Video Transcript
Which of the four diagrams
correctly shows the field lines for the magnetic field between two very wide flat
magnets that are placed near to each other? Red represents the north pole of
the magnets and blue the south pole.
Starting out, we see these four
different diagrams marked out a, b, c, and d. Each one has a representation of
the field lines between these two very wide flat magnets. When we say the magnets are very
wide and flat, here’s what that means. Well, we might normally see a
magnet that looks like this, with the north pole here and the south pole here. In this case, we’re working with
magnets that are more like this, where the red is the north and the blue is the
south pole of each one.
Once we’ve identified that red is
north and blue is south, as the problem statement tells us, that tells us what the
field lines from this magnet will look like. It’s always the case that magnetic
field lines point from north to south. They always move in that direction
and not the opposite way. This means as we evaluate our
answer options, we’ll be looking for one where the south pole, the blue part of the
magnet, has field lines going toward it. And the north pole, the red part,
has field lines moving away from it.
Let’s go one by one and see which
of our answer choices shows us a field like that. Starting with option a, we see
that, in this case, the south pole, the blue part of the magnets face one
another. The field lines are drawn such that
they’re going away from one of the south poles, the one in the upper magnet. This violates our rule that
magnetic field lines always move from north to south. This means option a can’t be our
final answer.
Looking at option b, we see that
this one has a north magnetic pole facing a south magnetic pole. But in this case, the magnetic
fields move from south to north. That’s opposite the direction they
move, which is from north to south. Therefore, b isn’t our answer
either. Looking next at option c, we notice
that this one has a south pole facing a south pole. And once again, like in option a,
we have field lines purportedly leaving a south pole. This can’t be, so neither can
option c.
Finally, at the last option, we see
a red north pole facing a blue south pole. And this time, the magnetic field
lines point from north to south. That indicates the true direction
of magnetic field lines. And therefore, we choose option d
as the diagram that correctly shows the field lines between these two magnets.
