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Ch 29: Magnetic Fields and ForcesWorksheetSee all chapters

# Summary of Magnetism Problems

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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

Concept #1: Summary of Magnetism Problems

Transcript

Hey guys. So, we're getting ready to start solving some more serious magnetism problems but before we start I wanted to do a video where I basically summarized 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 are going to see in this chapter and all the different situations but if you take a look at everything you're going to actually see in this video that it's quite simple, so this is my favorite video and after seeing it I think you're gonna feel much better about what's to come. So, let's get started.

Alright, so first of all, I want to remind you how electric charges both produce a field and feel a force, and this is a central thing we're going to talk about in this video. So, if you have a little charge here q1, it produces the field E1, and it goes in a bunch of different directions, I'm just going to draw a few of them, and then you can have a second charge right here q2, that q2 is going to feel a force and it's going to be repulsed, it's going to feel a force F2, okay? And the reason it feels a force is because q1 is sending these field lines that are communicating that, hey too 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, Q1 over r squared, where r is the distance between the two, and Q1 we called the producing, the producing charge, because it's creating a field and the force on F2 over here in this charge F2, if you know the fields E1, then it's just E1 times q2 and q2 is what we call the feeling charge, okay? So, if we 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 a force is because you're also sending it feels my way and telling me to feel a force. So, it's very important that you understand that are charge at the same time produces the field and will feel 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 going to get field lines from north to south like this remember, right? Let's make a really big one here. So, what happens is, this magnet here is producing, it produces a magnetic field, by the way, magnetic fields is given by the letter B instead of e, so it produces a B field 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, okay? They're both producing and feeling, that's why there's a mutual force. Now, the reason this is important is because almost every magnetism problem is going to ask you to calculate the magnitude of either a new magnetic field 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 field, existing magnetic field, okay? Most problems are like this. So, when you start solving a problem, the first question you should ask yourself is, are we dealing with an existing field or are we dealing with, or are we creating a field, okay? And we're going to talk a lot more about that. Alright, so we're going to 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 talked about magnets before, the only you know about magnets to their direction, you don't actually going calculate the magnitude of force between two magnets for example, okay? The key difference here, between magnets and these guys is that magnets will always produce fields and will always feel the force, two magnets will always do that, okay? But charges and wires don't always produce a field and they don't always feel a force, okay? Charges will only produce a field and feel force if they are moving okay, if they are moving. So, I'm going to draw this here because I think it's going to be helpful, if you have a charge going this way and then you have a charge going this way V1, V2 there will be, there will be some sort of magnetic force between them, but if charges are static there's only electric force between them no magnetic force, okay? The same thing happens with wires, electric wires will only produce a field or feel a force, if they have current, if they have current running through them, and if you think about it, what is current, currents is just charge that is moving. So, these two points are actually equivalence, right? So, charge 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. Alright, so in this chapter you need to see a lot of equations and don't worry about them for now, you'll see you guys later but I just want to kind of scare you ahead of time, this can also work for you as sort of an equation sheet that you can take through out the chapter, we're not going to talk about any of these equations, I just want to make the point that these seven boxes are most of what you're going to see, and I'm going to do one video on every single one of them but they're all very, very similar, I'm trying to sort of do demystify the entire chapter and what you're going to get is you're going to get problems where you're producing a new field or feeling a force in an existing field. So, again the first you are going to ask is, is this a charge moving to an existing field or is this a charge creating a field that doesn't already exist, and depending on the answer to that question, you're either going to be here or here. Notice how here, you're trying figure out what is the magnitude of the new field, right? That's why all these equations are B'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 going to make.

First you're going to have a moving charge. So, like a q that is, let's say moving this way with the magnitude of V and if you have a, if you have a moving charge that's moving through a magnetic field, I'm sorry, if you have a moving charge, if you have a moving charge it's going to produce a magnetic field, and you can find the magnitude of that magnetic field using this equation, we'll get into details of those equations in other videos, okay? But remember, charges can also move in a wire and if you're you're a wire you're essentially just trapping charges inside of it. So, you can have a moving charge or you can have any 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 i to the right, okay? And it's going to be similar that this wire is also going to create the magnetic field, so this is a quick example, right here at a point p there will be a magnetic field here, okay? So, this point here will have a magnetic field, let's call it, this is point P, point P, it will have a magnetic field a B field B, p, okay? Because the certain distance away from that table, 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 can do that as well, that's the third we're going to talk about, and you can find a magnetic field right through the middle right here, okay? Using that equation, and finally you can make a lot of loops, you can get a wire and do 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 leaps and you guys are called solenoids, something like this, and essentially you're making something where this is really, really long L, so that it's bigger than, much bigger than just the radius of the circle, and that's sort of gets different equation, okay? So, these are the four situations, we're going to talk about in this chapter, 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 see as we go through these that it's just the same crap over and over, cool? That's it for this one, let's keep going.