Pearson+ LogoPearson+ Logo
Start typing, then use the up and down arrows to select an option from the list.


Learn the toughest concepts covered in Physics with step-by-step video tutorials and practice problems by world-class tutors

28. Magnetic Fields and Forces

Summary of Magnetism Problems


Summary of Magnetism Problems

Play a video:
Was this helpful?
Hey, guys. So we're getting ready to start solving somewhere serious magnetism problems. But before we start, I wanted to do a video where I basically summarize the entire chapter. And the reason I want to do this is because in my experience, students tend to get very confused and overwhelmed with all the equations you're going to see in this chapter and all the different situations. But if you take a look at everything, um, you're going to actually seeing this video, that it's quite simple. So this is my favorite video. And after seeing this, I think you're gonna feel much better about what's to come. So let's get started. All right. So first of all, I want to remind you how electric charges, um, both produce of fields and field of force. And this is a central thing we're gonna talk about in this video. So if you have a little charge here, Q one, it produces a Fields E one and it goes in a bunch of different directions. I'm just gonna draw a few of them. And then if you have a second charge right here, Q to that cute too, is gonna fuel force and it's gonna be repulsed. It's gonna fuel Force F two. Okay? And the reason it feels of force is because key one is sending these field lines that are communicating that hey, to you're supposed to have a force. Okay, you're supposed to experience a force. So you may remember that this magnetic this electric field here is given by K Q one over R squared, where r is the distance between the two and key one. We called the producing the producing charge because it's creating of fields and the force on F two over here in this charge, F two. If you know the fields E one, then it's just the one Times Q two and Q two is what we called the feeling charge. Okay, so if you have two charges, I'm here, You're there. I'm sending a signal to you an electric field saying, Hey, you're supposed to experience a force and therefore you experience a force. The reason why I experience the forces because you are also sending a field my way and telling me to feel of force. So it's very important, you understand that are charged at the same time it produces a field and will fuel a force from other charges. So you do both at the same time, and that happens with magnets as well. So if you have two magnets and I'll make this part faster, north and north, they're supposed to repel. And you're gonna get field lines from north to south like this, remember? Right. Let's make a really big one here. So what happens is this magnetic This magnet here is producing. It produces a magnetic field, By the way, magnetic field is given by the letter B instead of e. So it produces a Byfield, and this guy here will feel a magnetic force. FB okay, and vice versa. Okay, so this guy here is producing and this guy's feeling, but actually they're both producing and feeling they're both producing feeling That's why there's a mutual force now. The reason this is important is because almost every magnetism problem is gonna ask you to calculate the magnitude of either a a new magnetic fields that is being produced. So there's no magnetic field, and then something happens that creates a new magnetic field or the force that you're going to feel from due to an existing magnetic fields, existing magnetic fields. Okay, most problems or like this. So when you start solving a problem, the first question you should ask yourself is, Are we dealing with an existing fields, or are we dealing with or are we creating a fields? Okay. And we're gonna talk a lot more about that. All right, so we're gonna calculate fields produced by okay, as well as forces felt by either electric charges or electric wires. Okay, We're actually not going to calculate anything with magnets. We've talked about magnets before. The only thing you need to know about magnets in their direction. You're not actually gonna calculate the magnitude of force between two magnets, for example. Okay, um, the key difference here between magnets and these guys is that magnets will always produce fields and will always feel a force to magnets. Will always do that. Okay, but charges and wires don't always produce the fields, and they don't always feel a force. Okay, Charges will Onley produce the fields and feel a force if they are moving. Okay, if they are moving. So I'm gonna draw this here because I think it's gonna be helpful if you have a charge going this way. And then you have a charge going this way. V one V two, there will be there will be some sort of magnetic force between them. Okay, but if charges are static, there's Onley electric force between them. No magnetic force. Okay, The same thing happens with wires. Electric wires will only produce the fields or fuel of force if they have current, if they have current running through them. And if you think about it, what is the current currency? Just charge that is moving. So these two points are actually equivalence, right? So charges have to be moving. So if you have a wire, it means that the wire has to have a current through it so that the charges are moving. That's a very important distinction. And you see this in the equations as well. All right, so in this chapter, you're gonna see a lot of equations, and don't worry about them For now, you'll see these guys later, but I just want to kind of scare you ahead of time. Um, this can also work for you as sort of an equation sheet that you could take throughout the chapter. We're not gonna talk about any of these equations. I just wanna make the point that the seven boxes are most of what you're going to see, and I'm gonna do one video on every single one of them, but they're all very, very similar. I'm trying to sort of demystify the entire chapter. And what you're gonna get is you're gonna get problems where you're producing a new fields or feeling of force in an existing fields. So again, the first question gonna ask is, Is this a charge moving through an existing fields? Or is this a charge creating a fields that doesn't already exist? And depending on the answer to that question, you're either gonna be here or here. Notice how Here you're trying to figure out what is the magnitude of the new field, right? That's why all these equations, Arby's and here you're trying to figure out the magnitude of the forces. Okay, so you have four different kinds of problems we're going to see, and this is the last point I'm gonna make first, you're gonna have a moving charge. So, like a cue, that is, let's say, moving this way with the magnitude of V. And if you have a, um if you have a moving charge that's moving through a magnetic fields I'm sorry if you have a moving charge. Um, if you have a moving charge, it's going to produce a magnetic fields, and you can find the magnitude of that magnetic field using this equation. We'll get into details off those equations and other videos. Okay, But remember, charges, um can also move in a wire. And if you're wired essentially just trapping charges inside of it so you can have a moving charge or you can have a wire with currents or a current carrying wire, and these situations are identical. So now you have a wire, and instead of V to the right, you have high to the right, okay? And it's gonna be similar that this wire is also going to create the magnetic fields. So just as a quick example, right here at appoints p, there will be a magnetic fields here. Okay, so this point here will have a magnetic field. Let's call it. This is point P points p. It will have a magnetic fields A B fields be p okay, because the certain distance away from that cable. You can also get a wire and make it into a loop. So imagine a wire and you just make a square out of it or a circle out of it. Something maybe like this, where you make a loop out of a wire so that the current goes this way, goes all the way around and then comes out this way. You could do that as well. That's the third thing we're gonna talk about. And you can find a magnetic field right through the middle right here. OK, using that equation and finally, you can make a lot of loops. Um, you didn't get a wire into a loop with it like this. That's a single loop. Or you can do three loops. That's three loops, or you can make really long loops. And these guys were called Sahlin Woods. Something like this. And essentially, you're making something where this is really, really long Hell, so that it's bigger than much bigger than just the radius of the circle. And that sort of gets a different equation. Okay, so these these are the four situations we're gonna talk about in this chapter Um some books actually break this up into two chapters where this will be one chapter, and this will be one chapter. Sometimes it's in the opposite order, but it's all the same crap over and over again. I really wanted to do this video because I think you'll see as we go through these that it's just the same crap over and over. So that's it for this one. Let's get going.