Lesson Video: Attraction and Repulsion between Permanent Magnets Physics

Video Transcript

In this video, we’re learning about the attraction and repulsion between permanent magnets. Before we talk about attraction and repulsion though, we can consider just what a permanent magnet is. And even before that, we can consider what a magnet is. Maybe the simplest way of saying it is that a magnet is an object that creates a magnetic field. And it’s at this immediate point that we can think of an analog, an analogy, with another physical phenomenon.

We know that electric charge is also something which creates a field around itself, in that case an electric field. Electric fields are invisible. But we know that they’re real, that they exist, because of the effects they have on other electric charges. And in a similar way, magnetic fields are themselves invisible. But we know that they exist because of the effect they have on other magnets.

Now we’ve said that magnetic fields are analogous to electrical fields. And that’s true to an extent. But there’s one very important difference. We know that when it comes to electric charge, there are two basic types: positive and negative charge. Similarly, when it comes to magnets, there are two types or two kinds of pole. There is a north magnetic pole and there is a south magnetic pole. And it’s here that the difference comes in.

Notice that our different types of electric charge, positive and negative, are separated from one another. In other words, we can isolate them. So we can have a charge that’s 100 percent positive or another charge which is 100 percent negative. With magnets, as far as we’ve been able to see, that’s not the case. They always come with a north and a south pole together. We might think, “Well, okay, if we start off with a magnet with a north and south pole, what if we broke it down the middle? Wouldn’t that then give us just a north pole by itself and just a south pole by itself?”

Well, people have tried that. And they found that when they break a magnet in half, what actually ends up happening is the two parts themselves become magnets with north and south poles. And then when they break those in half, they find the same thing, more magnet parts, all of them having a north and a south pole together. So that’s a main way that our analogy breaks down that magnets are not like electric charge.

So we found that a magnet is the type of thing that always has both kinds of poles, the north and the south pole, involved in it. And this reality affects what the field of a magnet will look like. Unlike the electric fields here, which look a bit like the spokes on the wheel of a bicycle, the magnetic field created by a magnet looks sort of like two gigantic lobes. Like the electric fields though, the magnetic field lines also have a direction to them. They point from the north pole of our magnet towards the south pole. This means that if we were to follow a magnetic field line from start to finish, we would go in this direction, starting at the north pole of the magnet, curling around along the line, and then joining back up at the south pole of the magnet.

Okay, so we’ve talked a bit about what magnets are and what their fields look like. And now let’s get on to the topic of permanent magnets. Thousands of years ago, people first encountered a type of stone that they found in the Earth that had some interesting properties. For example, they found that the stone always wanted to take a certain position relative to the rest of the Earth. They found that if they took one of these stones and suspended it by a thread, then if they just let the stone sit there completely still and no one touched it or moved it, then all by itself, as far as anyone can tell, the stone would slowly rotate. So that one end of it was pointing towards the north and the other end was pointing towards the south. And no matter how the stone started out in terms of its position, this is how it always seemed to end up along a north–south line. These stones, called lodestones, are the earliest examples of permanent magnets.

Now saying that a magnet is permanent brings up the question: Is it possible to have an impermanent magnet? The answer to that actually is yes. It is possible for an object to be a magnet at one time but then lose its magnetization. But lodestones and other permanent magnets aren’t like that. They’re always magnets, regardless of whether or not they’re being influenced by something else.

Now let’s imagine that we ourselves were doing this experiment with a suspended lodestone and we see the stone turn as it’s positioned. Based on our compass, the north pole of the Earth is that way and the south pole of the Earth is that way. And we can see that this part of our lodestone is pointing towards the north and this part is pointing towards the south pole.

We know that when it comes to magnets, just like with electric charge, opposites attract. That means that the north pole of one magnet will attract the south pole of another magnet. Looking at our experiment, we might say since this end of our lodestone is pointed to the north, that means it’s attracted to the north. Therefore, it must be a south pole on this stone. That reasoning seems to make sense. And it would be correct except for one tricky thing about the Earth’s north and south poles.

If we were looking at a globe, a representation of Earth, we would say that this here is the north pole and this here is the south pole. And that’s true. When it comes to magnetism though, there’s a bit of a confusing twist to it. It turns out that the north geographic pole, which we’ve identified here, is very near to what we call the south magnetic pole, the south magnetic pole being the place where the north pole on magnets is attracted to. And likewise, the south geographic pole here is very near to the north magnetic pole, that is, the place where the south pole on magnets is attracted to. So you can see this turns things almost completely backwards from what we might intuit.

The point we’re making here is that this part of our lodestone, which we might reasonably think to be a south pole because it’s pointed in the northern direction, is actually a north pole. Because as we said, the north geographic pole of the Earth is actually very near the south magnetic pole of the Earth. Again, very confusing, but once we know it, we know it.

Now if that top end of our lodestone is its north pole, then that must mean that this other end is its south pole. So we had this stone, and it’s a permanent magnet. It always has a north pole and a south pole to it. Let’s take this experiment one step further.

Say that we now have two lodestones suspended from thin strings. And we’ve already done the work to figure out what the north and the south pole on each stone is. So now with these two stones brought near one another and suspended from springs, what we wonder will happen to them?

Here’s what we see. We see that the north pole of this lodestone is near to the south pole of this other one. And as we mentioned earlier, magnetic poles are a little bit like electric charge, where opposites attract and like repel. In this case, we have two opposite magnetic poles, which means there will be a magnetic force drawing these lodestones toward one another.

But now let’s try something else. Say that we reset this experiment. But we flip one of the stones around so that now its north pole is facing to the left and its south pole to the right. In this case, we now have like poles, north and north, close to one another. And just like with like electric charges, these poles will repel one another. We find that, in this way, magnetic poles behave like electric charge. Like poles repel one another, and unlike poles attract.

Now, so far, we’ve talked about permanent magnets as though the only type of permanent magnet we might encounter is a kind of stone we would find buried in the Earth. In fact though, permanent magnets can be constructed by heating certain materials to a hot enough temperature that the magnetic fields within them are fixed or made permanent. We can make permanent magnets into a bar shape — that’s fairly common — or a horseshoe shape or really almost any shape we could imagine. Regardless of the shape a permanent magnet comes in, they interact with other magnets according to this rule. Like magnetic poles repel one another, and unlike magnetic poles attract. Now that we know a bit about permanent magnets and how they work, let’s try out a few examples.

A bar magnet is hung from a thread that is attached to a stand. The magnet can pivot freely. What will happen if the north pole of a second bar magnet is brought near to the north pole of the bar magnet that is hanging from the thread? What will happen if the north pole of the second bar magnet is brought near to the south pole of the bar magnet that is hanging from the thread?

Okay, let’s say that this is our bar magnet, this is the thread, and this is the stand that the thread is hanging from. We’ll say that the blue side of our bar magnet is the north pole and the pink side is the south pole. The first part of our question asks this. It says, “What if we take a second bar magnet and we bring the north pole of it near to the north pole of the bar magnet hanging from the thread?”

To figure out what will happen here, it’s helpful to remember that magnetic poles are a bit like electric charge. When it comes to electric charge, like charges repel one another and unlike charges attract. So we would say that positive is drawn to negative but repelled from positive, and negative is drawn to positive but repelled from negative. Well, the same sort of thing happens with magnetic north and south Poles. Unlike poles, a north and a south pole, attract one another, but like poles, for example, the north and north pole we have here, repel one another.

For these bar magnets, because the poles that are closest together are of a like type — they’re both north — that means these magnets will repel one another. If we were to hold this second bar magnet in place so that it couldn’t move, then the one that’s suspended by the string would swing a bit backward. It’s pushed away from the other bar magnet. We can write that out as a sentence this way. We can say that the north pole of the hanging bar magnet will be repelled by the north pole of the second magnet.

In part two of this question, we want to know what will happen if the north pole of the second bar magnet is brought near to the south pole of the bar magnet that is hanging from the thread. In this second scenario then, here’s what we have. We have our second bar magnet. But this time, the north pole of this bar magnet is being brought near to the south pole of the suspended magnet. In this case then, the two poles that are closest to one another, the north and the south poles of these two bar magnets, are of opposite types. Therefore, they’ll attract one another. In this case, the suspended bar magnet will be drawn towards the second bar magnet rather than repelled from it.

Let’s write that out in answer to this second part. Our answer is that the south pole of the hanging bar magnet will be attracted to the north pole of the second magnet. This is what will take place in these two instances of bringing the second bar magnet close to the suspended bar magnet.

Let’s look at one more example of permanent magnet attraction and repulsion.

Two bar magnets are placed on a flat table in the arrangement shown in the diagram. Will the magnets be attracted to each other or repelled by each other?

The diagram shows us the two bar magnets as well as the way the north and the south pole on each magnet are arranged. We can see that, for the first bar magnet, this one here, the pole that’s closest to the second bar magnet is the south pole. And for the second bar magnet, the pole that’s closest to the first bar magnet is the north pole.

It’s at this point that we can recall how magnetic poles interact with one another. We can recall that like magnetic poles, that is, either a north and a north or a south and a south, will repel one another, whereas unlike magnetic poles attract one another. And this gives us the answer for how these two bar magnets will interact. We see that because two unlike poles are nearest one another, they’ll be drawn together according to this rule.

For our answer then, we can say that they will be attracted to each other. This will be the way that these magnets interact.

This just about finishes up our talk on the attraction and repulsion of permanent magnets. Let’s summarize what we’ve seen so far.

We learned in this lesson that permanent magnets are objects that always create a magnetic field around themselves. And we saw examples of permanent magnets that are both natural as well as man-made. Additionally, we saw that all magnets consist of a north as well as a south pole. They never come just one by one. They’re always together. We saw that these poles helped to create a magnetic field around the magnet. And the direction of that field is to point from the north pole towards the south pole.

And then, vitally, we saw that like magnetic poles, that is north and north or south and south, repel one another, whereas unlike magnetic poles attract one another. And this governs how magnets interact among themselves.