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.
