Magnetic Poles

Magnetism is the third noncontact force we will study in physics. Where electricity has positive and negative charges, magnetism has poles: specifically, N poles and S poles. (These are commonly referred to as north poles and south poles, but this can get confusing as we'll see a little later.) Just as with charges, like poles repel one another, and unlike poles attract. Note that poles are not charges: a positive charge placed next to an N pole won't feel anything.

There is one big difference between magnetic poles and electric charges. If you take an electric dipole (two opposite charges stuck together) and break it in half, you get two separate charges. If you break a magnetic dipole (two opposite poles stuck together) in half, however, you get two new dipoles. You can never have an N pole without an S pole, or vice versa: we say there are no magnetic monopoles. *as far as we know! A number of physicists believe that magnetic monopoles do exist somewhere, and Paul Dirac showed that the existence of a magnetic monopole anywhere in the universe would explain why charge is quantized Section 1.2. But no one has ever discovered one, and we're unlikely to run into one in the lab, so let's just assume that they don't exist. Think of magnetic poles as if they were sides of a coin: behind every head is a tail, and behind every tail is a head. Breaking a magnet is like breaking a roll of coins in half.

In electricity we talked about charges a lot, but because there are no magnetic monopoles, our focus will be on magnetic dipoles. A dipole can be represented by a vector called the magnetic dipole moment \(\vec\mu\), that points from the S pole to the N pole; the needle of a compass is a good analogy. When we say a dipole "points" in a particular direction, we mean that its N pole faces that direction. (The magnitude \(\abs{\vec\mu}\) of the magnetic dipole moment tells you how strong that particular magnet is; we'll talk more about that later.)