 Alright, so, in case you're interested in the awesomeness that was that video, boy do I have a treat for you because, there we go, we actually have a course that explores pretty much everything you just saw in that video, both from a music and a physics perspective. So it's co-taught by a physicist and a musician from the School of the Arts. It goes by many numbers, it kind of depends what you need. If you need ways of knowing, it's a ways of knowing course. If you need physics, it's a physics course, okay. It's Physics 1320 or 3320 or KNW 2305 was one of the very first approved ways of knowing courses in the new undergraduate curriculum. It's called Musical Acoustics and there's a link to the previous semester course there. And it provides the foundation's ways of knowing and proficiencies and experiences quantitative reasoning. So, you too can take all the mystery out of life by taking this course. But we have, so we have some of the equipment that you saw in there. This will generate, let's see if I can get this. So this uses essentially friction and builds up an electric charge. Alright, so this is a tiny version of the thing that I have at the end that's electrocuting everybody in chain mail. The stuff that you saw at the end of the video is not fake, you can do that. In fact, it's routinely demonstrated at science museums. You can put on a giant metal suit and you can go right next to a lightning generator and you can be struck by lightning and you will be perfectly fine. And the reason is because of two things. One, you're surrounded by a conductor and two, the electric potentials in different states are vacant, okay. If you ever make physical contact however, if you become the more attractive path to zero electric potential, which is what the ground is from whatever the charge is on this thing here, then it'll go through your heart. And that's back. But if you put metal around your body, even a fine mesh or a coarse mesh of metal, and often at science museums they'll do this demonstration in a like a bird cage. So it's got bars and the bars are like that far apart and then they'll raise the cage up with the person inside of it and they'll put them right next to a big Vandegraaff generator, which will strike them with small scale lightning. And they can touch the inside of the cage, it's perfectly fine. Does anybody know why that is? Why is it okay to put your hand on the inside of the metal conductor and not get electrocuted? Has that ever been explained to anyone before? Well, what does a conductor do? It allows charge to do what? Yeah, move through it and freely. So if there's an electric field anywhere and a conductor is immersed in that, the free charges inside of it will all move out, for instance, to the surface of the cage. So since the electric field in that video at the end is coming from the big generator off to the side, it's making a big electric field that fills the space, guys walking around on a metal mat, which has been connected probably to plumbing or something like that. So to a point that's defined as zero electric potential. So we can call this zero electric potential. And if you create a higher electric potential over here, charges will want to go from the higher potential to the lower potential. It's just energy, right? You want to go from high to low. So if you put yourself in a conductor, all the charges on the conductor will move to the outside of the metal very quickly. And they will try to cancel out the electric field that's in the space around you, so you can put your hand on the inside. There's no electric field inside of the conductor. So there is no potential difference between you and the inside walls of the conductor. Reach your hand around the bar, however. And now you're over where there's a high amount of charge. And that charge would like to get to a lower potential. And if your feet are connected to ground, so zero electric potential, guess where the charges are going to go? So you never want to put your hand on the outside of one of these things. They're called Faraday cages, named in honor of Michael Faraday. And so that's how, so a car is a Faraday cage. If you're ever in a lightning storm and you have a metal body in the car or a metal frame in your car, it's okay to be struck by lightning in a car. Don't get out of the car in a lightning storm and try to find a place you think is safer. The car is actually very safe. You might blow your tires out because that lightning strike will go through the car and probably out through the metal rims of the tire and jump to the ground. That can blow your tires, but that's about the worst that's gonna happen. It's not gonna electrocute you. Just don't put your hand outside the car during a lightning storm or you're just begging for trouble. So, all right, so anyway, I'll send the link around to that. And Allison has one of the devices at the end here. It's a little, it basically creates an electric potential difference. And there's a gas inside of it that can be easily ionized. And when she touches it, she changes the potential. And the lightning will jump from the, the charge will jump from the inside glass container to the outside glass container. But it's an insulator, so she's perfectly safe touching it. What she's effectively doing when she touches that is changing something called the capacitance of the system. And we're about to start studying that next week, okay? So, yeah, so you can play with that to your hearts content. I'm gonna hand out the quizzes. Now, yellow quizzes don't mean anything. It means that a colleague of mine left yellow paper in the copier. And I didn't feel like killing more yellow paper or white paper trees to print these quizzes. So, because you're not special, you're not getting singled out. It doesn't mean anything. All right, a color, any symbols on things like this? All right, well, let's go ahead and get moving today. We're gonna have, I think, lots more time for problem solving today after I go through an example problem. So, again, some concepts to review, I'm just beating these to death. There are positive and negative electric charges. They're the sources and recipients of electric fields. Fields are vectors, just like forces are vectors. The charges exert forces on one another, and that's the whole base of everything we're beginning to study right now. Charges respond to the influence of external electric fields by changing their state of motion, right? You have a force, and if you have mass, you get acceleration. That's just Newton's stuff, all right? So those charges are accelerated by the fields, and that correspondingly alters their position and velocity over some period of time. So equations of motion relating v and changes in position and time. And you're gonna get homework three, it should be posted on Wiley by now. And you're gonna get lots of exercise and that stuff, okay? So your next assignment, a little bit more reading to do, but it's a weekend. So 24-3, 24-4, and 24-7, we're cherry picking our way through chapter 24 here. And I've got about an hour long video to accompany that, okay? Homework three's assigned, I said that, grand challenge problem, okay, team names. So I've got like something slightly over half of the teams have chosen a name. So getting close to the deadline, I'd like a name like today from your team. You're supposed to have met once in the last week with your team. If you're having problems meeting, I need to know. Okay, so don't fail silently, fail loudly, so I know what's going on. I need to know these things. And I'm not your parent, I'm not the NSA, I'm not keeping close tabs on you. So it's up to you to tell me if there's a problem, all right? You're supposed to choose a lead editor, an enthusiastic and stupid young person who's willing to organize the rest of the group. A leader, right, okay? So that's your job. Discuss how ideas in the course so far might inform a solution. Might inform a solution to the problem, you need to come up with three eventually. But we're not even a third of the way through the course yet, so. All right, so please, I want minutes, notes from your first meeting. I'd like to know what you discussed, okay, who was present. And I expect those to come from the lead editor. So the first task for the lead editor is, if you've ever had your meeting and you didn't take notes, jot down what you remember, ask your teammates to add things they remember, and then send them to me no later than 5 PM on Monday. If I don't get minutes, then I'll know who to start contacting. So this is my attempt to be a little parental, I guess, okay? And find out what's going on. So I will, don't make me email you, but I will email the whole team, all right? And so where are we in the team names? We've got Team Tachyon, Bodies Electric, Team Foxtrot, and the newly crowned force field, okay? So we have three to go, Charlie, Delta, Echo, WTF, let's go. Whiskey Tango Foxtrot, I want to know what's going on here, all right? All right, so we did quiz time. That was fun. Now let's go to the answers to the quiz questions. Okay, so what's the relationship between work W done by a conservative force field on a charge and the change in potential energy of the charge, Delta U? One, two, three, or four, is it one? Anyone say one? Raise your hand high if you think it's one. How about two? Raise your hand high if you think it's two. Work equals change in potential energy divided by charge. How about three? Raise your hand high if you think it's three. Awesome, that's great. All right, four. Boy, I just gave away what the answer is. All right, four, anyone want to be less awesome and say four? No? Okay, thank you, Thurman. You're our brave soul. No, it's fine, this is how we learn, right? So, all right, so we can take a look at this. So I tried to beat this to death in the video. The book tries to beat this to death too. Work is a form of energy. Change in potential energy is a form of energy. So the units for those things would be joules and joules, respectively. This thing would have joules equals coulombs times joules. That doesn't work. Joules equals joules divided by coulombs for two. That doesn't work. So you're kind of down to one in three. And this is what I want you to know. I want you to know that the work done by the field is the negative of the change in potential energy. Why? Because fields, when they do work on objects, they take them from regions of high potential energy to low potential energy. That's what they do. Okay, conservative force fields, that's what they do. So if this is a big number and this is a small number, the small number is the end result. The big number was the first result. So final minus initial is a negative number, but the field did positive work. So you got to relate those things by a minus sign. So that's one of the tricky little minus signs to be committed to memory or figure out how to figure it out if you don't remember that sort of thing. So that was three. Which of these is the definition of the electron volt? A new unit of energy we've introduced here. You probably encountered this in chemistry. I hope, since it's a lot easier to talk about chemical energy processes in electron volts than in joules. So one, the electron volt is the charge carried by an electron. No, okay, two, the electron volt is the energy of an electron in the ground state of a hydrogen atom. No, okay, three, the electron volt is the energy obtained when the electron in a hydrogen atom is released from the atom. That's a volt. Okay, no one for three. All right, four, the electron volt is the energy gained by an electron moving through the one volt electric potential difference. Raise your hand if you thought that was the correct answer. If you know if it was by process of elimination, obviously most of you thought that was the correct answer. Okay, yep, good. By the way, for all the boo-hooing at the end when I gave the quiz question answers last time, for everyone it was like freaking out, I didn't get that. On average, the class did a lot better on the last quiz than the second to last quiz. So I don't know what your standards for being upset are but you did really well overall on the last quiz. All right, so what's the definition of electric potential difference, delta V? This is what we saw being explored in real time in that music video at the beginning of the class, okay, which you missed if you rolled in five minutes ago. I'll send a link around. All right, so what's the definition of electric potential difference delta V? Is it the work done by the field? Number one, is the change in electric potential the work done by the field? Okay, how about the change in electric potential? Is that the change in potential energy per unit charge? You thought that was it? Okay, all right. Is the change in electric potential the negative of the change in potential energy? In other words, is it equal to work done by the field? No, okay, good. You didn't fall for one, you didn't fall for three. All right, and how about delta V equals Q times delta U, the charge times the change in potential energy? Nobody? No one courageous enough? All right, so the answer is two, okay? So it's the change in potential energy per unit charge. An electric potential exists in space independent of whether there's a charge there or not. Just like electric fields exist in space independent of whether there's actually a charge at that point in space, that point P we keep looking at in problems and so forth. This is, it's a little tough to visualize, but I like to think of the electric potential as kind of a well or a barrier that has to be escaped from or overcome. And actually when you look at pictures of this in the book, for instance, or if you look on Wikipedia, things like the single point charge electric potential, they'll have a 2D surface and a deep like black hole like well in the middle of the surface and deep at the bottom of that well in potential energy space is the charge that's causing the field. And so you can think of another charge coming along the surface, hoo, hoo, boo, and then it falls in the hole, right? So it's now caught in the potential, the electric potential of the charge and it's gonna take energy to get it back out again. So you can think of electric potentials as wells into which things can fall or barriers that have to be overcome. And in fact, going forward in physics for one more to continue studying it, when you do quantum physics, that's exactly how you think of atoms. You think of the Coulomb field as a barrier to be overcome. And if you want to liberate an electron from an atom, you've got to put in a corresponding amount of energy to get it out of the well. And that's in fact, the whole language of quantum physics, all right? So, let's talk about solving problems. These may seem not so bad today. I don't know, kind of depends on your comfort level with energy and work and potential energy, okay? But some basic things to keep in mind when you're solving problems involving energy. Work is a form of energy, whether it's work done by a conservative force field or work done by an applied force, for instance, me, okay? So I'm an applied force on converting breakfast, chemical energy, the ATP in my cells is getting converted into electrical signals to my body, from my brain to my arm, okay? And then I can do work against the field. So this is costing me energy. So now, if the field wants to do work, okay? Then all I have to do is let it go. I just have to, like the song from Frozen, I just have to let it go, okay? I hear somebody whistling one of the songs from Frozen in the hall the other day and not even one of the popular ones. So that's a devoted fan, right? All right, so all I have to do is let it go and this is costing me energy, like holding this thing here and I can feel the tension beginning now in my muscles. It's very hard to hold your hand up, even if there's not a lot of weight in it for too long. Interesting, if you wanna try this as a college, a fun college thing to do, try seeing how long you can hold your arm up in the air before basically your muscles freak out and your brain's like, stop it, cut it out. It's not as long as you think it would be. All right, so all I have to do is let go and now I let the force field do its job and it does work positive work. So that's costing the field energy to do that. I'm doing positive work to work against the field and it's having work done on it. So if it starts at rest and then I do work and I bring it to rest at the end, the work that I did as an applied force is equal to the negative of the work done by the field. I have stored the energy I gave up, WAP, in the field. So that becomes negative W field. And then if I want the field to do work and change this potential energy to kinetic energy, I just let it go, that's it. That's the gravitational analogy that you can think of for all this. The only difference really between electric fields and gravitational fields is that there are two kinds of charge in an electric field that can respond to the field. Negative charges will go against the direction of the electric field arrows. Positive charges will go with them. If there was negative mass in the universe, we could start it on the surface of the earth and it would fly off into space to get as far from the center of the earth as possible because it would respond in the same way that negative charge does to electric field lines. Gravitational field lines are all pointing in toward the center. If electric field lines are all pointing into the center and I put a negative charge here, it would fly away from the center of the electric field lines. So it wants to go toward where the positive charge appears to be in the field. Okay, so then we have the work done by a uniform electric force. So work is fundamentally force times displacement. If you have a nice uniform force, that's the same everywhere, you know, it's a nice maybe like, I don't know, one Newton or so force there, maybe probably less than that, I'm not that strong. All right, so you do a little, put a little constant force, do some constant acceleration on this thing and then stop. If you have a uniform force, then all you have to know is the displacement and you have to know the direction of the force and the direction of the displacement. All right, so keep in mind for negative charges that it's gonna be in the same direction or opposite directions. So you have to be a little bit careful with that. But then you get the work from that and get joules out of that. So you can figure out the work done by a conservative force field just by knowing the force F and the displacement Delta R. Be careful. In the case of a point charge electric field, which we'll see in the next lecture video, you have a non-uniform electric field. So you can't just say, oh, the force is constant from near the charge to far away from the charge. That'll get you into trouble. You have to use a little calculus to figure out what the work is. So it's not too bad. It's not the worst thing in the world. So it's an easy integral in the elements. All right, so the change in electric potential. All right, so electric potential, is Delta V, which is the change in potential energy per unit charge. So if you have a charge Q and it experiences a change in potential energy in the electric field, you can calculate the electric potential difference between the two points in the field. So if there's a change in electric potential between two points, I initial and F final, the, let's see. That's just to tell you whether or not there is a charge there. This is related to changes in potential energy as I've written up on the board here. So those are the basic things that we need to attack problems. So let's take a look at a problem. 9-volt battery on the tongue. Have you ever done that before? Don't be shy. I've done it like a thousand times. Parker, thank you. I knew there was a brave soul in here who's dumb enough like me to do stupid things like this. Actually, if you don't mind a tiny little bit of uncomfortable excruciating pain, then this is a great way to test to see if a 9-volt battery has any juice left in it. So all you have to do, 9-volt battery, there you go. It's one of those square ones or rectangular ones. And the reason it's so easy to test with the tongue is it's got its two positive and negative terminals. The positive terminal limits positive charge. The negative terminal limits negative charge. You hook them together and the charges will flow past each other. They're on the same side. So they're conveniently, roughly tongue sized. I don't know who thought this was a good idea, but please don't do this with a car battery. Actually, the good news about a car battery is they've made the terminals further than your tongue can typically reach unless you're in the band of kiss or something like that. So is that ace freely? Is that his name? Something like that. Gene Simmons. Sorry, Gene Simmons. What do I know? Look at me. I don't know anything. It's technically my generation too. That's sad. All right, so you can just touch that 9-volt battery right to your tongue. And Parker, what happens? Yeah, it doesn't feel like too good. Hang on. That's worth quoting. Parker, 2015, it doesn't feel like too good. That is like understatement of the year. It won't kill you, all right? I'll tell you a little bit more about the biology of touching a voltage to your salty, moist tongue in a moment, all right? But yeah, it's sort of, when I do it, it might be a little different for different people. But I get the taste of blood on my tongue. So it gets that coppery taste. That could just be the metal in the terminals. But you are running electric charge through your taste buds. So that may have something to do with it as well. I don't know. But it feels like somebody just bit you on the tongue for burning, biting sensation. And at least you'll know the battery's still good, right? So we're good enough to make your tongue hurt. So again, if you want to figure out if that thing still has any juice left in it, you just shove it on your tongue. It won't take more than a second, I promise you. And when you do this, something like a charge of negative 0.18 coulombs will be driven through your tongue in about one second, assuming you can hold it there that long, OK? So the question that I would like to answer is, for instance, what's the kinetic energy gained by the electrons? These are negative charges. So we can imagine this is all made of electrons at its heart, lots and lots of them. And actually, it might even be interesting to calculate how many electrons that is. Maybe we'll do that just to show you how you get to numbers like that. Of course, I've left my chalk someplace. There we go. All right, so the first thing we want to know is what's the kinetic energy gained by electrons during this process? So let's start with kinetic energy. That's the thing we want. We want to know the change in kinetic energy experienced by the negative charge. Negative charge, that's given to us. Q equals negative 0.18. Did I say 0? No, it was 1.8 coulombs, OK? We are told that this process takes a time, we'll call it delta t, t final minus t initial of 1 second. So conveniently already in meters, kilograms, second units. And coulombs is our new unit that also fits nicely into the MKS system. So we want to figure out what that delta k is. And to do this, we have to start relating kinetic energy, which we don't know right away, to other forms of energy. So to do that, we need to take advantage of the fact that we have to assume that this is a closed system that is energy in total is conserved. So this is an assumption. We won't need to revisit it, but in the real world, you might have to be careful with things like this. But that's technically an assumption. I've assumed that the battery tongue system is closed. Energy can't flow out of the system. That's actually maybe it would be worth pausing and thinking about that for a second. Is there a mechanism by which energy can be moved out of the tongue? All you medical types? How in general is heat moved around in the body? Yeah, Thurman. Contractions of? Muscles, yeah. And the flow of? Blood. So the heat in your brain is regulated by blood flow. That brain blood barrier is really important. For instance, if you start outside on a hot day and you didn't have a normal blood flow in your head, you pass out because your skull would heat up too much and essentially your brain would start to shut down, which is not good. This is also how you know, by the way, that mobile phone radiation, even though it's microwave radiation, can't cook your brain because, well, you pass out. If your blood flow in your brain ever stopped while you were talking on the phone, there's a chance that you might heat up a part of your brain over here and then it could start to shut down. But the sun can do a lot more damage. The sun delivers a whole lot more heat per unit area than a mobile phone does, which runs on a tiny little battery. So yeah, the tongue has blood flow in it. You cut it, it bleeds. So you know there's blood in the tongue and that blood is helping to redistribute energy in the body. So in fact, this isn't really a closed system. If you dump energy into the tongue, it can be pulled away by the blood. But we're going to just ignore that for now. So that's an assumption, closed system. And you can stop and think, well, is that really true? Do I have to cut the tongue off first? You know, then what's the point? Because you're not going to feel it anymore. You're going to be too busy screaming because your tongue's just been cut off, right? So yeah, so yeah, I think about these things, right? Okay. All right, so we need to relate forms of energy that might be present here. We need kinetic energy. Now, we're told that the battery provides a potential difference of nine volts. So this is this new unit, volt. And as a reminder, this is joules per coulomb. Volts are joules per coulomb. So if you ever want to check your units and you're not sure if you've got the right thing and you've got volts somewhere in there and you're like, oh, what cancels out of this? I know, I think I've got like meters and kilograms and a bunch of other things. Meters, kilograms and seconds are of course buried in joules. A joule is a kilogram meters squared per second squared or a Newton meter, okay? So Newtons are units of force, so units of force are buried in there as well. And then coulomb, coulomb is just a new unit. It's not related to anything else. So if you need to get rid of coulombs, that's how you might find a hidden coulomb somewhere in your units, like in volts, right? So this is nine joules per coulomb. That means this battery is capable of delivering nine joules of energy for every coulomb of charge that it has to do work on. And so you can already see there is probably a path to the answer from here. If the battery can give nine joules per coulomb of energy, there must be a way to relate charge and volts and get energy and then we can figure out kinetic energy. And of course the answer to that is well yeah, first of all, that relationship is through conservation of energy. I'll stop embarrassing Parker, I feel bad about it already. So we want a change in kinetic energy. Now changes in kinetic energy in a system are related to work done by the field. So this battery supplies an electric potential difference and that means it supplies some kind of electric field that can drive charge. So we know that there's a field involved here and the change in kinetic energy will definitely be related to the work done by the field. Any work done by the field will turn into kinetic energy for the charges. So we've already got one relationship we can write down. But we don't know the work done by the field, so we're not really getting anywhere. What is the work done by the field related to? Well that was on the quiz. That's the negative of the change in potential energy. Aha, a change in potential energy, that can be related to changes in electric potential. So this is just going to be equal to the negative of delta V over Q. All right, well this isn't so bad, right? Delta V was given to us, it's nine volts. Q was given to us, it's 0.18 coulombs. So all we have to do is plug and shove now. We can shove some numbers in and get some answers. So remember Q is a negative number, negative 0.18 coulombs. Delta V is 9.0 volts. So the change in kinetic energy for that charge will be 1.62 joules. All right, so you plug in the numbers, you do. No one caught that. I have to put a dollar in the jar. Oh, you guys are going to get donuts so fast. This is terrible. All right, hang on. That's six already. Okay, so we're like $2 away from you guys getting a dozen donuts. I guess I have to wait until I get three dozen because there are a lot of you in this class. All right, so what was my mistake? What is delta V equal to? Yeah, delta V is delta U over Q. I had this reversed. I said this was delta V over Q equals U. So delta U is equal to Q delta V. So bad algebra on my part, doing it mentally. I should never do that, okay? So this should have been Q delta V. Okay, all right, so we have a negative sign times a negative charge will give us a positive number times another positive number will give us a positive number, okay? All right, so the change in kinetic energy, we expect the charges to speed up when they go through the electric potential difference. After all, the field is doing work on them. They're gaining kinetic energy. We expect that changing kinetic energy to be positive but there are no other forces acting on the field or on the system, okay? So yeah, so in the end, you get a changing kinetic energy of 1.62 joules. So let's put that in perspective and that's sort of the second part of this. What's the power delivered to the tongue? Assuming no other losses of energy. All that energy goes into the tongue. So how do I calculate power? What's power defined to be in physics? Work over time. Work over time, excellent. That's its most basic definition. So it's work over some change in time. So whatever the work is during that period of time, well, we know the work. It's the change in kinetic energy over the change in time and I made the time like super simple, one second. All right, so if you just get back 1.62 joules per second, which is 1.62 watts. A watt is a joule per second. Watt equals joule per second. Okay, just a refresher on these units. All right, so basically, plugging a nine volt battery into your tongue is equivalent to plugging an almost two watt light bulb or licking a two watt light bulb. That doesn't sound terribly pleasant, but yeah, okay, two watt, who cares? 40 watt might hurt. I mean, 40 watt's gonna burn for sure. Two watt doesn't sound so bad, but if no energy is leaving the system, that means that basically 1.62 joules is being delivered to the tongue every second. So that's a new 1.62 joules being delivered by the battery to the tongue every second. Does anybody remember how much energy it takes to heat one gram, one cubic centimeter of water, one degree Celsius? 4.2 joules, 4,000 to 40,000 or something like that. 4.2, roughly. Yeah, 4.2 joules is all that's required to take one gram of water up by one degree Celsius, okay? Now, I'm more used to thinking in Fahrenheit. So human beings can detect temperature changes of basically two degrees Fahrenheit very easily, okay? So a change of one Celsius or two Celsius, you're gonna start to notice that pretty fast, especially if it happens every second or so. So part apart from the electrical nervous system effects that are happening in your tongue, you're basically running an electrical current through your tongue because your tongue is fundamentally an electromagnetic object that it's hard, that's already gonna cause problems. It's gonna make your nerves go crazy, okay? And I'll come to exactly what pain threshold is in a moment. But in addition, you're heating up your tongue because if all that energy's being dumped into your tongue, even if your pain receptors are shot, you're gonna cook your tongue eventually. You're gonna keep raising the water temperature in your tongue, all right? So this is just a bad thing to do for a long period of time. So lick the battery, if it hurts, stop, put it down, walk away, okay? Go get a drink of tea or coffee or water or something and rinse the taste out of your mouth, all right? So yeah, this does hurt. If you're gonna try it at home, please do this with extreme caution. Now, let's think about biological systems and electric current flow. So current is something we haven't talked about yet, but we're about to talk about it in the coming weeks. So electric current. It's denoted by the symbol I, little I or big I, depending on how many I's and I hats we have floating around, I may change notation, but basically the letter I is its symbol and it's the amount of charge that flows, say, past a point in space, delta Q, divided by the time during which that delta Q passes that point. So it's charge per unit time. Coulombs per second. Does anybody know how much coulombs per second, which, by the way, gets its own name? I'll revisit this again later. This is known as the ampere after a physicist will meet later, or just an amp or just A, okay? So it gets, you know, an A is an amp, it's an ampere. Does anybody know how many coulombs per second or amps are required to induce pain in a human being? No one? How do they teach you? Yeah, they're filming, okay, great, yeah. Seven. Seven amps. Okay, so somebody was courageous enough, all right? Seven amps will not only stop your heart, it will set you on fire. So all we wanna do is induce like pain. What was that? Who said that? I said milliamps, but I don't know. Yeah, great, okay, let's just say, we did that, that's great. So order of magnitude, 10 to the minus three amps or 10 to the minus three coulombs per second, that's it. It takes five to eight milliamps to cause pain in the human body. Now let's think about this a second, okay? This is 0.18 coulombs per second that the battery is driving through your tongue. That's 0.18 amps or 180 milliamps. So it's no wonder this hurts so much. This is well above pain threshold for a human being. In fact, by the time you get to about 11, 12, 13 milliamps, muscle contractions become so severe that if the current is flowing in one hand and like across your chest and out your leg or out the other hand, you basically can't let go. If you've accidentally grabbed ground or zero electric potential and high electric potential and that thing is capable of driving a lot of current through your body, above about 11 milliamps or so, 10 milliamps, you can't let go anymore. And so somebody's gonna have to physically knock you off of whatever you're holding, right? So this is why you need to be careful with things like the electric potential differences in these wall sockets. What are they? Electric potential differences involves roughly. Anyone know what comes out of the wall plugs? Sam? Maybe is it 120 or? Yeah, 120, 120 volts, 120 volts. In Europe it's like 240 or so, 220 to 40 kind of depends on you. We'll explore volts and what that means for the ability to do work very shortly in the course when we get into electric circuits. But basically every one of these boxes, every one of these pairs of prongs that you have on a wall or in a box like that, that's just an electric potential difference waiting to be utilized. The electric potential difference of 120 volts is there independent of whether or not we plug a lamp into that thing and make it do work, give off light, that sort of thing. Okay, that's all the stuff we're gonna explore when we talk about circuits. But the potential to do work is always there. So if I were to take this and plug my finger into one of these and shove this in the other, I would get 120 volts delivered across my body. And if you take into account the resistance of the human body, most of the currents gonna flow over your skin. And if you're a little sweaty or a little moist and salty on the surface of your skin, that's a pretty decent conductor. And it will cause muscle contractions. So since the voltages there change from positive to negative 120 times a second, you'll get like pulsed muscle contractions. It's really quite unpleasant. So I've done that before. I've touched 120 volt a few times in my life. Once accidentally in middle school, I was taking an electronics course. And then after that, accidentally, while I was taking apart a amplifier. So it's not fun, it hurts, it burns. And in your heart races afterward, because you're basically putting 11, 12, 13 milliamps of current across your heart. So you get these muscle contractions and spasms. And if you're holding tightly when you experience the voltage change, you may not be able to let go. So it's really important if you're ever working on exposed wires in a guitar amplifier or some other device like that and it's plugged in, unplug it or have a friend stand nearby and keep an eye on you in case you seize. Because they might have to knock you away in order to keep you from having basically game burned. So what's gonna happen is if you hold on to that long and if you'll get burns. If your heart's got a problem, it might stop eating or it might fibrillate and that's bad too. You don't want your heart rhythms to become irregular because then you will die, right? I have a question. So, yeah? Yeah, what is it 120 here and 240, like 220 in Europe? I don't know the exact reasons, but I suspect it's just a choice. It doesn't have any benefit like at hand. So it does actually. So for instance, if you buy an electric tea kettle in Europe that's rated for operating on their 240 volt, 220 volt system. Well, it will work here, but it has a much beefier heating element because that 240 volts is capable of driving a lot more charge through the same amount of conductor. So basically it can increase the work per unit charge by factor of two over what we get in the States. You can boil water a lot faster with an electric tea kettle in Europe simply because all it is is it's a metal heating element that's plugged straight into those prongs. There's nothing fancy about those heating elements unless they've got the computers in them that switch it on and off when the water boils. But most of them just have mechanical switches and they're just a loop of wire plugged straight into that thing. It's not fancy at all. But if you look at the heating element for like a tea boiler that you can dip in a cup of tea or at the bottom of an electric tea kettle, it's beefy. It's maybe almost three quarters of a centimeter diameter metal. And that's because it has to be capable of carrying a large current of something like four, five, six, seven amps through it. And that many Coulombs per second across that electric potential difference, you calculate the power, you can figure out how long it'll take to boil water by knowing how much energy it takes to increase water by one degree Celsius times the number of grams of water in the kettle. You can do all that stuff with this and you can calculate how long it'll take to boil water and it cuts it down by at least a factor of two. So you can boil water at least twice as fast in Europe. If you take an American hairdryer, which is also just a heating element with a fan in it, so it blows the heat from the heating element in your hair to dry it, or a tea kettle and you plug it into a European socket without a voltage adapter, what happens to it? It burns. It burns. Yes, it melts. So I recently had to, okay, this is being recorded. I recently had to talk down my spouse who is a physicist as well in this department. We were traveling together for conferences in Europe and the hairdryer in the hotel room we were staying in was just not functioning. And so she was getting really upset and she's like, that's it. Our voltage adapter had broken. And she's like, I'm just gonna plug this in with a wall plug adapter into the wall. I went, no, no, no, no, no, no, no, no, no, no. She's like, what's the worst that can happen? I'm like, it will melt. It is not rated, if I looked at the plug, it'll say it's rated for 120. If you plug that into a 240 volt outlet, at best it will melt and you'll get a fire, okay? I don't wanna think about the worst flames come shooting out when the fan turns on or something like that. That would be terrible. So I had to actually talk her down from, please don't plug that in, please. Well, we'll find a hairdryer that works in Europe, I promise you. They must have them around here someplace. They have stores in Europe we can find one. So these are things to be aware of. If you ever take something that's rated for a low electric potential difference like 120 volts and then you subject it to a high electric potential difference, there's no guarantee that the metal in there is rated for handling that kind of work per unit charge. And so you, again, best case scenario, you'll start a fire probably. So overvoltages and overcurrents are often things that start house fires. So you should never plug eight power hungry things into the same wall outlet. Because basically what you're asking that wall outlet to do is provide more coulombs per second for all of them. And you'll see why it's not more voltage, it's more coulombs per second. And the question is, can the metal in the wall, can the wiring in the wall handle that many coulombs of charge moving every second through the volume of the conductor? And if the answer is no, you'll start a silent house fire in the wall of the house which can burn the house down. So these are things to be aware of. So all this physics has practical implications. Okay, speaking of fibrillating, let's do a problem, all of you, all right? So portable defibrillator, these are extremely common these days. Laws have been passed, good Samaritan laws so that if you ever injure somebody by accident while trying to save their lives with a portable defibrillator, you can't be sued. So there's a whole legal domain to this. There's a whole medical domain and there's a physics domain, they all intersect. So this is an example of a portable defibrillator. So that tells you roughly a human knuckle-sized right there. So it's not that big, you can carry it around and it's got the classic paddles you see in TV or if you've done rotations or shadowing in a hospital, you may have seen one of these operatives. The ER or whatever the current medical show is, help me out, current medical show, houses off the earth. Yeah, Grey's Anatomy, that's it, that's the great, I was like, I know, everybody in my family watches it except me, what's it called, what's it called? Grey's Anatomy. All right, so yeah, no clear and then everyone steps back and boom, okay. The idea is that this is capable of delivering a large electric potential difference across your chest. So if you look on medical guides on these things or God forbid Wikipedia, you put basically one paddle here, one paddle here and you pull the trigger, this thing over a period of seconds will build up a huge charge creating a huge electric field and then it can deliver that charge through the electric field across your chest. The goal is to reboot the heart. So it's sinus rhythms have been interrupted, these are the regular rhythmic pulsations of the heart, they're electrical in origin and so by dumping that much charge across the heart, you're basically forcing the natural pacemaker that's in it to reboot. All right, so we're just wetware computers, right? And sometimes you gotta hit control all delete and that's it, that's control all delete for the human heart right there. All right, so a couple of things to know, that device is typically capable of moving something like 145 micro coulombs through an electric potential difference of 2.3 kilovolts. Okay, so just to chop these down so they're a little more readable. The charge involved here that it can move is negative 145 micro coulombs and the potential difference involved here is 2.3 kilovolts, so 10 to the three volts. So two questions, what's the change in energy of the charge as it moves across the potential difference? So it goes from that paddle to that paddle across your heart, what's the change in energy assuming no losses of energy in the body just drives right through the body and comes out the other side, okay? So that's an assumption you're free to make. What's the change in energy across the potential difference? And then finally, if we assume the electric field that's now across your chest momentarily is uniform in strength and direction, okay? So it's uniform. What is the strength of that electric field if the paddle's at 25.4 centimeters across? So it's roughly 10 inches, okay? Have fun, pair up, triple up, work together. I expect to hear talking, if I don't, I'll give you more things to calculate, so.