 So, let's first of all talk about what kinds of written questions I can ask you or multiple choice with point charges. And so the first thing we're going to look at is what can I ask you with two point charges. And I'm going to do a diagram. I'm going to say we have point charge number one, point charge number two. And I'll say that this is negative 15 micro coulombs. And this is positive 10 micro coulombs. And I'm going to say that this distance here, ooh, hey Mr. Dewitt, use some of your fancy tools. Oh yeah, I do, I like that. This distance here is 1.2 meters. This is where we started out. And so the first question that I could ask you is force. Some of these I'm not going to finish because I think some of these will reach the plug and chug stage. I'll just say plug and chug. So find the force. And I could say to you, well the force between point charges is K, Q1, Q2 over R squared. What direction would the 10 micro coulombs feel its force? Left or right? Victoria. Left, you're right. How come? Opposite to track. Okay? This is going to get tugged this way. This is going to get tugged this way. If I made one of these a fixed charge, then it would be this guy that would be moving. Or if I made this guy the fixed charge a little bit more angelic in place, then it would be this guy that was moving to the right. Okay? This is getting tugged to the right, this is getting tugged to the left. What else can I ask you about point charges? I can say find the potential energy between these two charges. The potential energy is going to be K, Q1, Q2 over R. By the way, you probably want a formula sheet handy. It really helps you to kind of spot things as we're going through. You can try and anticipate where I'm going on. Now here I would go 9 times 10 to the ninth. Here I would include the negative. You'll get a negative amount of energy. What's that saying? First of all, it depends which of these is your fixed charge. Let's suppose this was our fixed charge in place. I'm going to write the word here fixed in place. Which way does this guy want to fall to the right? If I want to move him out to infinity to zero, I'm going to have to plus add energy, which means he better start out with a negative amount of energy, which is why the answer to this is negative. Here I would not put the negative sign in. We decide the direction by like charges repel, unlike charges attract. What else can I ask you with a diagram like this lasso based? I can say if I give you a location, oh how about halfway so that this is 0.6 meters and that this is 0.6 meters and we'll call this location x. I like this question. I like this question. I like this question. At a location, I can say, see, find the net electric field. Find the net electric field, so I know what to do. How? What's the equation for electric field from point charges? Kq, only one charge at a time. What we're going to do is we're going to temporarily pretend this charge doesn't exist. I'm going to find the electric field from the left-hand charge. I'll call that EL for left, which is Kq over R squared. It's going to be 9 times 10 to the ninth, 15 times 10 to the negative 6. Why not negative? Electric field is a vector. We're going to decide the direction using our electric field and direction method. All over 0.6 squared, I get 9 times 10 to the ninth times 15 micro coulombs divided by 0.6 squared. When I get an electric field of 375,000 units, Greg, how can you figure that out by looking at your formula sheet? There is an equation with f and q and e in it. Yes? It is Newton's per coulomb. Direction. How did we figure out the direction of an electric field? Remember, we have temporarily covered up this guy, so pretend he's not there. We're only looking at this guy. How did we figure out the direction of the electric field? Remember, if not, you need to for tomorrow. Yep. We ask, which way would a positive want to move right there if it could? Ignoring this guy. Which way would a positive want to move if it could? We do the same thing now with the right hand. The electric field of the right hand charge is going to be Kq over R squared. It's going to be 9 times 10 to the ninth, 10 times 10 to the negative 6. All divided by same radius because I said we're exactly halfway, but don't assume these are always the same. I could have done like 0.7 and 0.5 or I did halfway just for the sake of an easy diagram. And also it makes my calculator easier because all I need to do is change the 15 to a 10. All the rest of the numbers are the same. And I get 250,000 Newton's per coulomb. And pretend this guy is gone. Direction based on this, which way would a positive want to move if it could? How would I find the net electric field if it's 375,000 left and 250,000 left? They're in the same direction. Add them up. What if this was to the right? Bigger minus smaller. So I'm going to add them up. The total electric field, the net electric field is that plus 375,000, 6.25 times 10 to the fifth Newton's per coulomb in the winning direction was left. What else can I do with two point charges? We're too far, Mr. Dewey. Again I can give you a location. I did halfway last time. I'll go somewhere different. I'll go, how about right there? What's that look like? I think 0.9 and 0.3, is that okay? And I can say to you, oh, let's give you a location again. Find the total, spelling the word voltage correctly, Mr. Dewey. Find the total voltage at x. How do I find voltage from point charges? I'm looking at your sheet somewhere. Miguel. Okay. So I'm going to find the voltage from the left charge, which is going to be kq over r all from the left. It's going to be 9 times 10 to the ninth, 15 times 10 to the negative 6 divided by 0.9. And I've made a mistake. What's my mistake? Look at your formula sheet. Voltage. What? Voltage is a scalar and it's in the bottom row there. You put the signs in. Negative. It's going to be a negative voltage from the left-hand charge. Negative 15 and it's 0.9 and delete the square, is that right? Negative 15 times 10 to the negative 6 divided by 0.9. The voltage from the left-hand charge is negative 150,000 volts. What about the right-hand charge? Okay, that's going to be 9 times 10 to the ninth. Positive 10 times 10 to the negative 6 all over and its distance is 0.3. It's going to be, get rid of the negative, make that a 10. Make that a 300,000 volts positive. So from this charge, there's positive 300,000 volts right there. From this charge, there's negative 150,000 volts right there. What is the total voltage right there? Add them up. Equals negative 150,000 plus 300,000 total voltage positive 150,000 volts direction. Miguel, no direction, it's a scaler, it's a scaler, it's a scaler. Now that I know the voltage there, what if I wanted to move a charge out from infinity all the way over to here? How much work would it take? I could go Q times the voltage, whatever the charge was. What else can I do from two point charges? I think that's it. What can I do from a single point charge all by itself? So go 12 micro coulombs. And one of the things that I could do is put a distance in the diagram, give you a location. Once again, I'll call it X. And if I tell you that this is 0.8 meters, I could say, I don't know, what letter am I going to write? Find the electric field at X. How do I find the electric field from a point charge? Sorry? KQ1Q2R squared, I'm pretty sure that's an F for force. I want electric field. Yeah, just KQ over R squared. The symbol for electric field is the E, KQ over R squared. It would be 9 times 10 to the ninth, 12 times 10 to the negative 6 divided by 0.8 squared. What would the direction of the electric, well, let's crunch the numbers since you guys are kind of nearly curious, 9 times 10 to the ninth times 12 times 10 to the negative 6 divided by 0.8 squared, 1.69 times 10 to the, 1.69 times 10 to the fifth. Nutrients for Coulomb. Direction. What's the direction of the electric field right there? Right, why? Which way would a positive right there want to move if it could? It would get repelled by this guy to the right. We can all see with the same diagram, say, find the voltage. Is that okay, Alexis? Sorry, my grade 12s have a test tomorrow, so they're all panicking. I know you guys have a test too, but go figure. Find the voltage. For me? Ah, the clever shortcut. Yeah, because it turns out the equation is KQ over R. So Ian just said, you know, if I multiply by R, that would cancel one. I would even go shorter than that. The second function, enter, and just delete the squared, because it's going to be the same equation. 9 times 10 to the ninth, see it? The only issue would be is if this had been a negative charge, you would put a negative in here now, because you do get a negative voltage, and that's okay. But 135,000 volts. No direction, because it's a scalar. The other question that we liked to ask involved one fixed charge and a moving charge and work. One fixed, one moving. Too far. Not far enough. Too far again. Oh, I see you're just taking me there no matter what. Okay. So, here is our fixed, and we're going to make it negative 12 micro coulombs. And we're going to start out right there. Excuse me. And we're going to have a little tiny charge Q equals negative 2 micro coulombs. Which way is this charge going to move? Left or right? I heard two answers. Convince me. Which is, I heard someone say left or someone say right. Which one? Right. Convince me. Like charge is repelled. It's going to get forced to the right. So, here's what we're going to say, and it's going to speed up. What we're going to say is after it travels 3.2 meters and gets to there, how fast will we travel? So, we can use energies here. Oh, we need the original distance too. You're right, my friends. I'm sorry. How about 3.0 meters? Good point. What happens when I make these ones up? That's why I was kind of hoping you guys would go with option A. Now, if they said how much work to move from infinity, let's say, right to here, I would say, oh, work is change in potential plus change in kinetic. This is not a work question, and we do have a change in speed as well as a change in height. This we're going to solve with energies. Kinetic energy initial plus potential energy initial equals kinetic energy final plus potential energy final. Initial kinetic is zero. Final kinetic, no. Initial potential, no. Final potential, no. Although I would move this guy by itself to get the kinetic. Well, let's actually write out the equation first, Mr. Dewick, so that we don't panic our brains. I would go, oh, what is potential energy between two charges? I would go k q1 q2 over r initial equals a half mv final squared plus k q1 q2 all over r final. I'm going to change this question just a tiny bit. This is what happens when I make these up. Can you all look up for a second? Instead of negative two micro coulombs, instead of negative two micro coulombs, I'm going to make it negative two electrons, two electrons somehow glued together with crazy glue. There is two protons. I think it's called an alpha particle, but I don't know if there's two electrons. Or in our magic make believe physics world anyways. I would move this guy over to here. So I'm going to have k q1 q2 all over r initial minus k q1 q2 all over r final. That's going to be a half mv final squared. Nine times ten to the ninth, q1 was negative 12. q2 is negative two electrons. Remember an electron is negative 1.6 times 10 to negative 19. Sorry, two electrons. I put the negative there divided by r initial, which was 0.3. Is that right? Oh, just three. I'm making these up. I have no idea what these numbers are going to end up being. With my luck will end up faster than the speed of light. Minus nine times 10 to the ninth. Oh heck, I'm just going to go like this. Minus, because I'm going to rush. The same thing, but divided by three point, what did I say? Two? Three point two? Yeah. That equals a half mv final squared. Sorry, six point two. Thank you, thank you, thank you. I don't have a diagram in front of me, but you would add the distances together. Would you not? The total distance from the center would be six point two. Yes? In my hideously drawn diagram. It's going to be nine times 10 to the ninth times negative 12. 10 to negative six times negative two times 1.6 times 10 to negative 19. Divided by, so the first one is a three. I get 1.152 times 10 to negative 14. 1.152 times 10 to negative 14. Minus, the same thing, but we're going to divide by a six point two. I get 5.574 times 10 to negative 15. 5.574 times 10 to negative 15. This equals a half mv final squared. Let's actually get an answer on the left. So 1.152 times 10 to negative 14 minus that answer. 5.946 times 10 to negative 15. 5.946, is that what it was? Times 10 to negative 15. That equals a half mv final squared. Before I get the b-files by itself, take this number and do what? Times by two and divide by the mass of two electrons, because I did say two electrons. Okay. So times by two, divide by the mass of two electrons. Two times 9.11 times 10 to negative 31 equals, that's vf squared. How do I get rid of this squared? And happy I am that this is going to be less than the speed of light. The final ends up being 8.08, 1, 2, 3, 4, 5, 6, 7. 8.08 times 10 to the 7th meters per second. Yeah, yeah, so far. Okay. I can also make this a how much work question if I start out from infinity with a negative charge, because if I move it this way, it doesn't want to go this way. I'm going to have to be doing work. Or instead of starting out at rest and saying how fast, what if I give you an initial incoming velocity and say when will, how far till it stops? When it stops, what will its final kinetic energy be? So there you'd start out with the same thing, except the initial would not be zero. The final would be zero. And if I asked you where it stopped, you'd be solving for our final. I'd still get everything to the other side and then cross multiply, get an answer and then cross multiply. What else have I seen variations on this diagram here? That's it for now. Yeah, fixed moving like that. One fixed one moving. You're either heading towards it, slowing down, or you're heading away from it, speeding up. Or if you're heading towards it and slowing down and it's an outside force doing the work, or you're losing energy and eventually it'll come to a stop. Is that okay so far? Oh, oh, oh, oh, oh, oh. All right, then we look to parallel plate force problems. And here I think I'm going to start to look at the review. And I'm going to start to give you some, I like these questions, I like these questions kind of hints. So I'm going to say, I sort of like 4A in that on your test somewhere, I'm going to give you electrons that are getting accelerated through a plate and say find the speed. So when it says that the electron beam is produced by accelerating electrons through an electric potential difference, fancy word for voltage, of 380 volts, what's their speed? That was where we had the equation with the two V's in it. Remember? In fact, we ended up with QV equals a half MV squared, except the V's were different from each other. So I like them before. Okay, so definitely something like number seven. Here to find the electric potential. What's another word for electric potential, by the way? Yeah, what don't they want you to find? Energy, not energy, voltage. But here location P is outside of both charges, but it'd be the same thing as the one right that I did where the location X was between the two charges. You're going to find the voltage from the first charge. Include the signs. Find the voltage from the second charge. Include the signs. Add them up. Oh, and you'd have to, the distance from the first charge, the one on the far left, all the way over to location P, looks like it's 0.35. I'd have to add the two small distances together. So I like number seven. In fact, to me, a great question would be to give you something like number seven and say, find, A, find the electric field, B, find the potential. So A, electric field would be cover up one charge, find the electric field, magnitude and direction from the other charge, and then repeat the procedure for the second charge. Add them up vectorially. Oh, here's your two charges, find the electric field. Oh, and this one's even halfway, just like the one that we did. So I like number 12. I want to talk about number 15 because there's some good nerdy stuff here. So we have this proton that's sailing along, and it hits a bunch of plates, and each plate is charged to a different voltage. This far voltage here, positive or negative? Positive. What about this one? But not as much. What about this one? But not as much. You know what? They're going to repel this proton. This proton, when it hits here, is going to start to slow down, slow down, slow down, slow down. For a split second, it's going to come to a stop, and it's going to come whizzing, shooting back the way it came, because light charges repel. And this question is saying the proton will be stopped momentarily in which region. Although it looks really nasty, it's actually not too bad. I start out by saying kinetic energy initial plus potential energy initial equals kinetic energy final plus potential energy final. Are any of these zero? And I'm going to start counting actually from initial right there. And to me, Miguel, this is like launching something straight up off the ground and having it fall back down. Do you know how much potential energy I have on the ground? I'm going to argue that that's zero. I'm going to say a half mv initial squared equals, and I'm going to use the equation that has the v in it. In fact, this is just like the earlier question I mentioned about the electron gun. What do they want me to find? Well, they want me to find which region. I think I'm going to figure out what voltage would I need to cancel out this proton with that much kinetic energy. Oh, and this is actually really nice. I don't need to go a half mv squared. They told me how much kinetic energy I have. I can actually go, oh, 2.1 times 10 to the negative 17. What's q proton? What is the charge on a proton? Well, this is nice. The voltage required to get it to stop and then start turning around and come back in the direction would be 2.1 times 10 to the negative 17 divided by 1.6 times 10 to the negative 16. That's what I said, 19. You can't read my own writing. Okay. Now I'm kind of curious. 2.1 times 10 to the negative 17. That's right. Divided by, so you've got me doing that, Greg. I get it's going to turn around and run out of steam at 131 volts. Which area is that region now? So a nice kind of outside of the box question. Sorry what? Yeah. Nope. So here's one just like we did. A proton initially at rest will have what speed at point y? So kinetic energy initial plus potential energy initial equals kinetic energy final plus potential energy final. Your initial kinetic is zero. Your final kinetic will have the v that you're trying to find. Number 17, if the electric field is this way to the right and this is an electron, which way would an electron want to move in an electric field that was pointing to the right? Left. West. Right? Because the electric field tells me which way a positive wants to move. We assume the electron wants to move in the opposite direction. West. Number 18, what's the electric potential? What's electric potential? Another word for voltage right there. This is the beauty of voltage. Earlier we did a voltage question between two point charges where I said find the total voltage. And I made them a nice straight line. The nice thing Leslie is because voltage is a scalar, it does not matter. That's the same diagram mathematically as the one where I had them in a nice straight line. You would go KQ1 over R plus KQ2 over R. Done. Get an answer. Here's another energy transformation question. So they told you how much kinetic energy, how much potential do I have just before I get to the plates? Nothing. Well actually I would argue that I could go Q change in voltage. That's when I get through. My final is zero. My initial is going to be QV. And I know it's from zero to negative. It's because we're moving a proton. I would just say I'm going to call it positive because I'm going to get a positive square root in my square root answer anyways. Another question we did with point charges was we asked if we had two of them. Not just how fast, but what if it didn't want to go in that direction? So this negative charge right there. Which way does this electron want to move? Left or right? It wants to move to the left. So we're going to do work. We're going to force it to go to the right. How will I do that? What's work equal to? It's not equal to change in potential. I never taught you that. I got to be very fussy. That's the law of conservation of energy. What is the work energy theorem? What is it that says work is equal to what? It's not change in potential. That's what Ian said. I've never taught you that. Oh boy. It's change in potential plus change in kinetic. Change in kinetic. Now in this case, we can assume starting and ending at rest, but don't you dare write that right away unless you've read the question and walked through that assumption. Work is change in potential plus change in kinetic. What if I had said that this particular electron was already moving? Then it would have some kinetic energy. In fact, a nice question would be to have this fall towards here, come to a stop and then turn around and say, oh, how far will it get until you have to turn around? Now that I know that, work is potential energy plus kinetic energy. Potential energy is final minus initial. Change in kinetic or kinetic energy is going to be zero, I think. Starting at rest, ending at rest. B, what's the potential difference? That's the second thing I did with point charges. I think find the voltage. Find the voltage from the first. Find the voltage from the second. Add. Or as a shortcut for two marks, if you know how much work, work is QV. So that means the voltage must be work divided by Q. Which Q? I think the one that's moving. You're going to get something like 21 as a multiple choice. I'm going to give you an electric field diagram of some type. Remember we said that the lines always point from what to what? The arrows always point from what to what? From positive to negative, that is correct, sir. We said that the closer the lines are together, the stronger electric field that location, and we said the further apart that we could be on. Boy, you got me yawning here. Suddenly I'm hitting the wall. We have to stand up, we'll see. See that proton? Which way does it want to fall? To the right, because it's getting repelled by the 200 and attracted by the negative 500. So there's a net change there of 700 volts. If they want me to find the maximum speed, start out kinetic energy initial, potential energy initial, could I get an energy final, potential energy final? That works with plates too. What is the magnitude of electric field and electric potential at a point P, midway between the two vex charges? So they like to ask the question that we saw a few minutes ago as a multiple choice as well. Let's do 24. No, nothing. I needed that one. A charge of 30 micro coulombs is moved from point X to point Y. How much work is done? The beauty is, because work is a scalar, I would start out by saying work is equal to what? Don't you dare say change in potential? And I should say potential energy, because change in potential really doesn't mean voltage. Yeah, it's change in potential energy, plus change in kinetic energy. Looking at this diagram, I'm pretty sure I can do that. Looks like there's V0 initial and V0 final for both of them. What's changing anything? Or I could go QV. Oh, do I know the voltage? Do I have two charges? I think I'm going to go final minus initial. I think I'm going to go KQ1, Q2 over R final minus KQ1, Q2 over R initial. Do you want me to put signs in here? Yes? Oh, wait a minute. Positive, positive. Mr. Dewick, why didn't you go all fancy with vectors and stuff? I don't care about the angle direction, because voltage and energy are scalars. All I need to know is, where are they? Where are they going to finish? Who cares what happens in between straight lines? So it would end up being KQ1, what was Q1, Mr. Dewick? Too far. 70 K, 70 times 10 to the negative 6, 30 times 10 to the negative 6, divided by R final, which I think was 8, minus 9 times 10 to the 9, 70 to the R initial, which was 3. And you're done. What makes that question fall apart is energy is a scalar, so I don't need to worry about all funky angles or anything like that. By the way, your attention was over quite a while ago. So now you're just learning? Oh, it feels good to give up your free time to improve your academic achievement. Yes? Nope, I want, which one, this one, Mr. Dewick? A little bit. 28, what's another word for electric potential? So how am I going to do that? Oh, KQ1 over R. And KQ, so KQ1 over 3, that's a part of it, and KQ1 over 5. BS, what's the potential difference? Which mathematical operation does difference generally subtract? Which mathematical operation does distance, does difference? Good gosh, Mr. Dewick. Generally refer to subtraction. Poor. She sells seashells by the seashore. The six sheets, she's sick. Can't even say that one. The six sheets, she's sick. She sells seashells by the seashore. Boy, having a tough time getting stuff out today. How much wood do you want? Which I could check with. Okay, I can do that one at least. I can do that one. Boy, apparently I can't say the word difference. So here, the potential difference, and I would go bigger minus smaller. See how much work, oh it says that much work is done. If it doesn't want to work, is it positive or negative, Charles? In C right there. Positive, because it's requiring you to do work to move there, that means it doesn't want to go that way. If it doesn't want to go that way, can you tell me, is it negative or positive, the mystery charge? Yes, think. Positive, yeah, because that's negative. So I already know the polarity, and I can crunch the numbers and find Q. That's on my solution key. 30 is a good question. So how much work must be done to move charge Q1 two meters closer? So here's where you would use work equals change in potential plus change in kinetic. Change in kinetic is zero. Change of potential, final minus initial. You need to know how to find the electric field between parallel plates. How do I find electric field between plates? Voltage divided by distance. Now Ian, especially on multiple choice, if they ever ask you this question, they'll almost always also give you this distance, because they need one more distance in the question to make up better wrong answers. Just be aware that they'll always give you how long the plates are, but it's how far apart the plates are that creates the electric field. There used to be. Greg asked, is there anything that ever needs the length of the plates? Remember the cathode ray tube? We used to actually figure out how much it got deflected by, so you need to know how fast it was traveling when it got there, how long it was experiencing the force for what its vertical acceleration was, and then you could draw a vector and figure out how much it got deflected by. And it was nearly cool, but that got pulled away. 1995, electric field lines. I'm going to give you a diagram, something like this, right? It'll be a picture and I'll ask you to figure out which ones are positive, which ones are negative, and maybe which ones bigger, which ones smaller. How can you figure out which ones bigger, which ones smaller? For example, how do you know both of these two charges are identical? Not distance between the lines, that's the electric field strength. It's the number of lines, right? So this first one has one, two, three, four, five, six, seven, eight. It has nine or eight. If this one had 16, it's twice as big a charge. If it had four, it'd be half as big. Guys, okay, again, with magnitude and direction of electric field for two charges. Okay. So I like that question, 35. 35B is kind of a nice touch. Let's go find that, Mr. Duke. So when I crunched the numbers, I got an electric field from Q1 of 2000 to the right, because it's positive, so it would want to push a positive charge to the right. When I crunched this one here, I got an electric field of 4,500 Newtons per Coulomb to the left, because a positive would want to move towards the negative. So my net electric field was to the left. That means which way would a positive want to move to the left? Which way would a negative want to move? So if in part B, they tell me an experience is a force to the right, I already know it's negative. So let's find the polarity. I said, well, electric field is forced on a charge, forced divided by the charge. So the charge is going to be the force divided by the electric field. 36, some other electric field diagram. Are you cluing in? Number 37, are you cluing in fairly often? They're asking you to find a net electric field or a net electric force. Now, this is the net electric force. For that, I need three charges. It wants me to find the net electric force on this guy. So I'll go F equals KQ1, Q2 over R squared. F equals KQ1, Q2 over R squared. Oh, direction, positive to the right. Direction, oh, positive to the left, repelling and repelling. So right minus, I'd go bigger, minus smaller. Cathode ray, a little bit of that. So we have an electron, which is negative. It's heading towards some plates. So this one does have some initial kinetic energy. It wants its impact speed. So it wants its v final. And the potential energy initial is going to, well, the change in potential energy is going to be that right there. Final potential energy is zero because that's the ground. Initial potential energy is going to be QvQ times 250. As a twist in number 43, still two charges, very similar diagram, location in between. Instead of giving you both charges and saying, find the electric potential, the voltage, it's telling you the voltage and saying, find the mystery charge. Okay, how did I do that one? I said, well, nope, too far. I said the total voltage is the voltage from charge one and the voltage from charge two. They told me the total voltage was 1.9 times 10 to the fifth. And I said, mystery charge one, I don't know. I'll minus this over to this side. So I went 1.9 times 10 to the fifth minus 9 times 10 to the ninth, 2.5 times 10 to the negative six divided by its distance equals, and I even stuck those numbers in. I got 115,000, nice round number, and I said, oh, times by 0.6, divide by 9 times 10 to the ninth, and I'll get a lovely 7.67 times 10 to the negative six. That was the magnitude. How did I figure out the direction? I said, well, I'm doing voltage. I better make sure I put signs in and if I put signs in, the direction should take care of itself. What you did? Sorry, the polarity, the direction, the polarity should take care of itself, the sign. You gonna make it, Kelvin? Because I'm not. This one here shows up every once in a while on the provincial. I don't think I have one like this on your written. I might have one on the multiple choice. They want the amount of work. Easy way to do this is going to be find the total potential energy right there, find the total right there. You don't need to worry about the angles or what, because it's a scalar, final minus initial. That's how much work. Oh, now we're on scholarship questions. Good gosh. Actually, 50 is fairly reasonable, because we did one like this a little bit for the Mickelson's oil droplet experiment. So similar to Miguel with his hanging pencil right there, gravity down, thread hanging it, holding it up. Here we have gravity down, electric force pushing it up. So that electric force is QE, right? Between plates, MG. And you're able to get that, oh, change in voltage over just, I don't know the electric field. And you can get the mass of the mystery charge. Oh, and now that you know the mass, B was a bit trickier, but I would say A is not scholarship. A is fair game. B was a bit yuckier, but nearly cool. 51 is no longer considered scholarship. Having said that, I didn't do one like this on your test, I don't think on the written, because the graphics were too tricky to draw. So I've rambled for a bit here, trying to salvage this, because I really have a tough time making these questions up on the fly. Give me a moment here. I'm going to pause the recording. So I just took a quick browse of the test with the video camera and the recorder turned off. One of the things I noticed in the multiple choice is lots of going backwards questions. For example, instead of me giving you two charges saying find the force, I'll give you the force and one of the charges. Please find the other one. In other words, for any of the equations, Calvin, I feel comfortable giving you the answer and find one of the missing terms as well, going backwards instead of giving you all the stuff on the right-hand side and say find the left thing. I have no problem mixing and matching. So I have no problem saying, oh, here's how much work you did. How big was one charge? Oh, okay, I'll find the Q instead. Or what could I do? Oh, here's the electric field. And here's how big the charge is. What's the radius? How far away are we from the big charge if the electric field is this? Okay, so there's about, as I glance through this, three or four of those as well. Mix and match, okay? I think you'll find the written very straightforward. You want to know what's on the written? If you look at both quizzes, I don't believe there's going to be any real surprise, aside from the using principles of physics right to explain question, but it's a pretty easy one in that if you want to, you can simply crunch the numbers. Or you can do it nerdily and algebraically. Oh, very cool. But if you want to, you can crunch the numbers and figure out what the correct answer is, okay? Multiple choice. In fact, I think you'll find the written easier than the multiple choice. I believe there's three written, times seven, 21 marks. Oh, but there's a using principle of physics right to explain. It might be 23 marks or something like that. Testes out of 48, so about half written, half multiple choice. You'll probably find the multiple choice tougher than the written, but of those multiple choice, and I think there's like 12 questions, four plug and chug, four or five are of the variety that I just mentioned to you. Oh, he gave me the force and he wants me to find the radius, sir. Oh, he gave me the voltage and he wants me to find the size of the charge instead of here's the charge, find the voltage. One more question on there, on the multiple choice. I think I gave you a big hint in lesson one about it in the notes. So I won't give much more of a hint than that. I think you'll find it okay. And if not, you can always make up reasonable numbers and crunch the answer and see what happens. Not really any surprises on the test. So it's four o'clock. Stick around for five minutes. I'm gonna in a couple of days be giving out a bunch of provincial exams. They're sitting right here. I was here for two hours on Sunday, but what I'll do right now really quickly is just look at the electrostatics written questions, what kind of stuff they can ask. So, desktop. Physics 12 right here. And we're gonna go really quickly just so you can kind of see, oh yeah, I can handle some of these old provincial exams. And one of them was August 2003. Fit with, okay. What kind of electrostatics questions can they ask you? The screen is still frozen. Thank you. What kind of, still frozen. Unfreeze. Oh, battery almost died. What kind of electrostatics questions can they ask you? So I've done like that on number 20. I think we did an electron shooting up between parallel plates. But number 20, same idea. You're gonna use kinetic energy initial, potential energy initial, kinetic energy final, potential energy final. Initially at rest, kinetic initial is zero. You're hitting the ground on the far end, so potential final is zero. Now I want to ask you cathode ray little questions. And, oh, now we're already on circuitry. So they must have had a written question on electrostatics here. Period. Oh, here we go. Two protons are held initially at rest that far apart. If one of the protons is released, what is its speed when we get to there? Kinetic initial, potential initial equals kinetic final plus potential final. Then I have January 04. Number 20. What's the answer? Read it. What does an electric field line indicate? What do you think the best answer there is? Miguel. Hey, because how did we figure out the direction of the electric field? What did we ask? Which way would a positive want to move it could? That's really our way of saying what way is the force on a positive charge, right? Number 21, there's the two charges. Find the net electric field. There's a million times. Here's an interesting one. Number 22 says, here's a positive charge Q located several meters from a fixed charge. Also positive. You're told to move charge Q a distance of one meter so as to cause the greatest increase in its electric potential energy. What way would cause it to increase its energy the most? Let's see. Direction one, two, three, or four. Direction four, because if you move it closer it's almost like winding up a spring. It really wants to go away. You would have to do work, which means you would have increased its energy because the work that you do would go into it. Now we're in circuits. The potential difference in moving from position A to position B in the diagram below is equal to 400 volts. So I guess when you go from A to B, you gain its 400 volts, determine the size and the polarity of the charge Q. Okay. The change in voltage starting at A going to B, I think you're going final minus initial. You can find the mystery charge. I would use KQ1 Q2 over R minus KQ1 Q2 over R. You'd have Q appearing twice, but you could do that. I've never seen that one in a while. I'm not going to ask you one quite like that, but that's a mini curveball, but I can handle it. Let's just look at the written. Oh, too far. Determine the electric potential and then sometimes they'll add the phrase relative to zero at infinity. That's saying use the voltage KQ1 Q2. At a point P, midway between the two charges is shown below. Okay. Cover up this one in your mind. KQ1 over R. Include the positive. Cover up this one in your mind. KQ1 over R. Include the negative. Add them up. Are you noticing a fairly repetitive after a while? How much work would it charge? Would it take to move a negative 15 micro coulomb charge from a point P to position infinitely far away? Work is changing. Oh, you know what? Work is QV because potential energy is Q changing voltage. And since I just figured out the total voltage, I bet you that'd be a shorter way to do it. Oh, and since it's asking how much work to move a negative, I'm willing to bet you got a negative voltage because it, sorry, I'm willing to bet you got a positive voltage because it doesn't want to move far away from the negative. I'd kind of be willing to bet that for part A I get a positive answer, just a gas, but probably. And one more. That big styrofoam ball carrying 50 micro coulombs of charges released from rest in position A as shown below. So we have a fixed charge. It's released from rest. Which way is it going to want to move? Yeah. So first thing it says, find the change in electric potential energy. Oh, and they were even nice enough to remind me it's not voltage. How nice of them. What's changing anything? Final minus initial. And I'll use the point charge electric potential energy with Q1, Q2 over R in it. Final minus initial. And then there's going to be a B. How much you want to bet B is the speed? Yeah. If Vi equals zero, what's the speed? So the questions get pretty repetitive. I hope that makes you feel a little bit better. I'm going to stop the video and press pause here. Or were there any questions that someone wanted me to go over? Get up your formula sheet. It really to me does also help look at a formula sheet. I'm really serious. I actually group it visually as well as out loud. Look at the electrostatic equations. Top row are all vectors. So no signs. What's the next row? I don't think the second row is voltage. Read them to me, please. That one there, although voltage is a scalar and I should technically put the sign in, I frequently don't because we were often square rooting and didn't want the negative. And a lot of it has to do with if you're a negative charge moving in a negative direction against a negative voltage, there's a bunch of negatives in there. That one there, I really don't often put the sign in. Keep going. What's the next one? Okay. And there's another one on that row. So that row there is the two definitions. The definition of electric field, nuisance per coulomb, and the definition of voltage, joules per coulomb. And since those are definitions, I don't put signs in those. Next. Okay. Signs in that one scalar. Signs in that one scalar. So if you're looking top row, no signs. Bottom, I can't say it's not bottom row, but last two signs. I haven't found a good acronym for that as well, but that's how I use it. The most flexible ones, you didn't read the electric field for between parallel plates, though, to me, did you, or did you? The voltage over distance? That's the middle row. So the most flexible ones are the E equals f over q, because that is actually where the electric field equation comes from. And energy equals sorry, voltage equals energy over q. Those ones really apply everywhere because those ones, they don't make the other equations into to generate a new wave. Any others? Good question. I tried to keep them pretty obvious. I thought of, it looks like in the mid 90s, they like to ask lots of polarity questions. They seem to have fallen away from that now. I think they've said, look, the language is archaic, let's just make sure the kids understand how to do this, but interpret the archaic language. So I've done the same thing. Most of my questions come from old provincial exams from around 2000 to 2005 with the same diagrams, clips, and different numbers. That mean if I look through the exams, oh please, I give you lots of review, you don't need to try and cheat to find my questions ahead of time. Although probably you could. I guess on that quizmedc.ca most of them would probably be on there somewhere if you tried every single question. Although I think by then it would be worth doing the review and studying, wouldn't it? Whatever. Any other questions? Okay, let me first of all hit stop.