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Q & A: testing magnets

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Most recent answer: 10/22/2007
Q:
My teacher has set me homework in class to find out what the method is to dtermine whether an object is a magnet or not. I thought that you just had to put something magnetic next to the object to see if it is a magnet but my teacher said it was wrong. Can you help?!
- Lucy (age 13)
Kent, England
A:
We don't like to do homework, but this sounds more like joining in an argument.

The answer depends a lot on what your teacher means by the word 'magnet'. I'm guessing she may mean 'something that keeps a big magnetic field for a long time'. In other words, she may mean what's called a permanent magnet. In that case, there will be strong forces between the magnet and either another magnet or a piece of magnetic material (like iron) which has not been made into a permanent magnet, but only magnetizes when in a magnetic field from something else. Unless you're careful, you may not notice the difference between magnetic material and another permanent magnet. The other permanent magnet could either repel or attract the first one, depending on how you turn it. Non-magnetized magnetic material will always be attracted to the magnet.

Of course, your teacher may have something else entirely in mind.

Mike W.

You can get the wrong answer if the thing you're testing isn't a magnet but is an unmagnetized piece of iron, and your "something magnetic put next to it" happens to be a permanent magnet.

One way to be sure that your test piece material isn't magnetic is if it's a wire. Moving a permanent magnet around a loop of wire will induce a voltage around that loop which can be read with a galvanometer (or any sensitive voltmeter) -- this takes advantage of Faraday's law of Induction.

A compass needle may also be handy, but as they are magnetized, they will be affected, even if just a little bit, by an unmagnetized piece of material (if for no other reason than that the unmagnetized piece of material may distort the local magnetic field from the Earth. Compasses are notoriously unrelaible on metal ships and inside cars made of steel).

Tom

(published on 10/22/2007)

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