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Ch 29: Magnetic Fields and ForcesWorksheetSee all chapters

# Magnets and Magnetic Fields

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Sections
Magnets and Magnetic Fields
Summary of Magnetism Problems
Force on Moving Charges & Right Hand Rule
Circular Motion of Charges in Magnetic Fields
Mass Spectrometer
Magnetic Force on Current-Carrying Wire
Force and Torque on Current Loops
###### How Magnets Work Concept:

Concept #1: How Magnets Work

Transcript

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. Alright, so a long time ago, we found out that there«s some metals that would 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, 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, same thing with magnetic forces. So, you're gonna 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, pole is just one of the sides of the bar. So, we could do something like, let's call the side A and B and then here with 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. 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 same thing with North and South, this could have been called the positive side, could have been call 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 metal. So, if you were to get to two magnets and do this experiment, 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 you have the A end or the A pole with the B pole they would attract, if you flip both so you have B and A they will also attract and finally, if you have B and B they would repel. So, hopefully you see a pattern here, which is that 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 same that opposites attract holds true for both electricity and magnetism and that's it for this one, let's keep going.

Concept #2: Magnetic Fields and Magnetic Dipoles

Concept #3: Compasses and Earth's Magnetic Field

Transcript

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 magnet, let me show you, so the Earth's North Pole somewhere around here, the south pole somewhere on here, it's actually angled because the Earth spins around a tilted axis, okay? So, the Earth spins around at that little line there, this is the North Pole, which by the way it's just a bunch of water, and then this is a 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 it has a, 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 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's 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 backwards they could have done it the other way around but they decided to say hey the 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, 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, an 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 the 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 that's, 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 to 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, magnetized metal bar, this way, so that things can be attracted to it's north, okay? Alright, so that's the first thing, cool. So, the earth of the magnet, weird, the second thing is, if you realize that opposites attract and the compass is north points to the Earth's north, 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, you just said it that's north, Well, this is the, what we called 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 a 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 are south you are a north seeking magnet or you're the north seeking side of the magnet in that case, cool? Another very important but the 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 magnet and the North magnet on the bottom, remember magnetic fields go from north to positive through the outside, high to low on the outside, it's going to 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 of the magnetic field around it. So, if you have 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'd 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're in outer 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? 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' south pole will point to the Earth's South Pole, which by the way, this is our geographic self, which will be our magnetic north, not morth, North, okay? So, to wrap it up, in the very beginning the video I've said the north side of a magnet points towards the north of the Earth but actually in southern hemisphere then the south side of the magnet points towards the south of the earth, the easiest way to remember this is just remember how good the lines look like and remember this one statement here, that's the direction of the magnetic fields or the direction of the magnet's 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 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's the north points in the direction of the magnetic field. So, what is the direction of the magnetic fields? 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 field, 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 all for this one, let's keep going,