Magnets and Magnetic Poles

Monday 7-15-96

Homework Problems : Chapter 18 - 8, 11, 18, 22, 33, 35, 37

The relevant section in the textbook is 18.1

Main concepts:

• there is a strong connection between electricity and magnetism. With electricity, there are positive and negative charges. With magnetism, there are north and south poles. Similar to the behavior of charges, like magnetic poles repel, while unlike poles attract.

• an important difference between electricity and magnetism is that in electricity, it is possible to have individual positive and negative charges. In magnetism, on the other hand, north and south poles are always found in pairs. Single magnetic poles, known as magnetic monopoles, have been proposed theoretically, but a magnetic monopole has never been observed.

• in the same way that electric charges create electric fields around them, north and south poles will set up magnetic fields around them. Again, there is a difference. While electric field lines begin on positive charges and end on negative charges, magnetic field lines are closed loops, extending from the south pole to the north pole and back again (or, equivalently, from the north pole to the south pole and back again).

• electric fields come from charges. So do magnetic fields, but from moving charges, or, in other words, from currents. In a permanent magnet, for instance, the magnetic field comes from the motion of the electrons inside the material, or, more precisely, from something called the electron spin.

• the magnetic field is a vector, the same way the electric field is. The electric field at a particular point is in the direction of the force a positive charge would experience if it were placed at that point. The magnetic field at a point is in the direction of the force a north pole of a magnet would experience if it were placed there.

• the symbol for magnetic field is the letter B.