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Magnetic Fields for AP Physics B & C

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By — McGraw-Hill Professional
Updated on Feb 12, 2011

Practice problems for these concepts can be found at:

Magnetism Practice Problems for AP Physics B & C

All magnets are dipoles, which means that they have two "poles," or ends. One is called the north pole, and the other is the south pole. Opposite poles attract, and like poles repel.

You can never create a magnet with just a north pole or just a south pole. If you took the magnet in Figure 22.1

and cut in down the middle, you would not separate the poles. Instead, you would create two magnets like those shown in Figure 22.2.

A magnet creates a magnetic field (see Figure 22.3). 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.

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 22.4a,

and when a magnetic field line is directed into the page, it is drawn as shown in Figure 22.4b.

Supposedly, the drawing in Figure 22.4a is intended to look like the tip of an arrow coming out of a page, and the drawing in Figure 22.4b is intended to look like the tail of an arrow going into a page.1 These symbols can be used to describe other ideas, such as electric fields going into or out of the page, or currents flowing into or out of the page, but they are most often used to describe magnetic fields.

Practice problems for these concepts can be found at:

Magnetism Practice Problems for AP Physics B & C

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