Video: Understanding the Magnetic Field between Two Flat Magnets

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. [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D

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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.

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