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Q & A: Light vs. Magnetism

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Most recent answer: 10/22/2007
Q:
I know it's possible to focus light (by use of a tube on the end of the flashlight) so it won't shine everywhere. Can you also do the same with a magnet, so it's field is directed at a certain point?
- Courtland (age 15)
Lawrenceville, GA, USA
A:

Courtland - I'm not really sure what it is you're asking here, but I think that the answer is 'not really.' I think that what you're suggesting is to somehow 'shine' a magnet through some sort of a tube so that the magnetic field points only in one direction. The reason that this doesn�t work as well as it does for light is because magnetic field lines do not travel in straight lines. It�s possible to focus light so that all of the separate waves of light from one light source are going in the same direction. A magnetic field is different - a bar magnet will produce a field that starts at the North pole of the magnet and goes towards the South pole, and the field lines look like this:

(from )

Although you can not focus all of the field from a magnet in only one direction, it is possible to affect the direction in which most of the field goes. You can do this by changing the shape of the magnet itself or by arranging several magnets around each other. A typical magnet used in a physics laboratory is really a pair of electromagnets with a small gap between the North pole of one of them and the South pole of the other. There is a big piece of iron or steel connecting the other sides of the magnets together so the field runs around in closed loops like it should. The strength and shape of the field is affected by the shapes of the pole tips in the center of the magnet. They are made of iron or steel and guide the field towards a small region for the experiments. These magnets are usually used to do experiments on materials: metal, ceramic, semiconductor, or whatever. The company sells different pole caps so the experimenter has some control over how the field lines arrange themselves. The more pointed the pole caps are, the stronger (but less uniform) the field will be, something like the focusing you are asking about.

And yes, you can make a tube out of a material called a superconductor, which, at low temperatures, not only will conduct electricity with no resistance, but which will expel the magnetic field from the material it is made out of (called the "Meissner effect"). Superconductors can then be used to shape magnetic fields. You can make a tube out of a material like lead or tin or niobium, cool it down to a few degres from absolute zero, and the total magnetic flux through the tube will be constant. Pinching the end of the tube will strengthen the filed at the pinched end (but pinching the metal that�s this cold will almost certainly crack it instead of bending it, and then it won't conduct electricity across the crack). If you cool such a tube down with no magnetic field in it and then place a bar magnet inside, the field lines will still go go from the N pole to the S pole on the bar magnet, but they will squeeze back and all go backwards through the tube. This doesn�t sound like focusing you asked for, but it is a change in the shape of the field nonetheless. Some of the strongest magnetic fields made in the lab are made with electromagnets with low resistance wire which acts sort of like a superconductor in that the total flux through a loop is more or less constant. Then the device is imploded by arranging dynamite around the outside and blowing the whole thing up -- don�t try this at home! The wire collapses, and the field strength goes up. But it doesn�t last very long! You have to do your experiment with the high field very quickly -- the time it takes to explode.

-Tamara (and Tom)


(published on 10/22/2007)

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