 Number one, first of all, it wants the quiet please. It wants the magnitude and the direction of the electric field. Okay, the electric field between parallel plates is equal to the change in voltage divided by how far apart the plates are. The change in voltage is not 500, it's 750. The distance between the plates is 0.055. And I get a magnitude of 750 divided by 0.055. 1.36 times 10 to the fourth. 1.36 times 10 to the fourth volts per meter or Newtons per Coulomb. That's the magnitude. What's the direction? If I'm a positive charge right there, which way do I want to move? Alright, down. One mark for the answer, one mark for the direction. B. Now B says, suppose we put an electron down there. Which way does the electron want to move? It wants to essentially fall to the ground, but the ground is up, hence the video that I showed you a few moments ago. Yes, I actually thought of that as a way to remind you, not everything is up as down, down as up. This is just like dropping an object and letting it hit the ground, except when you're dropping this electron, it's going to fall up. I'm going to solve this then using energies. I'm going to go kinetic energy initial and potential energy initial equals kinetic energy final and potential energy final. I think we're starting from rest. When you drop something, when you hit the ground, how much potential energy do you have left once you've hit the ground? Once the electron goes through the whole voltage, how much potential energy will it have left? Potential is going to be a final is going to be zero. In fact, we're going to get this. This was the equation with the two V's in it. We're going to get Q change in voltage equals a half MV final squared. We're going to get the final speed is going to end up being 2 QV. When I multiply the one half over here, it's going to become a 2 divided by the mass square root. It's going to be 2 times 1.6 times 10 to the negative 19 times the change in voltage of 750, divided by the mass of an electron, which is 9.11 times 10 to the negative 31. When you crunch the numbers, what do you get? Nobody, 2 times 1.6 scientific notation, but negative 19 times 750 divided by 750, Mr. Dewick. Come on. There we go, divided by 9.11 times 10 to the negative 31. What did I do wrong there? Oh, too big, square root. Do you get 1.6 times 10 to the 1, 2, 3, 4, 5, 6, 7? Yes. 1.6 times 10 to the 7th meters per second. If you got that, you get full marks. Otherwise, I would probably go something like 1 mark for that, 1 mark for that, 1 mark for that, and 1 mark for your answer. Question 2 is exactly the same question as question 1, except we're not starting from rest. Oh, and rather than let it just smash into the wall, we're going to cut a small slit in the wall. We're going to have it go through the hole and keep going. It's an electron gun. Suppose an electron is traveling at an initial speed of 2.1 times 10 to the 5th meters per second. Kinetic energy initial plus potential energy initial equals kinetic energy final plus potential energy final. The final potential energy is zero because it's falling all the way to the ground, but nothing else is zero. I think the easiest way is going to be to crunch the numbers on the left-hand side, and I'm going to cheat and just go to my answer key because it was a lot of typing, and if you got this far, most people managed to get the next few steps, I think. So, quiz 2, version 1 answers. Come on, computer. It would have been faster if I had the word open already. Oh well. Okay, it can get time. You're going to get a half mv initial squared plus q change in voltage equals a half mv final squared. Then when you crunch the numbers, I found it easier to actually work out what this side worked out to, 2.402 times 10 to the power of 17, and then I times by 2, or divide by a half, divide by the mass. That gave me v final squared, square root, 7.26 times 10 to the sixth meters per second. So what's this quiz out of? Okay, if you can give yourself a score on the quiz out of 13, however, I won't see you between now and the test. When's your test? Next class. When's the tutorial? Monday after school. So test Tuesday. Instead of me collecting the quiz, can the person on the end of each row put their name and their quiz score, and then pass it down, and everybody writes their name and their quiz score on a piece of paper. Circuits. Charles, today is going to be mostly terminology. You need to go ahead. When do you need to go right now? Okay. Circuits. Today is going to be mostly terminology. When I first started teaching circuitry, Matt, I really didn't have a good understanding of what was going on. I could solve it. I remember I got an A on this unit in first-year university physics, because I could do the math, but I did the math in my nerdy, complicated way. I used systems of equations, and it worked fine. And then my second or third year of teaching physics, 12, another physics teacher in the district and I were chatting, and I said to him, how do you explain circuits? Because I explained it, my kids that were strong math found it okay, but everybody else just found it tough. And he gave me an analogy. And I'm going to be using that analogy throughout this unit, because if you actually wrap your brain around the analogy, most of these questions will fall apart. On the first page you guys have is all the learning outcomes for this particular unit. In other words, at the very, very end when you start the test, you can say, hey, do I know how to do all this? If you do, you're good. And then we have basic circuit concepts. So as Phil in the following key facts about voltage, the definition of voltage was potential energy per charge. How much energy per Coulomb at any one location? And the units were Joules per Coulomb, or we gave that a name, we called it volts. What were some other words for voltage? What was the yucky one that we didn't like? Potential. Potential. Sorry, just trying to find something here in my handy-dandy handout. So the word that I didn't like, but they used it still, was the word potential. For this unit, almost always our voltage source is going to be a battery. It gives us a nice simple constant voltage. Well, no, actually the batteries get weaker over it. It's going to be constant. We're going to, in our magic physics world, going to have it be constant. What's going on inside a battery? Well, inside a battery, we have some kind of chemical reaction doing work. And what it's doing, Hannah, is it's separating the charges. It's separating positives from negatives. It's pushing negatives to the bottom and positives to the top. And then if we connect a wire, don't write this part down, but if we connect a wire with a resistor, the charges will happily flow through here because they are attracted to the positive side. But they can't go the shortest path because this chemical reaction makes it very, very difficult to go backwards. In terms of a circuit diagram, the circuit symbol for a voltage source, that's the negative, that's the positive. It looks like that. It's two parallel lines. The shortest part is the negative terminal. The longest part is the positive terminal. That's the symbol in a circuit for voltage source, for battery. And I said to you, I'm going to use some kind of an analogy, a mechanical analogy. And the analogy that we're going to use here is the idea that every circuit is actually a ski hill. What, really? Yeah, and it works great. You see, what a battery does is it raises charges, skiers, to a higher level on the mountain. In fact, the battery acts like a chairlift. How do the skiers get back to the bottom of the chairlift? They have to go through the ski run. Those are the wires and the resistors. And when they go through the ski run, they lose potential energy and they have to get rid of the potential energy for them to get back to the bottom of the chairlift and then the chairlift raises them back up to a higher potential energy again and they go through the circuit one more time. So, battery acts like a chairlift. And then when the skier, when the particle, when the charges get to the top of the mountain for them to lose their energy, they have to go through a resistor. How many volts would it, let's suppose this was a 6 volt battery. Don't write this down. But if this was a 6 volt battery, how many volts would the charges have right here? 6 volts. How many would they have right here? Zero volts because they're on the ground. They gain 6 again. How many do they lose going through here then? 6 volts, otherwise they couldn't get to the ground. Oh, that's a gain, lorally, if we pretend that our wires are perfect and have no resistance whatsoever. We're going to pretend that. What happens, next page. What happens if we connect a wire to one side of a voltage source? Well, the negatives would love to get to the positive, but when they go along here, they can't get to the positive. So once you have a couple of negatives here, this is negatively charged. Negative charges repel each other. You know what? Really, basically nothing happens at all. No current flows. The skiers are stuck at the bottom of the chairlift. What if the wire forms a continuous loop? Okay, now current flows. Oh, stop. Look up. Hannah, you too. Do you remember I said that Benjamin Franklin named our charges wrong? Even though in real life, Leslie, it's the electrons that move through the wire. We're going to decide from now on, we're going to pretend that it's the positive charges moving because we want to do math with positives. So I'm going to draw the current going this way from positive to negative. And that's the convention. In fact, we call that conventional current. That's what electricians decided a long time ago. We said, let's talk about positives moving. It makes much more sense because here's the top of the ski lift. The skiers have lots of energy and they have to end up at the bottom of the ski lift. So they have to end up with zero volts. The problem is the problem with this circuit here. So current will flow, current flows. But we call this a short circuit because this hill has no ski run, no resistor. These charges have to lose their energy somehow. So if they can't lose their energy going through a resistor, where will they lose their energy heat in the wires? That's how you cause electrical fire. So a short circuit is very bad, very dangerous. In fact, later on, we'll look at the difference in current and it's quite stunning. So a short circuit, energy goes into heating the wires. Why is example D a much better circuit? The vast majority of the energy will go into heating that light bulb. And burning that light, causing the light bulb to light. Or if you want to go to the ski hill analogy, we get off the mountain, we go down a ski run, get to the bottom of the chair lift. Get off the chair lift, go to the ski run, get to the bottom of the chair lift. We have a lovely ski run that works. By the way, as a circuit drawing, this would look like this. Battery, resistor, back to the battery. There is a symbol for light bulb. I can't remember it right now, but I think it looks like this. I think that's the symbol for a light bulb, but I can't remember. So current. Electric current is defined as the rate of flow of charge in a circuit. It's defined as how many charges come by any one section in a second. It's charge over time. And the units are coulombs per second, but we've given that a definition. We've defined that as one amp. Named after a scientist whose last name was ampere. Oh, and recall from this current unit that we just finished, any charge is actually a certain number of fundamental charges, electrons or protons. So if I tell you the charge is three coulombs and you divide by 1.6 times 10 to negative 19, you can figure out how many electrons are in those three coulombs because each one of them chips in a tiny bit or how many protons are in those three coulombs if each one of them chips in a bit. So we can actually relate current to a number of electrons that have moved through the circuit. Example three. A 1.5 volt AA battery is rated at 0.75 amp hours. What's that mean? We'll talk about it in a second. When connected to the motor in a portable CD player, the cell provides 0.25 amps of current. How much charge is stored in the cell? How many electrons are stored in the cell? How long will the cell power the motor? I'm going to skip A and B, and I just want to know how long will the cell power the motor? Well, the maximum this can do is 0.75 amps in one hour. If we're only drawing 0.25 amps, how many hours are we going to get? Three hours, because we're getting one-third of the load, so three times as much time. We're not going to do much with this type of a question. What we are going to do is analyze circuits like crazy, and that's going to start a little bit on the next page. So, I said that voltage is like the chair lift. I said that a resistor is like the ski hill where you lose energy where you get back down to ground level. What's current like? Current is like the number of skiers and how fast they can ski. It's like the flow of skiers up and down a ski hill in kilograms per second if you're talking about skiers in coulombs per second if you're talking about current. Since electrons flow at a constant speed, we will assume that the skiers use their skis to control their speed and move down the hill at a constant speed. The problem with this is if we have electrons doing the moving, we have the electrons skiing up the hill and going down the chair lift, which doesn't make sense. So, we would like to be able to say that the current flows downhill from higher voltage to lower voltage. For this reason, we choose to focus on the positive charges. In fact, what we call current is actually the downhill flow of positive charges. Many books will call this conventional current. And electron flow is in the opposite direction, example four. Malcolm, which part of this battery is the positive terminal? The top one or the bottom one? That's which way the current is flowing. Current is flowing this way. Current is flowing this way. Current is flowing this way. Current is flowing this way. Which way is the electron flow? Electrons are flowing this way. Electrons are flowing this way. Electrons are flowing this way. In the opposite direction of the conventional current. C. Ohm's law and resistance. Every circuit offers some resistance, symbol R, to the flow of charge. The circuit symbol for a resistor is a series of short wavy lines. Which hopefully you saw in Science 9 or Science 10. When did you do electricity? Science 9 or Science 10? You remember? I've heard both. I think it used to be 10. I think in the last few years it may have changed to 9. So some of you might have been exactly when it changed. What does resistance depend on? Well, resistance is how crowded the hallways are when we ring the tones. The worst hallways would be a narrow hallway. You couldn't get as many people, skiers, charges through. So one of the things that resistance depends on is the thickness of the wire. You have nice thick wire. The charges can go whipping through, hardly running into anything. Like a nice empty hallway. Or a nice wide hallway. If you have a very, very crowded or narrow hallway, you're going to bump into people all the time. You're going to be, if you're a charge, losing energy and collisions, and generating in those collisions, a fair bit of heat to be quite honest. Okay? So resistance is due to electric friction. When electrons collide with the atoms of the materials they are passing through. And we lose a bit of voltage, a voltage drop as we're going through a resistor. Example 5 says this. What can be said about the current flow in each case, right down either large current or small current flow? If we have a high resistance, if we have a very narrow, crowded hallway, will people be able to move quickly? Low current with no E? Gosh, Mr. J. Small current. What if we have low resistance, large or high current? What about for voltage? Well, if we have a large voltage drop, if we have a very steep ski hill, compared to a small voltage drop, a bunny hill, what's more popular? The bunny hill or a good ski run? What's more popular? You guys have been skiing. Is the bunny hill the one people really, really, really, really want? I can't wait to go on the bunny hill or do you go skiing to go on the other ones? Really. Really. Bigger voltage means large current. You got a nice big wide ski run. Long hill. We can get lots of people on that ski hill. Smaller voltage means smaller hill means, you know what? Small current. Scientists named Ohm wrote it this way. He said voltage equals I times R. We call that Ohm's law. Ohm's law says at current gets bigger, voltage gets bigger. As resistance gets bigger, voltage gets bigger. That's not so obvious. He initially wrote it in terms of resistance. He said as resistance gets bigger, current gets smaller. And as voltage gets bigger, current gets bigger. And then he re-raised the equation. This is the one we're going to remember. It's on your sheet, V equals I times R. If you're having to look that up, you're flunking the test, trust me. Technically, Matt, to be consistent with the unit we just finished, I should write change in voltage for voltage drop. And Ohm's law should really say change in voltage equals I times R. But this notation is cumbersome, annoying. And so the delta is dropped. But when we see the V in a circuit, we should really be thinking voltage drop or voltage gain if it's coming up the chair lift from the battery. No yawning. Got Leslie a little bit there. I know, you just get it to me. Cody, do you remember what the units were for current? What were the units for current? What were the units for current? Turn back a page if you need to. What are the units for current? Amps. What are the units for volts? Volts. What are the units for? Sorry, for voltage volts. What are the units for resistance? Ohm's, named after a scientist whose last name was Ohm. Good conductors have low resistance. Poor conductors, insulators, have high resistance. So it's always good to have an idea of what's a really big and a really small number. We're going to try and figure this out. So I'd like you to look on the left-hand side. There's six different objects mentioned. Which one of those do you think would have the very, very most resistance? Tell me the object. There's no, tell me the object. Copper, light bulb, which one? Glass. And what is the highest resistance on the right-hand side for a number to pick from? Okay. I'm fairly certain that I can reason that out. And I'm going to, that symbol for infinity is eight on its side, right? Which of these? Yeah. When I say sorry, you say the same volume. It doesn't help me at all. I heard, I can't just there. The eight on its side, that's symbol for infinity. What's it next to? What's it next to? Look to your left in that chart. Glass, so I asked, which of those, these six things has the most resistance? Glass does, glass does not conduct electricity at all. Glass does an infinite resistance. Okay. And now, Matt, I'm going to flip the question on its side of the remaining five things. Which of those do you think has the least amount of resistance? Copper wire. So what's the smallest resistance over on the right-hand side? I'm pretty sure copper wire has the resistance of about 0.1 ohms, although we're going to pretend it's zero for easy man. Now we have four things left. What of those four things do you think is the most resistance, the least conductive? Human skin. Human skin is reasonably good at insulating us from small shocks. So human skin is going to be the next biggest one, which is 100,000 ohms. And I'll cross that out. Hey there, Mr. Dewick. I've heard of people getting shocked. We're going to talk about how electric use and works at a second. Now I have light bulb electric motor, average lab resistor. Look at these two, the bulb and the motor. Which of those generates the most heat, a light bulb or an electric motor? A light bulb gets very hot, which must mean it has a bigger resistance than an electric motor, because if it had lower resistance, it wouldn't get so hot. Turns out the light bulb is 200 ohms, and the electric motor is 5 ohms, and the average lab resistor is about 2200 ohms, or 2.2 kilo ohms. Matt, do you need to know those? No, but I always try and show you what a really big and really small answer is, so you can kind of keep an eye out for calculated mistakes, right? Okay, technically, Leslie, you got me. The resistance actually changes with change in temperature, so as wires get hotter, they build up more resistance. We're going to ignore all that. Let's talk about electrocution, genre voltage. So here's our friend John, and what he's going to do is he's going to rub his foot on the carpet, pulling off electrons from the carpet and transferring them to his body. And they're going to repel each other. They're going to try and gather along the surface of John. And when enough of them gather, there's enough potential energy, enough voltage, enough energy per coulomb for them to ionize the air and make the jump. If I move the finger further back, way more electrons are required, and you can see most electrons are gathering on the outside because they want to repel each other. If I shrink the distance, that's a pretty nasty shock. If I start out right here, I only need a few electrons, and they already have enough voltage to jump. Now that's when we are generating the voltage source, when we are generating the electrons. What about when this voltage source is from an outside source? So human skin is a pretty good insulator, typically tens or hundreds of thousands of ohms. It will only protect us up to a certain point. When the voltage exceeds a certain value, the skin will break down and ionize, just like air does to create lightning. On the inside, we are basically salty water, and this conducts electricity rather well. Once beyond the skin, the danger to our health lies in two effects. First of all, we are not very good conductors. So if we're getting a bunch of voltage and it's going through us, we have a high resistance, those particles are going to run into molecules in our body, and when they do that, when those charges run into molecules in our body, what's going to happen to some of their potential energy? Heat. We're going to get burned. That's why electrocution burns people. The more serious one, and the more subtle one though, is this one. The flow of electricity through the body can also disrupt the body's own electrical nerve impulses, leading to temporary freezing or paralysis of the muscles. That's how a taser gun works. A taser gun is a very carefully calculated voltage, and the idea is that it's supposed to cause you to become rigid so that handlers can grab you. It is not supposed to be fatal, although here in BC, we certainly saw a few years ago at the Vancouver Airport that a taser can be fatal. The real danger would be, let's suppose you were doing some work outside and you were standing in a puddle of water. An electrical cord landed in the puddle and it gave you a shock, and you fell down with your face in the puddle, so your mouth and nose were under the water. The electricity, as long as that shock is there, it would not allow you to regain muscle control, or even if your face and mouth were above the water, your diaphragm is a muscle to breathe with, and your heart is a muscle. It could cause your heart muscle to fibulate, or your lungs, your diaphragm, not to be able to work and you would die. And a current for that is very small. As a matter of fact, the current required is only about 100 milliamps. It's not big. You can get it from a car battery quite easily. It's called the death current. If you have such a current flowing through the heart, the heart will stop or twitch helplessly. Now, if you're on a car and you shock one of your hands accidentally, you're hoping it arcs across your hands. You probably get a burn and a sting, but hopefully the electrons wouldn't go through your heart. Or if your feet got shocked, hopefully you'd be okay. But if you, well, clear paddles, they put the paddles on the chest for a reason. They're creating, your heart is basically dead center in your chest. It's not off to the left, what people think. And they're sending a current through your heart to shock it into fibulating and changing its rhythm. Okay? 100 milliamps, Kim, is not very big. Be careful when you're dealing with electricity. In our ski hill analogy, resistors act like ski slopes that connect the top of the hill to the bottom. The amount of resistance, electric friction, is sort of like how much powder there is. If there's lots of powder, you can't ski very fast. Maybe good skiing, but you can't ski very fast. If there's barely any powder, it's icy conditions, and although the skiing sucks, you can go fast. Low resistance means more current. High resistance means less current. Example seven. It says, find the current in each case. So the current is going to go this way. How many volts do the charges have right here? They just got off the chairlift. How high are they? Six. When they get back to here, how high are they? Zero. So where did they lose that height? How high must this little ski slope be? Yeah, and we would say it's a voltage drop. You could say it's negative six volts. We're just going to say this is six volts. We lose six volts. And Ohm's law says V equals I times R. What does example seven want me to find, Alex? Current. Get the current by itself. Current is going to be what, Alex? How many volts do we lose going through this hill? Six. What's the resistance? 25. How many amps will be flowing through this circuit? What'd you get, Carly? 0.24 amps. Compare that with this very dangerous short circuit. So here we're going to have the current is the voltage divided by the resistance. We still want to lose six volts, but because there's not a true resistor, it's only the resistance of the wire and the wires have resistance of 0.001. How much current will flow through this? Is it 6,000? What would happen with that thing on the right? Well, if it was a six-volt battery, you'd probably, well, first of all, you'd bleed the battery drive very quickly. It'd burn out probably about five minutes or so because that's way more amps in terms of the amp hours that they assigned. They were not expecting 6,000 amps. They were expecting 0.5 amps. And depending on how resistant your wires were, you're probably starting to generate some heat in those wires. You could start to melt the rubber, the insulation off of the wires. So technical comment. The second diagram is an example of what's known as a short circuit. It's called short because it has a near zero resistance and therefore will experience very large currents. If you short-circuit a AA cell, you'll completely, quickly deplete the cell, but you won't die. If you short-circuit a car battery using lab wires, the car battery has enough charge stored in it to make the wires red-hot and possibly even melt them. And if you try short-circuiting BC Hydro, you can kill yourself. Although most houses have, in theory, a safety system. They either have a fuse box or a circuit breaker box so that when there's a spike in current, boom, before you can even twitch, a physical switch snaps over and shuts down the current. So you might get a shock, but hopefully not a fatal shock. And the fuse box or the circuit breaker, hey, save thousands of thousands, maybe even hundreds of thousands of lives. Works great. So we've been talking about current and voltage. How do we measure current and voltage? Well, current is measured through something called an ammeter and voltage is measured through something called a voltmeter. Resistance is measured through something called an ohmeter. Remember, we said this. Current is how many skeers? So here's our circuit. We've got a six-volt battery and a light bulb or as a circuit diagram, six-volt battery, external resistor, lovely circuit. They would leave the battery six volts high. They would lose the six volts in the light bulb, come back down zero, and they would ride this ski hill all day long, happily, happily, happily. What if we wanted to measure, not calculate, but measure the current? If I just attached a wire, don't write this down, like this and this, and have my ammeter right here, the problem is that if the ammeter is supposed to measure, how many skeers don't write that down because that's wrong? If the ammeter is supposed to measure how many skeers, some skeers can bypass the gate and I won't even know they're live. If you want to measure current properly, you have to put the ammeter right in the circuit as a gate so that all of the skeers for the area that you want to measure have to go through it. Good? Bad. Oh, and getting technical, you would also want the ammeter to have near zero resistance because if it had a resistance, it accidentally added one more ski hill to your mount and now all the numbers change. So ammeter, you want to put that inside the circuit where, well, it depends on what you're trying to measure for more complicated ones we'll have to ask ourselves that. The second type of device is a voltmeter. A voltmeter wants to measure the height of the hill. To measure the height of something, you need to find out how high the bottom is and how high the top is. A voltmeter would have to be installed outside of the circuit and just touching the circuit. How high am I there? How high am I there? Subtract. Oh, and because we don't want any current to go through here, technically, Hanna, a voltmeter should have an infinite resistance so that no current wants to go through that section. Realistically... So this is good. Bad. They all know it was you, Malcolm. Just grab it. Trying to look innocent and natural, ain't gonna help. You see the difference between the two? Voltmeter, because I really want to measure a change in height, I've got to clip on to two separate spots and say, what's the difference in altitude between these two spots? Ammeter, I'm counting skiers. I better make them go through a gate. My gate. So far, I've been talking about direct current. Most of what I'm gonna teach you also works for alternating current. Alternating current has a magnitude that varies as a sine function. Who's in math 12? Current goes this way, current goes that way. Current goes this way, current goes that way. It is a lovely sine function. And in fact, because we get 60 hertz and that's the period, the frequency is 1 over 60. I can even tell you what... Sorry, the frequency is 60 hertz. The period is 1 over 60. I can tell you how long if you're trying to graph it, the period would be. And the amplitude is 110. Usually we say 120 volts, because it's a nice round number. It's actually, I think it's 114 point something volts. But it's either said 110, 110 or 120. What's your homework? None. Yes, unit review. Start working on the great big electrostatics unit review. When's your tutorial? Monday. Study for the test this weekend. Test Wednesday.