Magnetism

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Magnetism

Postby Naazima on July 23rd, 2014, 1:50 pm 

Dear Sirs
In between the black lines of iron filings there are clean stripes. Why iron filings avoid that space?
Regards.
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Re: Magnetism

Postby Dave_Oblad on July 23rd, 2014, 8:42 pm 

Hi Naazima,

That is not exactly what is happening. Without any iron filings, the Magnetic Field has no discernible lines of force. If they exist, they are too small to be detected with Iron Filings. But when iron filings are introduced to a Magnetic Field, they become Magnetized and attach to each other, end to end. This concentrates the Field around that strand. Further away, the Field gains strength again and the next strand links itself up. Treating each strand as a set of individual Magnetic grains, all with poles lined up in the same direction, then each strand pushes the next strand away from it.. creating the gaps you question between the strands.

http://en.wikipedia.org/wiki/Field_line

Wikipedia wrote:The iron filings appear to be aligning themselves with discrete field lines, but the situation is more complex. It is easy to visualize as a two stage-process: first, the filings are spread evenly over the magnetic field but all aligned in the direction of the field. Then, based on the scale and ferromagnetic properties of the filings they damp the field to either side, creating the apparent spaces between the lines that we see. Of course the two stages described here happen concurrently until an equilibrium is achieved. Because the intrinsic magnetism of the filings modifies the field, the lines shown by the filings are only an approximation of the field lines of the original magnetic field. Magnetic fields are continuous, and do not have discrete lines.


Hope this helps,

Regards,

Dave :^)
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Re: Magnetism

Postby zetreque on July 23rd, 2014, 11:53 pm 

I never thought about it, but from the sounds of that I imagine the effect of the magnetic field on the iron creates iron's own field, and each different substance or size and shape of the substance would create it's own unique dampening field properties?
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Re: Magnetism

Postby CanadysPeak on July 24th, 2014, 6:21 am 

Well, I've waited a very long to make this pun, so I'll inflict it on you. The field is reluctant to go between the filings.

Seriously, if you watch water run downhill, you will see the answer. Consider the area around the Colorado River. Why does most of that water run down the Colorado rather than beside it? For iron filings, there will be a small area where there are more filings touching each other, the iron filings have a much higher permeability than the surrounding air, so the magnetic induction will be higher there. The higher induction will attract more iron filings, making the overall reluctance smaller, increasing the induction, attracting more filings, ad infinitum.

You can also think of it as similar to the rich getting richer.
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Re: Iron filing compasses

Postby Faradave on July 24th, 2014, 2:20 pm 

Dave_O has it correct.

A typical compass is a small, needle-shaped magnet mounted on a pin which allows it to rotate in a plane. If you hold a compass near a large bar magnet, the needle will align itself along lines of magnetic flux but oppositely pointed. If the bar magnet has north pointing left then when parallel to it, the compass will have a right pointing north. By moving the compass around, it can be seen that the magnetic field of the large magnet is continuous. No gaps are found.
Image
Image
Its different however, when more than one magnet is present. With two large anti-parallel bar magnets, a gap appears in a region between the two magnets where equal and opposite field lines cancel.
Image
Image
Image

When iron filings are dropped into a magnetic field they become tiny magnets themselves, orienting anti-parallel to the prevailing field and creating adjacent gaps where their own field cancels the prevailing field. Additional iron filings will prefer to accumulate in regions where the magnetic field is stronger. This becomes clear in tapping the surface. Any filings that landed in a gap will tend to vibrate out of it and stay out.
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Re: Magnetism

Postby Naazima on July 31st, 2014, 1:57 pm 

Dear Sirs,
Keeping the bar magnet undisturbed if we sprinkle and collect the filings around it say 3-4 times, would exactly the same spaces be left vacant every time?
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Re: Combined magnetic fields.

Postby Faradave on July 31st, 2014, 2:29 pm 

Naazima » July 31st, 2014, 1:57 pm wrote:Keeping the bar magnet undisturbed if we sprinkle and collect the filings around it say 3-4 times, would exactly the same spaces be left vacant every time?


The first filings to fall in an area land at random locations with their long axes anti-parallel to the original magnet's field. Those first filings immediately become magnetized. Though the filings are only small magnets, since their magnetic strength increases with proximity and the large magnet's field strength decreases with distance, there is always a region between them where the fields cancel (equal and opposite).

So, the first filings to land alter the combined field seen by subsequent filings. The first ones are more random. Subsequent filings tend to fall in line with them.
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Re: Magnetism

Postby Dave_Oblad on July 31st, 2014, 3:46 pm 

Hi Naazima,

Faradave is saying "No" to your question. Lines and Gaps are basically random each time.

Best wishes,

Dave :^)
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Re: Magnetism

Postby zetreque on July 31st, 2014, 3:50 pm 

Maybe it would be kind of boring, but I would like to see this experiment done just for the fun of it.
It would be hard to clean up the filings after dropped though without moving the magnets. I guess you could use electromagnets, but could you guarantee that the electricity supply is equal on each attempt?
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Re: Magnetism

Postby CanadysPeak on July 31st, 2014, 5:54 pm 

zetreque » Thu Jul 31, 2014 3:50 pm wrote:Maybe it would be kind of boring, but I would like to see this experiment done just for the fun of it.
It would be hard to clean up the filings after dropped though without moving the magnets. I guess you could use electromagnets, but could you guarantee that the electricity supply is equal on each attempt?


Been there, done that. It's easy to keep I constant. The filings are random each time. You should, however, use degaussed filings as they may retain M, depending on their material. However, if you take the time to do this experiment, and you do it on a large scale, with maybe 0.1 T or so magnet, you'll find something interesting - in the white spaces where H is said to be 0, the filings will start to line up, i.e., you can fill in the blank spaces.
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Re: Magnetism

Postby zetreque on August 1st, 2014, 4:04 am 

Thanks CanadysPeak,

This thread has got me thinking about the aurora borealis as a different kind of example. I think we see the same kind of thing. We see gaps, but they are constantly moving around based on the gas particles and air currents acting in the same way as iron filings. The formation of the gaps are based on the properties and characteristics of the particles in the magnetic field.
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Re: Iron filing compasses

Postby sciencem on November 11th, 2014, 7:26 am 

Faradave » July 24th, 2014, 1:20 pm wrote:Dave_O has it correct.

A typical compass is a small, needle-shaped magnet mounted on a pin which allows it to rotate in a plane. If you hold a compass near a large bar magnet, the needle will align itself along lines of magnetic flux but oppositely pointed. If the bar magnet has north pointing left then when parallel to it, the compass will have a right pointing north. By moving the compass around, it can be seen that the magnetic field of the large magnet is continuous. No gaps are found.
Image
Image
Its different however, when more than one magnet is present. With two large anti-parallel bar magnets, a gap appears in a region between the two magnets where equal and opposite field lines cancel.
Image
Image
Image

When iron filings are dropped into a magnetic field they become tiny magnets themselves, orienting anti-parallel to the prevailing field and creating adjacent gaps where their own field cancels the prevailing field. Additional iron filings will prefer to accumulate in regions where the magnetic field is stronger. This becomes clear in tapping the surface. Any filings that landed in a gap will tend to vibrate out of it and stay out.


Very helpful & quite easy to understand, thanks for the post.
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Re: Magnetism

Postby optodsl on February 1st, 2016, 6:36 am 

Very interesting, although it's an old thread I have some additional questions.

How will the magnetic field be affected if the magnets are put in an angle, lets say 15 degrees, like this?
Image


How will the magnetic field be affected if the magnets have a notch in them like this?
Image


Lastly how will the magnetic field be affected if the magnets have a notch in them and at an angle like the above?
Image

Thanks
Magnus
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Re: Willy Nilly

Postby Faradave on February 1st, 2016, 2:19 pm 

I think the differences in field lines will be small. Generally from North pole to the closest South pole. I think you should get some magnets, a Wolly Willy and play.
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