Site hosted by Angelfire.com: Build your free website today!

<< home  < Articles

Magnets & Magnetism

Threads - Magnets & Magnetism
On 23/8/2006, Kevin Phyland posted:

Sorry if this has been hashed to death prior to my joining...but one of my Yr. 12 Physics students asked me how long a permanent magnet lasted...

It seemed like a doddle to find out...but the internet contains more contradictory responses than I thought possible.

My first thought was: forever? but then I got to thinking about work done...

I have a very sketchy understanding of domain theory - i.e. I think magnetism is due to electron-spin alignment (though what *spin* actually is is sadly beyond my ken).

I tried to nut it out from first principles but keep getting lost in the mire of external energy sources for electrons
(i.e. because the bar magnets I was using are clearly above absolute zero there would probably be thermal energy available for replenishing any energy loss from the work that the magnets do)...or am I totally off the mark?

Magnets have fascinated me for ages but I can't confess to really being any closer to understanding the action at a distance thing despite teaching Physics for 20 years...
Lynne Kelly replied:

Even if it has been previously hashed (before my time), I am keen to know the answer. In umpteen years of teaching physics, I was never asked this - yet it is such an obvious question.

Longing to hear the answer.

Peter Schmedding responded:

"A magnet is always a magnet. An electromagnet can be switched on and  switched off".

This sentence I preached to my young students now for over two  decades. However,with the interest of your 12-year student, look into  it more closely and other factors emerge.

What material does the magnet consist of? Take an ordinary nail, say  five or six inches long. Unless it is for special purposes it is of  the 'soft iron' variety. You can magnetise it with another, hopefully  strong, magnet. You can also magnetise it by a strong magnetic field  produced by a suitably shaped coil, energised by electricity.  In  either case it will lose most of its magnetism as soon as the  activating force is removed. But then, if not disturbed by other  magnetic fields it may never completely lose a certain amount of  magnetism.

Now take a harder material such as a darning needle. This, being hard  steel, will need a stronger magnetic force to magnetise it. (Put it  on a piece of paper or similar floating in a bowl of water and you  have a compass.) I would expect this to keep its magnetism forever.

The search by engineers in the field found that the addition of some  other metals to steel improve their magnetic strength. Here we talk  about the 'permanent magnet'.

A different material again you will find for example in most  loudspeakers that were manufactured in the last few tens of years:  The ceramic magnet. These are stronger than their steel counterparts  and I would be surprised if they ever get weaker, no matter how old  they get.

The strongest magnets you can buy are the 'rare earth' variety. They  are so strong, of certain size they are dangerous. They can pinch and  hurt your fingers if they are allowed to snap together.

The invention of the above-mentioned Electromagnet I have mentioned  above I always have classified in importance to technology similar to  the invention of the wheel. Why this is so might be an interesting  research project for your students.

and:


My first thought was: forever? but then I got to thinking
about work done...

Hi Kevin,

In that context I just had a brain wave. Is it not amasing when you  consider magnetic tape recordings. No matter if it is video or sound,  or data for that matter, every time the tape goes over the head, it  produces electricity. Of course it's "only" a signal and has ??? no  power??? It still has to push around the magnetic particles in the  head core, and every time.

No matter how often you play the tape, the signal strength remains.  The tape material and the coating will perish with time, but the  magnetism - would it last forever?

Just wondering.

Morris Grey added:

One of the problems with tape especially the big 1700 foot reel to reels we used in the 60s was 'print through' where one layer of tape magnetizes adjacent layers. Left long enough I think you would end up with just a lot of noise as the magnetic domains became more or less randomized.

Brian Lloyd noted:

Sorry if this has been hashed to death prior to my
joining...but one of my Yr. 12 Physics students asked me how
long a permanent magnet lasted...

It seemed like a doddle to find out...but the internet
contains more contradictory responses than I thought possible.

My first thought was: forever? but then I got to thinking
about work done...

I have a very sketchy understanding of domain theory - i.e. I
think magnetism is due to electron-spin alignment (though what
*spin* actually is is sadly beyond my ken).

It is beyond just about everyone else too but since it can be  described mathematically we can continue to use it.

I tried to nut it out from first principles but keep getting
lost in the mire of external energy sources for electrons
(i.e. because the bar magnets I was using are clearly above
absolute zero there would probably be thermal energy available
for replenishing any energy loss from the work that the
magnets do)...or am I totally off the mark?

Magnets have fascinated me for ages but I can't confess to
really being any closer to understanding the action at a
distance thing despite teaching Physics for 20 years...

Oh, man, what a FANTASTIC opportunity to teach real science!

Well, there seem to be two parts to this. How does it work and why  does it work. We can answer the former but the latter is more difficult.

Experience says that magnetic systems degrade with time. Magnetic  tapes and disk degrade.

But getting back to permanent magnets, you preserve them by  containing their lines of force and prevent outside effects. That is  why we put a "keeper" on a horseshoe magnet.

Now if you want to actually show your students something about how to  find things out, do this experiment:

Prepare a solenoid coil. Place various magnetic materials in the  solenoid and pulse the coil with DC (always in the same polarity.  Measure the residual magnetism in the material. This will let you  learn something about the magnetic susceptibility of the material.  You will figure out how easy it is to magnetize something. Also,  applying a fixed number of pulses of known duration and known  magnitude (current) lets you quantitize the application of magnetic  field to the material so you can draw some useful quantitative  conclusions.

Now reverse the process. Take various "permanent" magnets and place  them in your solenoid. Apply *AC* to the solenoid. Do this for  controlled lengths of time. Remove the magnets and measure the  residual magnetism. I suspect you will find that *ALL* "permanent"  magnets lose some of their magnetism. Some will even lose it all.  This is proof that "permanent" magnets aren't but you will see some  that will last for a long, long time.

BTW, if you are looking for a cheap way to measure magnetic field  directly, use a hall-effect device with a meter. If you don't want to  build one (they are pretty easy) you can get one pretty cheaply from  the Magnetic Shield Company (http://www.magnetic-shield.com/). Their  magnetic shielding kit (which includes a digital gauss meter) is a  treasure trove for experimenting with and understanding how magnetism  works and how magnetic lines of force are propagated.

Oh, and use some of the shielding material to "protect" your magnet  and repeat the experiments. Ah, cool idea.

(I had to deal with shielding a magnetic compass in an airplane from  influences from the electrical system. I ended up with a fairly good  empirical understanding of what works and what doesn't.)

Oh, and the other part of the question, the "why" of magnetism? I  haven't got a clue.

Hmm, other stuff: materials lose their magnetic field when heated  (Curie effect). Show that. Also, magnets can lose their magnetism by  striking them. (Keep dropping a magnet on a hard floor and see what  happens to its magnetism.) Make a magnet by placing magnetic but  unmagnetized material in a weak field and hit it with a hammer  several times. (The Earth's magnetic field will probably do.) Orient  it along the lines of force as you pound on it. It will acquire a  "permanent" magnetic field.

Thought: random thermal impact of one atom on another probably has an  effect on reducing overall magnetism with time.

Ah, I am thinking too much again.

Jim Edwards commented:
Sorry if this has been hashed to death prior to my
joining...but one of my Yr. 12 Physics students asked me how
long a permanent magnet lasted...


Over twelve hundred years according to this "reliable" source.

http://www.lutec.com.au/how.htm


Kristin Harris replied:

I'm glad you are, Brian, that was a great little synopsis of magnetic fun... shall have to try some of it out with my kids (we home ed). I'm for every telling them to not drop their magnets as it will shorten the time they are magnetic! Now I have some 'official' back up for my requests ;-)