 Review, kind of putting fine touches on everything. First of all, when you're solving equations, when you're solving problems, double check to make sure that you convert charges correctly into coulombs. So the most common one that we used was one micro-coulomb. What was one micro-coulomb the same as? I'll give you a hint, ten. It's ten to the negative six. Millie was ten to the negative three, but on your test it'll probably, micro was the one we used the most. And then we had the electron charge, often abbreviated as negative E. It was negative, what was the charge on an electron? 1.6 times ten to the negative nineteen. And the charge on a proton was positive 1.6 times ten to the negative nineteen coulombs and coulombs. And once again, you may find this lesson helpful to have a formula sheet in front of you because what we're going to be doing now is kind of dissecting what to use what. So example two says, solve for the correct quantity. Justin, it's amazing how often I say, find the electric field and they find me the force. Or I say, find the electric field, it begins with an E and they find the E on their sheet and they find me the energy. That's also got to, I know it's got an E with a little P for potential energy. So let's see if we can keep some strip, stop straight. It says, fill in the variables and the units in the column below. So what variable do we use for force? What have we used all of our physics career? F and what are the units? Newtons. And it's a vector. What variable do we use for electric field, for field E? What were the units for electric field? You can figure it out by looking at your formula sheet. There's two of them, I believe. Newtons per Coulomb and volts per meter. Those are both hidden in there in those two equations with a capital E in it. Potential energy, what's the formula? Now I use PE on the formula sheet. Unfortunately, they use EP. What are the units for energy? Now I'm going to write Joules, but I think we also noticed, did we not? Volt's Coulombs, did that work? Was potential energy QV? I'm just going to write that to remind us that that was an equation that we use very often. I don't think I've ever seen anyone write Volt's Coulombs, but if you ever did see, as Joules it is. Voltage, what was another word for voltage, unfortunately? Potential. Symbol, a V, but I always added that because I couldn't tell whether that was a lower case or an uppercase V if I didn't do that, if the V was by itself. Units, volts, and then the very last one is work. What letter have we used for work traditionally? By the way, part of the confusion here is the fact that we use the wrong variable for energy. Most of you, if you take university physics, you'll find they use the letter U, capital letter U for energy, because E is taken up by other stuff. But if I threw that at you in physics 11, it would have terrified you. What's the U there for? Then, example three says, use correct point charge parallel plates formulas, okay. Wait a minute, how do we find the direction? How do we find the direction for forces between point charges? Did we put in negatives or positives? I say no, no. How do we figure out the direction? So if this is positive and this is negative, what direction does this guy want to move? Okay, how do we figure out the direction for forces? Okay, light charges, unlike charges, let's write that down. Did I hit record? Yeah, I did, okay. Like charges, repel, unlike. What was the force equation for point charges? So find the equation that has an F in it and has more than one charge in it. Which equation do we use for point charges? Okay, so we used F equals K, Q1, Q2 over R squared. What force equation did we use for parallel plates? I'll give you a hint. Find the other equation with an F in it, but you're going to have to get the F by itself. We used force equals QE, although often we didn't know the electric field. What was the electric field between parallel plates? What's the electric field between parallel plates? Voltage over distance, see it there? I'm going to write that on the next line. And then what did we do when we had more than one charge? We went two at a time. If they were in a nice straight line, we just asked ourselves left or right and we would go bigger minus smaller. If they were at angles, we would add them together, tip to tail. I will not give you one like this is a written, but this is totally fair game as a multiple choice asking you to do a little bit of tip to tail adding the forces together. How did we find the direction for electric field because it was also a vector? What was the direction for an electric field? We asked the question, what was the question? Which way I'm going to write from positive to negative, which was true, but it's which way would a positive tiny test charge want to move if it could, which is from positive to negative? What was the equation for electric field from a point charge? Electric field was KQ over R squared. In fact, if you put this right there, don't you get, think about it, KQ1Q2 over R squared, that's where the force equation came from. In fact, it's this front part here is the electric field. This is sort of the MG of the electric field world. Ooh, what was the electric field between plate charges? We said electric field was equal to the voltage divided by how far apart the plates are. What if they gave us several different points? You went one at a time. You covered up every other charge. You found the electric field at that location from one point, magnitude and direction. Then you covered up every other charge except another one found the electric field from that point, magnitude and direction, and then you added them vectorially. I'm going to put a little star next to this one because I like it. I like it. I like it. I like it. I'm not sure if I like, well, I'm not going to give you one on your written section that is at 90-degree angles. I guarantee I'm going to give you one on your written section though with more than one point, but they'll be in a nice straight line with each other. That would be a great multiple choice. Now, did potential energy have a direction? Energy, scalar or vector? Scalar. So, did it have a direction? No, but what we did say for this is we said include the plus and the minuses in the equation. We don't include the positive and negative charges in the vector ones because we decide the direction independently either afterwards or ahead of time, to be quite honest. But for scalars, which is if you're looking at your formula sheet, the bottom third, bottom two thirds, kind of the bottom half sort of, not quite, but sort of, put pluses and minuses in. Potential energy, and I'm going to use PE because I hate using EP, potential energy between point charges was KQ1, Q2 over R. What was the potential energy between parallel plates? And this is the one we pulled out last day. It's on your sheet but in disguise, QV, technically Q change in voltage, but we got kind of sloppy. And the reason we liked energy so much, Dylan, is if they gave us a configuration like this, if they gave us a configuration like this, no trig, no vectors, their scalars just go two at a time and add them up, that was the beauty. Then we had potential or voltage. This also was a scalar, so we include the plus or minus, in fact, you know what, I'm going to add here, vector, scalar, scalar, because you know one of the questions they love to ask is which of the following are vectors or which of the following are scalars, right? What was the equation for voltage from a point charge, V equals KQ over R? What was the equation for voltage between parallel plates? I think electric field times the distance if you know that, but the reason you were so hesitant is usually they gave it to you like it was labeled on the diagram. They tell you how big the battery was. We didn't often bother finding the voltage between parallel plates. What we did notice was that the voltage was linear, so if you had 500 volts between the plates and you went halfway between them, how many volts had you traveled through if the whole distance was 500 and you've gone halfway, 250. We said it was a nice linear relationship that can come in handy. The gain for voltages, if they gave me two points and they said find the overall voltage right there, go one at a time, add them up. The last thing is work. Work is a scalar. Because it's a scalar, we're going to include plus or minus. For all of these, it's going to be work equals change in potential plus change in kinetic. That's why I wrote it underneath because it goes in both, or you can write it in both boxes really, really big, but it's both of those. Then Justin, if I'm dealing with point charges and they say how much work, this is the one that I would use for potential energy. If I'm dealing with parallel plates and one charge and they say how much work, this is the one that I use for potential energy. Of course, Dylan, my friend, what's changing anything? I don't mind it's initial. Read the question carefully. Did he start at rest? Did you end at rest or not? And away you go. So hopefully that kind of stabilizes, clears things up. Says to solve, oh, I should double check to make sure I covered everything they wanted me to at the top here. So correctly find vector direction for force and field. Don't assign a direction. Include plus or minus for positive potential energy and voltage, but not for force or field. Fill in column two below. We already have. Fill in the final row. We already have. Why can't we use work equals force times distance? Why must we use work equals change in energy for point charges? Because the force is changing. You can, between parallel plates, use work equals force times distance or change in energy. Either of those is fine because the force is constant between parallel plates because electric field is constant between parallel plates. So I almost always went to this because QE was pretty easy to type and that way I didn't have to learn a whole new formula. Then it says example six add vectors to find the total force and field. So in the far column on the previous page, on the first two we said add, you're going to add them tip to tail. Let's do one. Example seven, for this charge system below, first of all, A, it says find the force magnitude and direction on the positive charge. This is the positive charge. Once again, remember these little things are supposed to be microcoulombs. I'm probably going to have to break down and retype this whole thing. All of my notes over this summer, I don't really look forward to that at all because these diagrams take a long time to do, but it's probably going to have to happen eventually. So I'm going to freeze the screen, find the force, oh, and it wants the force on the positive so you can already tell me the direction. Which way is this positive going to feel a force left, right, up or down? Why left? And unlike charges of track. So find the force. I'm small. 1.5, negative 6, 4.2, negative, no, I think I'm right. Is that it? Yeah. I got 0.4375, oh, 0.438 Newton's left. B. Find the voltage at the location at the dot. Well, there are two voltages in this equation. This charge is creating a voltage and this charge is creating a voltage. I think you're going to have to go one charge at a time and the total voltage is going to be, add them up. Try that one. You calculate or die? Your calculator or my calculator? The battery's completely dead. I'm getting a net voltage, a total, come on, 45,000 volts. 45,000 volts. Direction, it's a scalar, no direction. C, the work required to place a charge at the dot. So what we're saying is if you started with this charge out at the dot and if you started with this charge out at infinity, how much work would you have to do to put it there, to bring it from infinity right to there? Okay. Well, work is change in potential plus change in kinetic. What's change in anything? Yeah, and you know what? I think my change in kinetic is 0 because I don't think we're starting with speed or ending with speed. And what's my initial energy if I'm starting out at infinity? I'm just going to say it's going to be potential energy final. What's my final potential energy? Well, here's the convenient part. What did I just figure out in part B? You know, if I know the voltage, the easiest way to find the energy is to multiply the charge that I'm moving times the voltage at that location. That's how much energy is at that location. And since I'm starting out with zero energy at infinity, that must mean how much energy I have to add. What was the charge? I've scrolled down. 4.75 times 10 to the negative 8. Sorry. Times 45,000 volts. Times 4.75 times 10 to the negative 8. And I get 2.1 times 10 to the negative 3 joules. 2.1 times 10 to the negative 3 joules. We can just leave it blank and we'll get the rest later. Turn the page. So here's our cathode ray tube. What we looked at a little bit last day. Over here is our electron gun. So this will be charged. This wall here will be very, very negatively charged. This wall here will be very, very positively charged. So the electron wants to move this way. Oh, we've cut a very, very small slit so that we end up having them come through that slit. We get a nice electron gun. Okay. It says, where will the electron dot end up? So right now with both of these plates, we have four plates with them all turned off. We're hitting dead center on our screen over here. And we're looking at this. Here's our viewpoint. So we're seeing it this way. Here's the person sitting in the chair. Okay. Where will the electron dot end up? Location A, B, C, D, or E. If we make the right plate positive, if we made that plate positive right there, where would the electron get deflected to? A, B, C, D, or E. What would this person see? So the electron's coming through here. So when it gets into here, this plate is positive. Is that going to pull it towards the plate or away from the plate? Towards the plate. So it's going to go into the page, which means based on this guy's viewpoint, he's going to see the electron beam get deflected to B. What if we make the right plate and the upper plate positive? So if we make the right plate positive and we also make the upper plate positive? E, yeah. It's going to pull it up and pull it to the right. So E. This is how you can deflect an electron wherever you want to. This is how you can make a TV screen. Okay. Now we're going to adjust the voltage so that the electron beam is hitting a point above the middle of the screen due to the upper plate already having some positive voltage. So we readjust things so that we have the beam hitting right there. Where will the electron dot end up if we increase the deflecting voltage? So if we jack this voltage up, where will the dot end up? A, B, C, D, E, F, or G? I think if we increase it instead of hitting right there, it's going to get deflected even more. It's going to hit right there. I think the answer is G. What if we reverse the polarity of the deflecting voltage? In other words, same voltage, but make the bottom positive, not the top? Where will it end up? Are you guys getting this okay? I'm only getting one response. I can't tell if I've lost you guys or not. See what we're doing? Justin, yeah, you know, yes. So C, right? Okay, Pat, Trish. Need some other feedback, man. Hey, what if we increase the accelerating voltage so that they're coming out of here even faster? What would happen when they go through here if they're going through even faster? I think they'd get deflected as much because they're not in the voltage for as long a time. Does that make sense? So if they come out even faster, which way would they get deflected? Where would they end up? Hey, does that make sense? Tyler, you were away. Make sure you watched the lessons. But this was the idea behind how a TV screen works. The old-style cathode ray tube TV screen. Not the plasma, not the flat screen, but the old one. So example nine. In what way does our simple model of a cathode ray tube explain how a black and white TV works? Only one dot or pixel is hit at a time. By sweeping the Y voltage from positive to negative, we can sweep the screen. We can step the beam over using a small X voltage and then sweep the beam up and down again so you can move things left and right up and down. The electrons hit the fluorescent paint on the inside of the glass and from our view, Dylan, we see the screen glow at that little dot and if it's fast enough, we see a moving picture. If we gradually add increments of horizontal voltage and the signal turns the cathode ray beam on and off as it sweeps the screen, beam on as a white dot, beam off as a black dot, beam partially on as gray and this manner of picture is created. What's your homework? Your homework is not this, although you can try a few. Your homework is the great big unit review and the take home quiz.