Magnet Shapes

Most recent answer: 10/22/2007

My dad and I would like to know into what shapes a magnet can be made, and how ? Are the metals poured into a mold or are they cut out of a solid block. Can they be made into any shape ? Thank you - David PS: My dad helped me with the spelling and can you tell us about any books or websites on the subject - Thank you again !
- David (age 6)
Longmeadow, MA
Hi David,

Very nice question! Magnets are really a lot of fun for a lot of reasons, and many of us never get tired of experimenting with them.

Magnets come in all shapes and sizes, and are used for a great variety of purposes. The shape of a magnet determines how the magnetic field lines are arranged outside of the magnet, which affects what the magnet can be used for.

All of the methods you mention have been and continue to be used to manufacture magnets. Some small ones may be stamped out of sheet metal. Others may be cut from blocks of metal. Others still may be poured into molds. Refrigerator magnets are probably made of molten plastic with iron powder mixed in, poured on a flat surface, and then cut into rectangular (or other) shapes. The magnetization procedure usually involves heating up the magnet and then cooling it off with an external magnetic field applied.

Some magnets are electromagnets. If you flow an electrical current through a coiled wire, it will produce a magnetic field. The strongest and largest magnets ever made are electromagnets. Very large magnets are used in high-energy physics experiments and can be several stories tall. They usually consist of many turns of very thick wire around in the shape of a big cylinder. The wire is often superconducting, so the whole thing has to be kept at liquid helium temperatures (we’re not so good yet at building wires out of high-temperature superconductors). Superconductors are chosen to reduce the cost of flowing the thousands of amperes of electrical current through the wire -- no resistance, no energy gets lost!

That may pretty much answer your question, but as long as we’re on the topic, here are some more thoughts about strange magnet shapes. Some of these may not be easy to understand except for readers who’ve had a physics course.

Other magnets can be made in other shapes to accomplish other things. Some magnets have more than one North pole and more than one South pole. A simple extension is a "quadrupole", with two of each. These tend to be square on the outside with alternating N and S poles pointing inwards. They are used to focus beams of high energy particles. You can make sextupoles and octupoles with three and four each of the North and South poles. A refrigerator magnet also has many North and South poles so the magnetic field lines can leave the magnet, flow through your refrigerator door and come back in again so it will stick well even if only part of the refrigerator magnet is in contact with the door, or if the magnet is cut up in pieces, the pieces will still stick well to the door (you can’t cut it up so there’s only a N pole or just an S pole -- there will always be two!).

Even your television (if it’s not one of the new plasma or LCD kind) has an electromagnet inside to steer the electron beams across the television screen.

Tom (w. mike)

(published on 10/22/2007)

Follow-Up #1: S-shaped magnets

Very interesting subject. If you cut or from a magnet into a S shape will the poles be consistent(N at one end and S at the other)?
- Joe (age 22)
That depends on when it is magnetized, and how. If you start with a simple bar magnet, and gradually deform it into an S shape, you might think that the magnetization lines will just twist along with the magnet. However, if it’s made of some ordinary metal you probably have to heat it up to deform it, and it’s likely to demagnetize when it’s hot. If you re-magnetize it with a coil wrapped around it, the magnetization lines will pretty much follow the S shape, and the poles will be at the ends. If instead you stick it in a big magnet with a uniform filed, the magnetization will be a compromise between following the S shape and simply lining up with the applied field.That could end up with the poles being near the top and bottom of the S, rather than at the ends.

If you carefully cut a big magnetic sheet, without getting it hot, the magnetization will stay almost the same s before cutting. If "N" was up, then the N pole will be near the top of the S.

Mike W.

Magnets do not necessarily need to have just one N and one S pole.  Hard disk drives, magnetic tape, and plastic refrigerator magnets all have many of each kind.  If you cut the magnet out gently (that is, without heating it up past its Curie temperature) the magnetization pattern should stay as it was before you cut it.  Some people shred hard disk drives -- the magnetization patterns should still be present on the shreds, but they should be (nearly) impossible to read since the jigsaw puzzle is hard to put back together again, and the mechanical deformation may destroy a large fraction of the bits or at least make them unreadable.


(published on 10/22/2007)