Joined Jun 17, 2014
7,758
Hi,

Still not entirely sure where you are putting the second bar (i guess that is what you mean) but whatever is closest to the magnet will experience the most attractive force. Anything magnetically active not by itself a 'magnet' will have reluctance and that acts like a resistance to flux similarly to how a resistor acts like a resistance to current flow. The permeability of the iron dictates how much force will be felt at the far end. Something with high permeability will show more force at the far end while something with low permeability will show less force. For example transformer metal will show much more force than air because air has very low permeability compared to transformer metal. If you have a gap say 10 feet of air, there will be very little force at the far end but if you bridge that with iron the force will go up because the iron is a better 'conductor' of flux than the air. If we had a perfect 'conductor' it would have to have very high permeability so it could concentrate all the 'lines of flux' inside the material and thus transfer the force at a great distance.
In the real world to keep forces high usually small distances are maintained because everything real limits flux so the shorter the distance the greater the force. A good example of that is a relay, where the pole pulls on the arm at a distance d1 with some force F1 but once the arm closes and touches the pole piece the distance d2 is much smaller so the force goes up to F2 and since the distance d2 is less than the distance d1 the force F2 is greater than F1 so the arm can be held against the pole piece with less current in the electromagnet.

Finally, if you use your configuration with another long piece not a magnet itself and allow the two far ends to touch, you have the electrical equivalent of a voltage divider which as you know is two resistors in series connected to a battery. The far end of the 'triangle' is where the two long pieces meet and that is like the junction of the two resistors in the electrical analog.

In any case though, the farther from the magnet the less the force given any isotropic material.

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