We have seen that an electric current is surrounded by a magnetic field; and also that, if a wire carrying a current is situated in an external magnetic field, it experiences a force at right angles to the current. It is therefore not surprising that two current-carrying wires exert forces upon each other. Most of us are familiar with the more obvious properties of magnets and compass needles. A magnet, often in the form of a short iron bar, will attract small pieces of iron such as nails and paper clips. Two magnets will either attract each other or repel each other, depending upon their orientation. If a bar magnet is placed on a sheet of paper and iron filings are scattered around the magnet, the iron filings arrange themselves in a manner that reminds us of the electric field lines surrounding an electric dipole. All in all, a bar magnet has some properties that are quite similar to those of an electric dipole. The region of space around a magnet within which it exerts its magic influence is called a magnetic field, and its geometry is rather similar to that of the electric field around an electric dipole – although its nature seems a little different, in that it interacts with iron filings and small bits of iron rather than with scraps of paper or pitch-balls.
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