magnet

Discussion in 'Physics' started by samjesse, Feb 19, 2010.

1. samjesse Thread Starter Senior Member

Sep 14, 2008
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0
Hi

If I bring a piece of iron bar and a permanent magnet "PM" and put them together where the iron will be attracted to the N pole. then the other end of the iron bar will exibit the force of the S pole.
Keeping the PM size constant. If I increase the size of the iron bar by 1000 times, what effect on the force of the S pole will size-change do? what formula does one uses?

thx

Last edited: Feb 19, 2010
2. BillO Distinguished Member

Nov 24, 2008
992
139
I believe the opposite is true, it will be a North pole.

3. beenthere Retired Moderator

Apr 20, 2004
15,808
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The bar does not amplify the magnetism. Otherwise, a small refrigerator magnet applied to the Brooklyn Bridge would attract and trap cargo ships.

4. samjesse Thread Starter Senior Member

Sep 14, 2008
212
0
True, the other end of the iron bar will be N pole and not S pole.

By the same logic, my intuition further tells me that the "magnetism" will be reduced by an amount proportional to the size of the iron rod.

Is there a formula for this behavior?

5. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
My assumption would be that the magnetic flux is constant for the magnetic "source" [the bar magnet] but the flux density changes as the flux redistributes over the iron bar CSA [cross sectional area]. So yes I would agree - the greater the CSA of the iron portion, the lesser the magnetic field strength.

The term "magnetism" of itself doesn't carry any sense quantification of magnetic strength or degree of magnetic attraction / repulsion. Although, I'm not sure what I sense as I bring two magnets near each other. I think I sense the effect of flux density in terms of the changing force I experience ....?

6. samjesse Thread Starter Senior Member

Sep 14, 2008
212
0
Let the CSA constant and let the rod length be x. move a paper clip near the rod at different lengths from the end where the Permanent Magnet is.
What is the value of the pulling force at distance x from the PM?

7. beenthere Retired Moderator

Apr 20, 2004
15,808
295
The classic experiment that shows lines of force is done by sprinkling iron filings onto a piece of paper. Held over the poles of a horseshoe magnet, it shows the lines of force running between the poles. Held in the path between poles of a bar magnet, it shows those force lines.

It is interesting, though, to place a piece of iron across the horseshoe magnet poles ( known as a keeper, as it preserver the strength of the field). With it in place, the filings on paper hardly show any external field at all. Place the bar magnet on a piece of iron, and the external field also seems to go away.

By the way, a magnetic field is very exponential in its strength. You can arrange an experiment with a fixed magnet and one that moves. Use a spring scale to measure the attractive (or repulsive) force, and plot that against distance.

8. Duane P Wetick AAC Fanatic!

Apr 23, 2009
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When one pole of a magnet (N) attaches to an iron object, the iron object is slightly magnetized to the opposite pole(S). Opposites attract. Now, if you quickly removed the (N) pole and replaced it by a (S) pole, the iron object would be repelled, (if it were allowed to move, ie, like poles repel).
There are many commercial uses of this simple principle.

Cheers, DPW [ Everything has limitations...and I hate limitations.]

9. BillO Distinguished Member

Nov 24, 2008
992
139
Pure, annealed iron has permeability some 150,000 times greater than free space. It will indeed concentrate the magnetic field. It will also attenuate it, or more properly allow it to diverge. However, by exactly how much per unit length, I don’t know. Suffice it to say, if you could come across some really pure and really well annealed iron, I think you’d need a lot of it to conduct a decent experiment. This is provided, of course, that the iron is of sufficient C.S.A. not to be pushed into saturation by the applied field.

What are some of the circumstances around your question?