 So, listen three, consider Zach the following situation. What we're going to do is we're going to move a magnet to the right. Now, don't write this part down, I'm going to really muck up this diagram. So right now we have magnetic field lines going out, which means are some of the magnetic field lines going through this coil? As I move to the right, will the number of magnetic field lines going through this coil change? Will the flux change? Will I induce a current? Yes. The question we're going to ask today is not will I induce a current? We're going to ask what direction? What direction? Now there's two possibilities, don't write this down. The first possibility would be the current could go this way. If the current went that way, using your right hand solenoid rule, which side of this would be the north pole, the left side or the right side? Are you with me? If the current went this way, which is wrong, don't write this down, which way would be the north pole, the left side or the right side? Now if that was north, what would this be? What would this south pole do to this north pole? What would make this magnet move slower or faster? What would make the flux change slower or faster? What would make the current bigger or smaller? What would make the magnetic field bigger or smaller? Bigger? What would make this move faster or slower? Faster? What would make the flux change faster or slower? Bigger? What would make the current bigger or smaller? Bigger? That's why this is wrong. This would be a perpetual motion machine. Nature, the universe, says nuh-uh. And it was Lenz, a scientist whose last name was Lenz, who first pointed this out. He said, you know what? If you're trying to figure out what direction the current is going to run, ask which way would slow things down that way, except he phrased it this way. The magnetic field is, sorry, the current is caused by a north pole moving to the right. Zack, the current is caused by a north pole moving to the right. What would resist a north pole moving to the right? A north pole right there. That's the direction of the magnetic field that's going to be created. Write that down. Put a north there and a south there. And now that you know what direction the magnetic field has to be for nature not to have perpetual motion. What direction must the current flow? Use your right hand, rule, point your thumb to the left this time. And you know what? The current has to be going this way. That's Lenz's law. It's a weird law because, Evan, it's not an equation. It's English. Here is Lenz's law. It says an induced current will always create a magnetic field that opposes the change in flux that created the induced current in the first place. What was the change in flux here? North pole moving right? What would resist that? A north pole right there pushing against it. What's Lenz's law? A change in flux, sorry, an induced current will always create a magnetic field that opposes or resists the changing flux that created it in the first place. In other words, you get nothing for free, ever, except my affection. No, I made you earn it most of you. Never mind. I'll take that back. Oh, except maybe your mother's love. Yeah, OK. This also really explains the negative in Faraday's law. Remember when we did it yesterday? We left to space last day, and then we put a negative in front, and I said, why? It's to remind us, Vitaly, that the voltage you're getting when you're spinning a coil is in the opposite direction of the voltage that would cause an electric motor to speed up. You get nothing for free. For example, suppose you have a loop, and it's placed inside a magnetic field where the magnetic field is going into the page. And then maybe it's an electromagnet. We, boom, flip the magnetic field so that now the magnetic field is going out of the page. It's this magnetic field that's inducing the current. We say that again. Look up, please. It's this magnetic field that's inducing the current. What direction is this magnetic field out of the page? You know what the current's going to do? The current in this coil is going to create a magnetic field that resists out of the page. What will resist out of the page, Ian, into the page? The current in this coil has to create a magnetic field into the page. So point your thumbs into the page and tell me, is the current clockwise or counterclockwise? As it turns out, the current must flow. Why? Because out of the page was trying to create it, so it's going to create into the page. Here's the copper pipe, but on its side, and only a short piece of pipe, not a long piece of pipe. So it says this. The diagram below shows an aluminum ring, and it shows the current that's induced in it by the nearby magnet. So we know which way the current is flowing. Which way is the current flowing this way? Which way is the north pole from this ring? Left side or right side? Use your solenoid rule. Right side? So you're saying the north pole is right there and the south pole is right there? That was created in direct response to the way this magnet is moving. Which way would this magnet be moving so that a north pole was created right there? I think a south pole moving away from it would, nature would put a north pole there to say, come back, stay. It would resist. So the magnet, this magnet here, must be what? Stationary, moving to the left, moving to the right, or spinning? It's got to be, oops, moving to the right. If I knew the current was going upwards, this magnet would be moving to the left. Oh, and that's what's happening when I drop the magnet. If you flip this on its side, supposing you have a north pole going down, the current that's created inside, and it's a small current, you don't get a shock from it, but the current that's created is creating a magnetic field with a north pole pointing up and that's what's slowing down, the free fall magnet. That's the three dimensional version of lens as well. Example three. Here we have a coil inside a magnetic field. Is the flux changing? Well, let's see. Is the area changing? No. Is the magnetic field strength changing? Well, the direction is. So will there be current? Yes. Which way will the current flow from X to Y or from Y to X? Well, look at your final picture. What direction is the final magnetic field up out of the page towards you? Is that correct, Ari? The dots out of the page towards you? So the current in the coil will resist that. What kind of magnetic field would resist out of the page towards me? The current in the coil is going to generate an induced current, a magnetic field, that creates a magnetic field into the page. Point your thumb into the page in the coil which way is the current flowing? This way, this way, this way. And now follow it through the circuit. This way, this way, this way, this way. From Y to X, from Y to X. What's Lenz's law? Ask, hey, what's causing the current? Lenz's law says your induced current will resist the cause. Will oppose the cause. That also explains why if you were pulling the bar, you'd feel a force resisting it because with the bar, it's the area behind it that's getting bigger. What will resist an area getting bigger? Tugging in the opposite direction, trying to make it smaller. Example six. If we are moving this magnet to the left, find the coil's induced north pole and the direction of the induced current. So let's look at the pole that's closest to the coil, this guy. Is it north or south? North. And which way is it moving, left or right? Left. What would resist right there, what would resist this north pole moving away from it? South pole right there, north pole right there. Now that you know that, what direction would the current in the coil be? This, yes? Curling over the top downwards, not over the top upwards. Says, Mr. Dewick will now show you a demonstration that's as close to magic as you'll ever see unless you know about Lenz's law. I've already, I couldn't resist, I brought that copper tube and magnet out earlier because it is so nerdily cool. What's that? You wanna see it again? Okay. If you insist. Homework. By the way, you'll notice I'm giving you homework from the review because we're towards the end of the year the review is gonna be your homework for this unit. Didn't wanna give you homework twice. From review, here are questions that ask about Lenz's law. Here are questions that ask about Lenz's law. Number nine, number nine. Number 22, 25, 27, 27. I guess that's technically it. So, lesson five. It says the final lesson, but actually I'm gonna do lesson four tomorrow. That gives you one day of review before your test. I think that's beneficial. It lets me give you guys the take home quiz and actually mark it in class and give you feedback. So what's a transformer? Well, first of all, if you look at the telephone poles, have you seen those big barrel shaped things on the telephone pole? Those are transformers. Or in some subdivisions, it's the big metal box that's on the ground that you can sometimes hear humming. That's a transformer. What a transformer does is it takes voltage or current at a certain voltage or certain amperage and it changes it for your house. You see the voltage from the power lines is not 120. It's about 20,000 volts, I think. Which means if we tried to run that through your house, fire bad. What a transformer does is it steps down the voltage for a house or it can step up the voltage if you need to. It's a very simple device. So here's how it works. If you take a big laminated iron core in the shape of a hollow rectangle and you wrap a certain number of coils of wire around one side and we use the subscript P for primary. This is your primary source voltage and you wrap another different number of wires around a different side. We use S for secondary voltage. If you run an alternating current through here, it's gonna generate a magnetic field pointing up, then pointing down, then pointing up, then pointing down. In fact, very quickly you're gonna have an oscillating magnetic field. Is that okay so far? What that's gonna do then, Zach, is you're gonna have a magnetic field going, this is gonna send up a magnetic field line like this and dozens of times each second, in fact 120 times a second on alternating current, the magnetic field's gonna go down, then up, then down, then up, then down, then up, then down, then up, then down, then up. What you're going to have is a very rapidly changing flux. Oh, if you have a changing flux, you will induce a current. It's an easy way to pass current from one part to another without having to have a circuit connector in between. These wires do not actually touch each other, it's the magnetic field from this guy, Evan, that induces a current over here. The input side is usually called the primary, the output is called the secondary, the transformer works as follows and alternating current produces technically a sine wave current for those of you that are in math 12. This changing current produces a changing magnetic field in the primary coils. And because the magnetic field here is changing and the magnetic field airy spreads out and goes through this coil over here, we have a changing magnetic field or a changing flux, it conducts it through the secondary coils. And the changing magnetic field passing through the secondary coils creates an induced voltage. Here's the key, Nick. If the number of loops of wire are different, you'll get two different voltages and two different currents. So it lets you turn any current into any other current or any voltage into any other voltage. Within reason, you have a tough time turning one volt into a million volts, but it could be done, just be a really complicated transformer. Would a transformer work with direct current? No. Why not? If you hook it up to a battery, you don't have a changing magnetic field. Your magnetic field would always just be in one direction. And we said last day, it wasn't magnets that caused current, it was changing flux that caused current. And how can you change the flux, change the area or change the magnetic field? So no changing magnetic field, no changing flux, no transformer. What you could do, Evan, is turn the battery off and on really, really fast. That would work because you'd have the magnetic field going from max to zero, max to zero, max, that's stupid. It's much easier. One of the many reasons why we went alternating current is alternating current, Evan allows us to pass voltage and current from one coil to another with great control. Is that okay? So no, using principles of physics, explain your answer. You have no change in flux, so no EMF, so no current. We find many transformers in our homes since most of our household appliances don't work with 120 volts. Show you a transformer. Think about all your electronic devices that have one of these somewhere either in the cord or in the plug itself, the big, big, huge square plugs, those have built-in transformers. Feel it. It's working hard, it's hot. Almost hot enough to burn me, but not quite. There's lots of electrons bumping into each other in that system there, creating friction in the wires, heat. So these are transformers. These are transformers. These are transformers, your calculators, your pocket-organized, your cell phones, CD players, almost all of those will contain a transformer, perhaps not your cell phones. Well, yes, in the charging system, there would be a transformer, but then the battery would deliver whatever current you wanted. Also seen on power poles or within the green metallic enclosures at ground level because BC Hydro uses them to step down from the thousands of volts in the main wires, which is why, by the way, and you guys probably know this, you do not mess around with those power lines. You don't try and climb a telephone pole or a power pole, even though I showed you the video of the elderly gentleman that was working in his Faraday cage suit on the power pole. Ah, you don't mess with them. And it steps it down to the 110 volts we supply in our homes. Here is the transformer equation. Turn the page if you haven't already, boys and girls. Here's the equations. Don't write this down yet. I'm gonna do it as three separate equations and then I'm gonna combine them. The equations are as follows. If you know your primary voltage, don't write this down yet, and you divide it by your secondary voltage, that's gonna give you the same answer as the number of loops of wire in the first solenoid divided by the number of loops of wire in the second solenoid. Also, if you know your primary voltage and you know your secondary voltage, that's going to give you with current it's opposite. Primary on, oh, not primary on top. Primary doesn't go with primary. Secondary on top, primary on the bottom. We're gonna combine this as one great big equation. This is what you're gonna write down. V primary over V secondary equals N primary over N secondary equals I secondary over I primary. And Evan, this means we're gonna be cross multiplying a lot, which is really all the math in this particular lesson. Where VS stands for whatever your secondary voltage is or you need it to be. VP stands for whatever your primary voltage is or you want it to be. IS is the secondary current in amps. Primary current in amps. The number of loops of that secondary coil, the number of loops in the primary coil. Is this what it is on your formula sheet? Is the letter P on top or is the letter S on top at the beginning? So S over P, S over P over S, I flipped it, it's the same equation. Where do you use this? Plug and chug. Example two. Supposing our car battery, which is 12 volts, we need to step it up to 11,000 volts because that's what our spark plug needs, okay? We have an induction coil. Yes, we're talking mechanics now. If the induction coil battali has 100 turns in the primary, how many turns do I have to have in the secondary so that from my 12 volt battery, I can get 11,000 volts to the spark plug? This question is talking about voltage and number of turns of wire. So we're gonna use the first two, but not the third one. Let's write it down just to jog our memory. VP over VS equals NP over NS. What's this question asking us to find? Louder? Oh, NS, that must mean they told us everything else. Let's see, did they tell me the primary voltage? What's our source voltage? 12. Over. What do we want the final voltage to be? 11,000. How many coils in our induction coil in our source voltage transformer section? How will I solve this? Now, before we go further, technically this won't work because the battery is a direct. Do cars have something called an alternator? What do you think an alternator does? It's an ingenious circuit, which regularly interrupts the flow of current, thereby from a direct current source, producing a changing flux, enabling us to transform the voltage. Oh, that's what it's there for, yeah. How do I solve this cross multiply? So if we're designing this vehicle, our induction coil, the second part of the coil better have, let's see, 100 times 11,000 divided by 12. How many coils of wire must we have? 91,700? Did I go to three sig figs? Is that okay? 91,700 coils, which in a factory is very doable. You have a machine just wrapping the wire around, no problem. You guys, when you built your speakers, probably did a couple of hundred. And realize, Brennan, how much control this gives us? What if you wanted 10,500 volts? A few less coils? Complete control. We're stuck with in a car the 12 volt battery input, but that's why it hasn't changed over the years, despite the fact that more and more electronic devices have been added to cars, we can get the voltage to whatever we need it to be. Little note, terminology. Transformers that increase voltage are called step up, transform. Oh, let's try that again. It's a highlighter this time. Transformers that increase voltage are called step up transformers. Transformers that decrease voltage are called step down. Most of the big barrels that lead to your house are step down, because your house can't handle 20,000 volts. I don't know, I heard somebody remember it, for the stove and the dryer that need 220 volts, I don't know if your house has a transformer or if they just run a second wire from the big transformer and also have a 220 volt attachment. I suspect they would do that, because that's still a lot of voltage to be having in your house. It'd be nicer to have an isolated circuit. Now, technically by the way, what I've told you is garbage. Troy, when you grab the transformer from my computer, what did you feel? Okay, this is not a perfect system. So often they'll say an ideal transformer. You know what they mean by an ideal transformer? We're back in our magic physics world, no friction, no heat. So an ideal transformer has 160 turns of wire in the primary and 800 turns in the secondary. The primary circuit is connected to 120 volts alternating current. What's the EMF across the secondary? And is it step up or step down? If the secondary is bigger, it's step up. Secondary is smaller, it's step down. Let's find out. Once again, we're using voltage and number of coils. So V primary over V secondary equals N primary over N secondary. What do they want us to find? Listen, V secondary, okay. That means they told me the primary, oh yeah, 120. They must have told me the primary, the number of turns of wire on the primary. Oh yeah, 160 and 800. I'm gonna pretend that we've cross-multiplied enough times that you can go straight to the answer. What is the secondary voltage that we'd be getting on the other side of the transformer? Sorry, 600? Even? Okay. And Erwin, is that a step up or step down transformer? Step up. You know what the circuit symbol for a transformer is? That, because they aren't, this is why we like transformers. They aren't actually connected. You're inducing a current over here. You're passing it on through the air, if you will. You know what the symbol for alternating current is? Well, technically it's supposed to be that, but this was the best that my friend who typed this could do in his stick figures and I'm not gonna complain. So it isn't alternating. Be careful. Every once in a while, Zach, they'll put a battery there to trick you on a multiple choice. And then the answer is zero. No current's gonna move. Okay. So it says find the unknown currents and voltages and then label the transformer as step up or step down. Look at what they gave me. Did they tell me both voltages? No. Did they tell me both currents? No. They say both ends. That's gonna be my starting point. N primary over N secondary equals. Miguel, do you wanna find voltage first or current first? I don't care. So it's gonna be voltage primary over voltage secondary and they want us to find the secondary voltage. That must mean they told us everything else. Let's see. N primary is 30. N secondary is 120. Voltage primary is 120 divided by voltage secondary. What's the secondary voltage? 120 times 120 divided by 30. 120 divided by 30, 4,800? Or am I totally wrong? 480, 480. Extra zero in there. Nope. That seemed high. That'd be good. 480 volts. By the way, step up or step down transformer. Step up. Okay. That's the voltages. And what else did they ask us to find? Current. So I'm gonna go back to using the ends, but the difference with the current equation, primary over secondary is current does not line up directly. Evan, current, if I have primary on top, secondary is on top with current. And if I have secondary on top, primary is on top. That's what's on your formula sheet. Current is the reciprocal, basically. All right, let's plug in what we know. It's still gonna be 30 over 120 equals, they want me to find the secondary current. Evan, what's the primary current? Look at the circuit diagram. Two. Go. Oh, I can do this in my head. Two times 30 is 60 divided by 120. Is it 0.5 amps? Step up transformer gives you a bigger voltage, but a step up transformer doesn't give you a bigger current. It doesn't give you smaller current. It's just, we're more interested in the voltage, okay? On your own right now, really quickly be, find the secondary voltage, go. I'll freeze the screen. Is that the answer? 1000 volts? Step up or step down? The step up transformer. No, but they love to ask that as a multiple choice question, and I've learned if I don't keep repeating it, kids get the fact that they'll think bigger current means a step up transformer. So bigger voltage. Step up. See. Find the secondary voltage and the current. Yeah. Matt, louder. What's on the next page? We're gonna talk about how the power lines work. Well played, sir. Well anticipated. Can you hold that question for about 60 seconds? Okay, can you hold that question for about five minutes? The disrespect I get from you guys at the end of the year. Good, solve both, solve this by the way, believe it or not, has once in a while on the provincial been a seven mark written question. And if you're writing this year, you hope it is. It's cross multiplying. Oh, make sure you remember that current is backwards, which is really the most only common mistake. That's grade eight, Matt. In fact, I've often argued that you could do this in science age, cross multiplying. And I think kids find electricity inherently interesting because you own electronic devices and you kind of wonder, how do these work? Can I put my tongue into a power outlet? No, Brett, no, you can't. You get 10 volts, I'm doing this all in my head. And I actually plugged the numbers in right away without showing my work, which I would never do on a test, but I'm trying to get two lessons done today. And for the secondary current, do you get 600 divided by five? It's gonna be 1200 divided by, no, sorry, 120. This lowers the voltage. It's a step down transformer. It does raise the current, okay? Are you guys okay on that? Then we'll skip D, and let's go straight to here. Let's talk about how the big power lines work, the ones that you see going over the mountains with the big metal towers that you can, on a rainy day, occasionally hear buzzing. Those ones, okay? Matt asked a great question about current. Now you remember in our circuitry unit, I said that current was sort of like number of skiers. You see, the problem is, if we wanted to ship power across country, and we had a high current in the power lines, what that would mean is in those power lines, there was a lot more skiers, a lot more charges. And they would all be bumping into each other like crazy. And you know what that would cause? Lots of heat. Our power lines would glow red hot, and that's energy that we're losing. And so power lines are shipped deliberately at very high voltage and lower current. Then when they get to the city grid, they're stepped down to a slightly lower voltage, but still high current. And then when they reach your house transformer there, steps it down to 110, I think it's actually 114 volts. So we traditionally say either 110, or we often say 120 volts, because it's a nice round number to your household voltage. So transformers are essential to the long distance transmission of electricity, because the hydroelectric dam is probably creating power at high current, low voltage. You guys have seen the big fenced off power stations occasionally in Maple Ridge. I know there's one out by 240th where I live. There's one near, I think Laity on Dudney Trunk, that you see them. Okay, those are stepping down or stepping up the power, probably also storing it and just giving it a bit because it's not quite a perfect system. We do lose some energy, but you're going to be surprised at just how efficient this is. Suppose we delivered power at a low voltage of 5,000 watts. The current running through the cables would be power divided by V. If I have, let's say, one million watts of power to distribute, one million divided by 5,000, I would have 200 amps of power in the current, sorry, in the wires. How much power would I lose going through the wires? Now in this circuit, it's the wires themselves that are the resistors. So if the wires have a resistance of 10 ohms, the power lost as heat would be I squared R, not VI, that's looking at the very, very ends. Now we're looking at the actual resistor. We don't know the voltage drop in the resistor. We know the current going through the resistor and we know the resistance. And we find that we would lose to heat that much. Of one million watts, 400,000 watts would go to thermal energy. We'd lose 40%. If we go at a higher voltage, say 500,000 equals VI, so the current required to ship a million watts at 500,000 volts is only two amps. How much would we lose inside the wires? Well, the power loss inside the wires is current times resist squared times resistance of the wires. It's I squared R, you lose 40 watts. Of one million watts of power, only 40 watts would go to heat, which means it's 99.996% efficient. Pretty good. And that explains your question or answers your question. Why do we ship stuff at low current, high voltage, way more efficient because the wires don't get so hot? However, most of our electric devices do require us to move a bunch of heat, stove, microwave, so we do need to get a big current eventually, transformers, it's a pretty good system. The electric devices, Miguel, not the toys. So suppose the Duick Hydroelectric Power Company decides to ship, we got 150,000 watts of power that we've generated over the past while. We're storing it in big capacitors or batteries, we're ready to ship it. We wanna ship it over wires that have a two ohm resistance, we're gonna ship it at 20,000 volts. How much power is lost? Well, power is VI, so the current that it has to be shipped at is, how do I get the I by itself fatality? 150,000 divided by 20,000 volts. How many amps am I shipping this at? 7.5 amps? Now, consider the amount of power lost. This involves the current heating up the two ohm lines which is why I have to use my second definition of power that has the resistance of the wires in it. I can't use VI, because I haven't taken into account the resistance of the wires. I have to use I squared R. What's I squared times, what's the resistance of the wires? How much power do I lose to heat? 113-ish, how efficient is this? Well, we're starting with 150,000. We're losing 113,000, which means we're keeping 149,887, 149,887, and then times 100% to make it a percentage. How efficient is this system? Pretty good, what do you get? As a percentage, so when you times by 100. No, no, no, that, how efficient, not how much do we lose? The efficiency is gonna be how much you kept divided by 150,000 times 100. 99.9, oh, it's great. I mean that in our real world, that's as close to perfect as we can get. The flip side would be, and this is what Edison was initially suggesting, because he wanted direct current. He said, ship it at high amperage. The problem then is basically every one of your power lines is a stove element blowing red hot. You're losing a lot of energy. So what if we shipped it at 560 volts? Well, the current required would be power divided by voltage, 150,000 watts divided by 560 volts. I'd be shipping it at 268 amps. How much power would I lose? Well, power loss is I squared R. So I'll take this number right here, square it, times what was the resistance of the lines two ohms? I would lose 143,495, wow. How much would be left over? Well, 150,000 minus what I lost. I'd have 6,505. The efficiency would be 6,505 watts left over, starting with 150,000 watts, 4.3% efficient. Dylan, I'd get fired. Transformers. So two lessons, but two shorter lessons. And that gives us one full day of review. And Evan, I like to think that the Transformers lesson, it's cross multiplying, suck it up if you're tired. It's cross multiplying. Finish the course, actually no, I cheated because I got one more to do. But we just gained an extra day, which is nice. You can read the little comic right there. What's your homework? From review, you can also now do, and this, you don't have to do this all tonight, but this is what you're capable of doing now. Number three is a Transformer question. Number 14 is a Transformer question. Number 19, that was the written question one year for seven hallmarks. It was what you guys just did in one hour. I thought it was a little too easy. 26, and that Troy is where as a part B, they're asking, is it a step up or a step down? That's why I kept going. Over at 33, motor, motor, motor, motor, 45, there you go. Thanks for being cooperative. I know last block asking you which through two lessons is a bit tough, but hopefully both of them stuck. What's Lenz's law? If they want to say what direction is the induced current, figure out which magnetic field is causing it, point your thumb in the opposite direction, that's which way the current's gonna go.