Video: Understanding the Magnetic Field of a Bar Magnet

The diagram shows a bar magnet. The bar magnet creates a magnetic field around it. At which point marked on the diagram is the magnetic field strongest? At which point marked on the diagram is the magnetic field weakest?

02:23

Video Transcript

The diagram shows a bar magnet. The bar magnet creates a magnetic field around it. At which point marked on the diagram is the magnetic field strongest? At which point marked on the diagram is the magnetic field weakest?

Taking a look at our diagram, we see this bar magnet with north and south poles and the four points — A, B, C, and D — marked out around it. We’re told that this bar magnet creates a magnetic field around it. And in representing that magnetic field using magnetic field lines, we’ll find the answer to our question of where that field is strongest and where it’s weakest.

We can start out by sketching in the magnetic field lines representing the field created by this magnet. We can recall that, in general, these field lines move from the north pole of the magnet to the south pole. With these example field lines drawn in, notice that each one has a direction associated with it and has an arrowhead on the field line telling which way the magnetic field points.

However, it’s not the magnetic field direction we’re interested in, but rather its strength. We want to know where the field is strongest as well as weakest. To find this out, we can recall that magnetic field line density, that is, how many magnetic field lines pass through a given space, is an indication of magnetic field strength. The more field lines within a certain fixed volume, the greater the strength of the magnetic field.

This means we can look at the field lines we’ve drawn in on this diagram and scan for regions of high concentration as well as low concentration. The places where field lines are spaced more closely together, say around here, indicate a stronger magnetic field. On the other hand, places where there aren’t many magnetic field lines in a given space, say out here, indicate a weaker field.

Knowing this, we want to evaluate our four locations — A, B, C, and D — on this diagram. To start out, let’s look for the point where the field lines are densest, most closely together. That will be where the field is strongest. Comparing the four points, we see that it’s point A that exists where there are more field lines per unit space than any other. That indicates a relatively stronger magnetic field. So we’ll say that this point is the one at which the field is strongest.

Now what about the point at which the field is weakest? If we consider the space around points B, C, and D, we see that it’s point D which has the fewest field lines nearby. The lowest magnetic field line density indicates the smallest magnetic field strength. Therefore, of the four points, the magnetic field is weakest at point D.

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