Magnets and Magnetic Fields - Online Tutor, Practice Problems & Exam Prep

Magnets possess two poles, North and South, which interact through magnetic forces. These forces only occur between magnetic materials, similar to electric charges. Magnetic field lines emanate from the North pole to the South pole, forming loops. Notably, magnets cannot exist as monopoles; cutting a magnet results in two smaller magnets, each with a North and South pole. Compasses align with Earth's magnetic field, where the North pole of a compass needle points toward the Earth's magnetic South, illustrating the principle that opposites attract.

How Magnets Work

Hey guys. So now we're going to start talking about magnetism. And in this first video, I'm going to keep it really simple and briefly explain to you how magnets work. Let's check it out. Right. So a long time ago, we found out that there are some metals that will sometimes attract each other. They sort of magically get stuck. And this was first found in the Greek island of Magnesia, so these magic metals were called magnets. The metals that are most commonly, that most commonly have this magnetic property of getting stuck to each other are iron, cobalt, and nickel, but you should know that not all pieces of iron, cobalt, and nickel are always magnetic. Okay?

Electricity and magnetism are very similar and there's a lot of analogies we're going to draw between the two. The first one here is that forces, electric forces can only exist between charged materials. Meaning if you have two objects next to each other, they're only going to interact electrically if both of them are charged if they both have charges. The same thing with magnetic forces. So, you're only going to have magnetic forces if the two objects have this magnetic property. So if you have two objects close to each other that are non-magnetic, you get no force. One magnetic and one non-magnetic, you get no force. And only in a situation like this, you're going to get a magnetic force. Very straightforward. Remember also the electricity in electricity, the forces could be attractive or repulsive. The same thing is going to happen with magnetic forces between magnets depending on the ends of the magnets. Okay?

So let's say you have an iron bar here and then you have another iron bar here and when you bring them close to each other, they are attractive. They attract each other. There's an attractive force. Now let's say you flip, you get the second bar and now you flip it. Now you flip that bar and then you see that this force is actually now repulsive. So these guys will repel each other. And just from this observation, you can conclude that there must be two different types of sides. Okay. Because the two sides are behaving differently, there must be two types of sides which are called magnetic poles. So a pole is just one of the sides of the bar. So we could do something like let's call this side A and B and then here we flipped and then this is going to be B and A. So that's the first thing you need to know about magnets is that they have two different sides. Okay. Now in electricity, these sides were called, you had positive and negative charges. And in magnetism these sides are going to be called North and South Poles. A pole again is just a word for side or end of the magnet bar. You may remember that these names are arbitrary. Positive could have been called good and negative could have been called evil or yellow or blue or whatever, but that's what they chose. And the same thing with North and South. Could have been called the positive side, it's going to be called the negative side, but they chose North and South. So those are the names. And the last thing I want to talk about is what happens if you get two, metals, two magnets that are identical. So these guys here are same as left. What happens if you face different faces, different ends of the metals? So if you were to get to two magnets and do this experimentally, what you would see is that if you have the A side with the A side, these guys would repel each other. But then if you flip this and then you'll have the A pole with the B pole, they would attract. If you flip both, so you have B and A, they would also attract. And finally, if you had B and B, they would repel. So hopefully you see a pattern here, which is that whenever the whenever the sides are different, A and B, B and A, they will have an attractive force. And you may remember this from electricity. In electricity, opposite charges would always attract. Similarly, in magnetism, opposite poles will also attract. Cool? So the saying that opposites attract holds true for both electricity and magnetism. And that's it for this one. Let's keep going.

Magnetic Fields and Magnetic Dipoles

Hey everyone, so in this video we're going to talk about how magnets work. We're going to talk about magnetic fields and magnetic dipoles. Let's check it out here. So remember that electric charges produce electric fields, and electric field lines that go from positive to negative charges. Like, if you have a charge that's by itself, a positive one, you're going to have electric field lines that go outwards like this. If it was a negative charge, then all these field lines would point inwards. Now if you have an electric dipole, right, when we have a positive and a negative charge, then you're going to have field lines that sort of go in arcs like this, but the electric field lines are always going to go from positive to negative like that.

Now similarly, magnets also produce lines. They're magnetic field lines given by the letter b, but they're directed from north to south on the outside. One kind of easy way to remember this is that almost everything in nature goes from high to low. So we have electric field lines that went from positive to negative and you can think of north and south as being positive and negative like on an axis or something like that. Right? So that's north and this is south. Alright? So what happens here is that you would expect the field lines to sort of look like this inside the magnet, but they're actually going to form loops that travel on the outside of the magnet. So you're basically going to go here from north and you're going to go in a loop around until you hit south and you're going to go in bigger and bigger loops like this.

Alright, so here's how the magnetic field lines work and look like for magnets. Alright, so you follow these sort of arrows like this and they always go from north to south. Now if you keep sort of going along this line here, what you'll see is that they actually sort of go from south to north on the inside of the bar magnet. So that's kind of important here. So I'm going to write here that it's south to north on the inside. Alright, so that's kind of important. Alright. So one key difference between electric fields and magnetic fields is that you could have single charges that could exist alone with electric fields. They were called electric monopoles. So for example, we just drew the electric field lines for a single positive charge and that's totally fine. So in other words, this is also known as a monopole. Right?

Now magnets on the other hand cannot just have one pole. That's really important here. Magnets, you can't just have a North or a South Pole. So magnetic monopoles cannot exist, only dipoles. And so in other words, magnets always have to come in north and south pairs. Now one important consequence of this is if you were to ever cut a magnet in half, I don't know why you would, but you would expect to see that a magnet would basically, turn like this. Right? You would expect that if you cut a magnet in half, it would sort of look like this at the end with a south and a north by itself, but that actually would not happen. This is wrong. Instead, what happens is if you split a magnet in half, you just end up with 2 smaller magnets. So if you cut a magnet in half, it would actually just look like this. You would have a south and a north, and then you would have a south and a north. Alright. So this is pretty much what would happen here. Alright. It's kind of a weird thing, but that's just the way that magnets work. Alright? Let's go ahead and take a look at our example here. So we're going to have both magnets that are fixed in place, but they can rotate about their central axis. Basically, you're going to hold these things in place here like with a pin, but they're able to freely rotate. Alright?

Now in the first thing, in the first part here, we're going to release only the bottom magnets. So in other words, the top magnet is going to remain sort of in place and it's going to look exactly how it did before. So in other words, it's going to be north and south and we're going to ask to find out what its new orientation would look like. Alright. So basically, if we kept this North one in place well, the top one, what would happen to the bottom magnet? How would it spin? Alright. So hopefully you guys realize that the way it would spin, is it wants to sort of orient itself so that opposites attract. So it's basically, what's going to happen here. So you're going to have this bar magnet, it's going to sort of line up with this. The key thing to remember is that opposites attract just like in electricity, right? It's positives with negatives, north with south, opposites attract. Alright. So if this is the south side, then what's going to happen is that this side here actually has to be the north side. So what's going to happen is you're going to keep this thing in place here and the bottom one's actually going to flip around like this and it's going to sort of orient itself so that the north side points to the south side. Alright?

Now let's look at part b here. Part b is a little bit tricky because it asks if you release both magnets simultaneously. So in other words, now both of them are able to spin, what's the new orientation going to look like? Alright. Well, hopefully you guys realize that unlike this first one here, now both of them are actually going to rotate and they're going to orient themselves in such a way that they point along towards each other. So the top one is going to rotate like this, the bottom one is going to rotate like this, so they're kind of like aligned like that. Now, what's tricky about this is that there's actually 2 different possibilities here. Alright? So they're going to sort of orient themselves like that. And one possibility is you could have north and south, and in this case what's going to happen is just like we had over here in part a, this one's going to have north and south. Or you also could have that you have the top and bottom magnet and basically you could have a situation where you could have the top magnet flips around and South becomes North. It's basically the opposite of this. And in this case, what happens here is that you have South and North. Right? Basically, just the opposites so that the opposites always attract here. So these are the sort of two possibilities you could have with these magnets. Alright folks. That's it for this one. Let me know if you have any questions.

Compasses and Earth's Magnetic Field

Hey guys, so in this video, we're going to talk about compasses and the Earth's magnetic field. Let's check it out. Alright, so remember magnets have ends or sides or poles that are called north and south. So something like this, this end is north, this end is south, but how do you know which one is which? How do you know which one to label north, for example? Well, the end of the magnet that points to the Earth's north is the one that gets labeled to be the north pole of the magnets. Let me show you. So the Earth's North Pole is somewhere around here, the South Pole somewhere around here. It's actually angled because the earth spins around a tilted axis. Okay. So the earth spins around that little line there. This is the North Pole, which by the way is just a bunch of water. And then this is the South Pole which is a bunch of ice and Antarctica. And you have sort of like the equator over here. So the way this works is, let's say you have a magnet and you paint one side red and then you paint one side blue and then you bring this over here and you're in the US somewhere, and you notice that when you're here this thing always orients itself like this. With the red side this way, and the blue side this way. And then you move over to Europe over here and then you'll notice that it always orients itself, with the red side pointing to north and the blue side pointing the other way. So what you're going to do is you're going to say, well, this one is pointing north, red is pointing north. So the red side must be what we're going to call the north side. Okay. So north and south. This is completely arbitrary, they could have done it backward, they could have done the other way around. But they decided to say, hey if it points north we'll call it north. That makes sense, right? And that is, by the way, how compasses work.

Okay? So this is what a compass looks like. It has a magnetic needle. So a very thin metal, a very tiny metal inside that is magnetized. And the end of that needle, that points to the earth's north is labeled north. Okay? So you can't see here, but this is North right there. Okay? And you may not be able to see this, but this tip is red. Okay? So on old school magnets, one side is red. And the red side is the one that is the north side of the needle. Okay? The north side of the needle. By the way, sometimes if you don't have colors, you may see this as an arrow. You may see something drawn like this. So for example here, I could have drawn, if I had something over here, I could also have just made an arrow this way, which means that that is a direction of North. Okay? By the way, if you have a magnet here that's pointing directly north, it means that you're probably somewhere close to this line so that it's pointing straight up. Okay? And similarly, if you had a compass that the north, that the north arrow or the north side of the magnet was pointing that way, it means that you're probably somewhere over here. So this is why compasses are able to be used, or used to be used as navigating devices. Cool. So if it points north, it's north, that's that.

Another important thing to realize is remembering that magnetic forces can only exist between two magnets. Okay? They can only exist between two magnets or more generally between two things that are magnetized. Okay? So, if this needle here is attracted to the top of the earth, if that needle is attracted to the top of the earth, and you can only have attraction between two magnets, it must be that not only the needle is a magnet, which it is, but that the earth is also a magnet, which it is. So the earth, it's not a magnet in the typical sense that there's a huge metal bar through it, but it behaves like a magnet. A gigantic magnet. Okay? So you can think of the earth as though it had a huge metal, but magnetized metal bar, this way so that things can be attracted to its north. Okay? Alright. So that's the first thing.

Cool. So the earth is a magnet. Weird. The second thing is, if you realize that opposites attract and the compass's north points to the earth's north, so let's do that slowly. Let's say I have a compass here and the north of the compass is pointing towards the north of the Earth. Remember, opposites attract. So if this end is attracting this end, and we call this North, this must be South. Okay? This must be south. Now you might be thinking, no that's north. We just said that that's north. Well, this is the, what we call, the geographic north. Okay? Which one way to think about this is that it's up there on the map. It's on top of the map. Okay? That's the locational, the geographic North. But it must be the magnetic south. Meaning the earth behaves like a magnet that has its north over here and its south over here. So the thing is if we wanted to call these magnets, this side of the magnet north because it's pointing to the north of the Earth, we must then recognize that this has to be called the south side of this big imaginary magnet that sits inside of the earth. All this stuff is just convention, but that's how it works. Okay? So north and south. So please get that difference down. And because of this, the north pole of a needle, so here's the compass needle, the north one, the arrow, this is the north side or end of the needle, is also sometimes called south seeking. And again, it's because opposites attract. So if you are the north, you want south. So if you're south, you are a north-seeking magnet. Or you're the north-seeking side of the magnets, in that case. Cool? Another very important but general point is that any magnet's North is always going to point in the direction of the magnetic field around it. So what does that look like?

So I want to redraw the earth over here, and we're going to draw the magnetic field on the earth. So the top of the earth is going to be the south magnets and the north magnet on the bottom. Remember magnetic fields go from north to positive through the outside. High to low on the outside. So it's going to look like this. High to low. High to low. Like that. Right? North to South on the outside. So the magnetic field lines will look like this. Any magnet's north will point in the direction exactly in this direction here. So if you had a magnet right here or a compass, it will point exactly in this direction here. Okay? Exactly in this direction here. That means that if you have a magnet, if you have, let's do a different color, if you have something over here it's actually going to point like that. Okay. But then you wouldn't really be navigating. Now you run out of space. But just to make the point that, now you got bigger problems, right? But just to make the point that it's always going to follow this line. So it actually doesn't always directly point north, right? If you're here, if you're in the equator, right here, it's actually going to sort of go up like this. It doesn't really point north directly. Depends on where you are. So you have to sort of adjust for your sort of height on the earth. Okay? Similarly, if you look here it actually points if you draw it down here, it actually points away. It doesn't point towards the north of the earth. What it actually does is it points away from the south. Okay? So that's the last point I'll make and then we'll do a quick example.

So it says here, if you, by the way, if you notice in these examples, I was careful to always draw stuff on the northern hemisphere, but if you are on the south of the earth, if you are here on the southern hemisphere, then the compass's south pole will point to the earth's south pole. Which by the way, this is our our geographic South, which will be our magnetic North. Oops, not North. North. Okay? So to wrap it up, in the very beginning of the video I said the North side of a magnet points towards the north of the Earth. But actually, if you're in the southern hemisphere, then the south side of the magnet points toward the south of the earth. The easiest way to remember this is just remember how what the lines look like and remember this one statement here that the direction of the magnetic field or the direction of the magnets north is always going to follow the blue lines. Okay? Let's do a quick example here. So the green magnet below is fixed in place and you have a ton of small compasses located around it. We want to draw the approximate orientation of the needles. And we're going to use an arrow to indicate the north direction. So what we're going to do here is we're going to first draw, we're going to use this principle right here which is super important, that the North will point in the direction of magnetic field. So what is the direction of the magnetic field? Well, magnetic field is always North to South through the outside. So I'm going to do north to south like this. And then I'm going to be very careful to do this here. Obviously, I laid out these guys there for a reason, so it all just looks real cute. And it looks like that. You can't really see the last one, but that's cool. So north to south, so it looks like this. North to south, it looks like that. Cool? So that means that this needle here is going to be pointed this needle here is going to be pointed in the direction of the field lines. So the needle is going to point like this. This needle is going to point like this. This needle is going to point like this. This needle is going to point like this, and then finally this needle is going to point down right there. The reason we said approximate is because it depends on how precisely you draw this thing. I just want to make the point super important that the fields, the direction of north on a magnet will follow the direction of the field. And by the way, this works in any hemisphere. Cool? That's it for this one. Let's get going.

Here’s what students ask on this topic:

How do magnets work and what are magnetic poles?

Magnets work by generating a magnetic field, which is a region around the magnet where magnetic forces can be observed. This field is created by the movement of electric charges within the magnet. Magnets have two poles: North and South. These poles are the points where the magnetic force is strongest. When two magnets are brought close to each other, opposite poles (North and South) attract, while like poles (North and North or South and South) repel. This behavior is similar to electric charges, where opposite charges attract and like charges repel. The magnetic field lines emanate from the North pole and loop around to the South pole, forming continuous loops.

What is the difference between electric fields and magnetic fields?

Electric fields are produced by electric charges and can exist as monopoles, meaning a single charge can create an electric field. The field lines for a positive charge radiate outward, while for a negative charge, they point inward. Magnetic fields, on the other hand, are produced by moving electric charges or magnetic dipoles and cannot exist as monopoles. Magnetic field lines form closed loops, emanating from the North pole of a magnet and returning to the South pole. Inside the magnet, the field lines travel from the South pole back to the North pole. This continuous loop is a key characteristic of magnetic fields.

How does a compass work and what is the Earth's magnetic field?

A compass works by aligning its magnetic needle with the Earth's magnetic field. The needle has a North and South pole, and the North pole of the needle points towards the Earth's magnetic South pole, which is near the geographic North pole. This alignment occurs because opposite magnetic poles attract. The Earth itself behaves like a giant magnet with a magnetic field that extends from its magnetic South pole to its magnetic North pole. This field is generated by the movement of molten iron within the Earth's outer core, creating a dynamo effect.

What happens if you cut a magnet in half?

If you cut a magnet in half, you do not get a separate North pole and South pole. Instead, each half becomes a smaller magnet with its own North and South poles. This is because magnetic monopoles do not exist; magnets always have both a North and a South pole. When a magnet is divided, the magnetic domains within the material realign to form two new magnets, each with a complete set of poles. This phenomenon illustrates the intrinsic dipole nature of magnetic materials.

Why do opposite poles of a magnet attract each other?

Opposite poles of a magnet attract each other due to the nature of magnetic field lines. Magnetic field lines emerge from the North pole and enter the South pole. When two magnets are brought close together, the field lines from the North pole of one magnet naturally seek to connect with the South pole of the other magnet, creating an attractive force. This is similar to electric charges, where opposite charges attract because the electric field lines from a positive charge seek to connect with a negative charge. The attraction minimizes the potential energy of the system, leading to a stable configuration.