A magnet creates a magnetic field. (See Figure 20.3 .) Unlike electric field lines, which either go from a
positive charge to a negative charge or extend infinitely into space, magnetic field lines form loops. These
loops point away from the north end of a magnet, and toward the south end. Near the magnet the lines
point nearly straight into or out of the pole.
Figure 20.3 Magnetic field lines created by a bar magnet.Just as we talk about the value of an electric field at a certain point, we can also talk about the value of a
magnetic field at a certain point. The value of a magnetic field is a vector quantity, and it is abbreviated
with the letter B . The value of a magnetic field is measured in teslas.
Often, the writers of the AP exam like to get funky about how they draw magnetic field lines. Rather
than putting a magnetic field in the plane of the page, so that the field would point up or down or left or
right, the AP writers will put magnetic fields perpendicular to the page. This means that the magnetic field
either shoots out toward you or shoots down into the page.
When a magnetic field line is directed out of the page, it is drawn as shown in Figure 20.4a ,
Figure 20.4a Symbol for a magnetic field line directed out of the page.and when a magnetic field line is directed into the page, it is drawn as shown in Figure 20.4b .
Figure 20.4b Symbol for a magnetic field line directed into the page.