 We're looking at static electricity. What's going on when you rub a balloon on something like your hair or a sweater? Well what you're doing is electrons are free to move and this is why we said that Ben Franklin named the charges wrong. You're kind of nicer if positives were free to move that would just mathematically be a little more convenient. But if you rub a balloon on a sweater it gathers some electrons from the sweater. Now as I move close to this wall keep an eye on the charges in the wall. Now right now the wall is neutral but like charges a repel unlike charges attract and as I move closer and closer you're going to notice the electrons want to try and get far away if they can and the positives technically don't move but if electrons are moving to the right mathematically that's the same as positives moving to the left so we kind of say and the positives want to get closer even though that's not quite what's happening once again this is why Ben Franklin should have named the electrons positives anyways and we end up with the negatives right here being attracted to the positives right here and the attraction is enough to overcome the repulsion force of the electrons right there and you can get more electrons the more you rub if you get lots of electrons you notice it very very very much that's the difference between you rub it a couple of times it barely sticks to the ceiling you rub it a bunch it really sticks on the ceiling and it works better on a really dry day because on a wet day there are water molecules in the air and water is a polarized molecule it can actually snag an electron or two along the way which is why it's harder to build a fistatic charge on a damp day like today okay what if we have two balloons so I can rub some electrons here oh what's going on now so here is your static electricity day if you pulled something out of the dryer it either has a shortage or an extra number of electrons and so if there's something else that is either neutrally charged or oppositely charged it wants to attract right the real question is this all right mr. duick I've done this a bunch of times I've actually played around with this I've noticed how this works the question is what the heck is it that's pulling this thing over won't see electrons well no no no they're not touching there's a big gap between them and we have something in physics in physics we say you can't have action at a distance if I want to make Tyler move I have to somehow come in physical contact with them so what the heck is going on well this comes back to the same answer for why this golf ball drops so when I drop this golf ball what's pulling it to the ground but it's not touching me or how can the earth pull it to the ground what we said was between here and here there's something that we call the gravitational field it's invisible but it's the gravitational field that's exerting a force on this golf ball it's an electric field an invisible electric field that's exerting a force on the balloon to pull it to either the sweater or the wall we say you can't have action at a distance if a negative and a positive charge are being pulled together they are touching each other they're each sending out an invisible electric field they're touching kello it's just that our eyes can't see it but just as though if I'm in a tug of war I'm pulling on you with the rope they're pulling on each other with a electric rope an electric field it's invisible it's a really key concept and in fact that's what today's lesson is going to be about so since none of you did the homework I won't take questions about section one gee do you think I'll put questions from this on the test what with nobody having done the homework would that be kind of a teaching moment that mr. duet might take advantage of I'll let you think about that or you can certainly on Tuesday ask me questions from both of these and we're going to move on to lesson two which is electric field and I get to show you some pretty cool stuff maybe my best toy just a reminder we said last day that charge is quantized that was the fancy word for it comes in chunks what was the elementary or the fundamental charge how big was it you remember it's on your formula sheet but I'm also encouraging you to practice finding it so you know where it is what was it no what was the elementary charge 1.6 times 10 to the negative 19 that's the charge on a proton what's the charge on an electron too slow tyler same but negative okay that's why we call it the elementary charge it's a charge on a proton or an electron okay now we said we measure charge in coulombs one coulomb is huge one coulomb is so big it would probably kill you if you shocked you so usually we use micro coulombs that was that symbol mu times 10 to the negative six read along with me example or lesson two gravitational field reflects the effect that a mass has on the space around it if another mass is placed in that gravitational field it will experience a force so here you have for example the sun all by itself it's sending out to the edge of the universe a gravitational field this gravitational field gets weaker and weaker and weaker the further you get away but it's pretty strong I mean it's enough to hold Pluto in orbit so it's feel pretty strong the field only becomes a force when you place another mass nearby the field is what exerts the force on this mass you say that again because some people were giggling and this is important the field is what exerts the force on this mass so fill in the appropriate relationships universal gravity was big g big m little m all over r squared what was the definition of gravitational field was on your test yesterday big g big m over r squared what we really said was this if big g big m little m over r squared is the same as mg here on earth we really said oh i guess g is big g big m over r squared that was not on your sheet it was a logical outcome of the universal gravitation formula but also gravity is also as a ratio it's also the force divided by the mass you can get it like that force divided by mass or you can get it as an equation and remember the units for gravitational field were well what's force measured in newtons what's mass measured in so you can do it in newtons per kilogram and it was also because it was also an acceleration meters per second square okay for example if you wanted to find the gravitational field strength that far from the earth's center you would say this gravitational field strength is big g big m over r squared it's 6.67 times 10 to the negative 11 the earth's mass is 5.98 times 10 to the 24th this is the big letter m because it's the planet that's sending out the gravitational field divided by 8.38 times 10 to the sixth squared what's g that far from the center of the earth can someone crunch the numbers please we're far from the center far from the earth's surface i'm expecting an answer less than 9.8 5.67 5.68 which went 5.68 units newtons per kilogram i can also calculate the gravitational field this is if i know the planet but i don't know the force i can also calculate it if i know the force and i know the little test mass or the satellite mass that we're placing near the planet in other words it says find the gravitational field if a 10 kilogram test mass has a weight of 750 here what they want me to use is this here what they want me to use is this they want me to go 750 divided by 10 and they want me to realize that the gravity field that we're talking about here is 75 newtons per kilogram probably jupiter or something like that so there's two ways justin that i can find the field strength if i know the great big planet i use big g big game over r squared if i don't know the planet but i know the satellite at that location i can figure out the force if i they tell me the force on that satellite i can figure out its acceleration which also happens to be g and newtons per kilogram see the see the difference between these two approaches if you know what's causing the field we use this if you know what's in the field at that location we use this big deal huge deal next page we're going to define electric field in the similar manner we say that the electric field reflects the effect that a charge has on the space around it as soon as you put a charge anywhere in the universe it's sending out an invisible electric field in all directions and if you place another charge in that field it will experience a force if they're like charges it will experience a force away if they're unlike charges it will experience a force toward in fact unlike you can almost think of this as gravity it wants to fall to the planet the only thing is because there's two types of charge it can also fall up away from the planet we talked about that last class saying why aren't there two types of gravity we're looking be nice says fill in the appropriate relationships the force of electricity we said it was k big q little q over r squared where k was nine times 10 to the ninth in gravity it was 6.67 times 10 to negative 11 big q is the big planetary charge little q is the little tiny positive or negative test charge and r squared would be that distance squared what was the definition of electric field then well the same way that gravitational field said there's your little mass this is your field for electric field capital e it's actually look up for a second please we found gravity as a ratio by dividing the force by little m sorry maybe rephrase that we found gravitational field as a ratio by dividing the force by little m what's the equivalent of mass when we're talking about electric force charge as a matter of fact we're going to say the electric field is f divided by little q and what we're really saying here justin is that little q would cancel the electric field and the symbol for electric field is capital e don't confuse it with energy it's equal to k big q over r squared where k is nine times 10 to the ninth big q is the charge that's creating the field and r is the distance from q to any location that we're trying to figure out the strength of the field at a field point in meters it's a scalar equation we're going to define electric field distance in a bit electric field as a ratio electric field equals f over q take a look at your formula sheet and see if you can find this one it's good to know where it is because all of these equations look the same so i want you to start training your eyes where to look find the electric field see it i think it's the very very right side yes i can't remember is this one also on there or not oh it is oh okay a few years ago it wasn't so they must have added that actually a few years ago i think all they gave you was this they didn't give you this one and you had to realize that this was that and when a q cancels it is that this is the equivalent for gravity jordan of little g in other words the 9.8 we experience on earth that's how you can figure out the equivalent for electricity for electric field strength what are the units for electric field well what do i measure force in newtons per what do i measure charge in coulombs units for electric field or newtons per coulomb oh do you remember gravitational field actually had two units there was newtons per kilogram and meters per second squared there's going to be a second unit for electric field as well but that's later on turn the page as was the case with electric force electric fields cannot be negative we don't put a negative sign in even if your charge is negative kellen we don't put a negative sign in in fact a lot of the time in a probably in a university textbook you'll see an absolute value sign around the chart to remind you don't put the negative sign in so at a certain point in space a negative point five micro coulomb charge experiences a force that big towards the south find the electric field strength at this point so dylan i've given you two equations for electric field i've said it's this and i've said it's this which one do you use when and this you'll just have to know i'm using a big q this is the planetary charge that's causing the electric field here little q this is the charge that's at that location the satellite around the earth if you will so what you need to do is read this question very very very very carefully and ask question begin right these are the two ones we just wrote in the box okay so what you need to do is read this question very very carefully and you ask yourself this charge here is it the planetary charge that's causing the electric field or is it the charge that's at this location experiencing the electric field they use a capital q but it's not they don't distinguish between letters there okay yeah they use a big q on theirs trust me it's the test charge we're talking about i'm trying to tie into gravity so i'm going to say planet big charge causing it satellite read this question again are we talking this charge here is it the big planetary or is it the tiny satellite charge and your hint is that word there a test charge is a charge a tiny charge placed at that location it's the satellite charge for me to find the electric field i'm going to use e equals f over q now my other hint is they gave me the force in this question see it it's going to be 5.2 times 10 to the negative 4 all over 0.5 and micro we said means 10 to the negative 6 why didn't you put a negative it we said for electric field we don't put negatives in for charge you get 1.04 times 10 to the 3 let me see here 5.2 negative 4 divided by 0.5 negative 6 1040 units units newtons per coulomb right b find the electric field strength 0.45 meters to the right of a 2.3 micro coulomb charge now this charge here is the planetary charge we're going to use this one how do i know they've told us to find the field strength to the right of this so we're not actually this charge is not at the location we're talking about i also know because they didn't give me a force i have to use this one the electric field is going to be k q over r squared it's going to be 9 times 10 to the 9th 2.3 times 10 to the negative 6 all over 0.45 squared and you're going to find electric field answers are usually in the thousands tens of thousands or sometimes even hundreds of thousands how big is the electric field here 1.02 times 10 to the 5th just over a hundred thousand units Tyler units for cool in the next page we need to talk about direction because electric field is a vector it's a vector it's a vector you see gravity field direction was not much of an issue because which way do things always move in gravity towards the center of mass they fall unfortunately in electricity not only do things fall down they can also fall up they can repel so we need to decide what the direction is and you absolutely need to star or memorize or asterisk or this is hugely important pay attention you need to know this because you're going to lose marks if you don't here is how we're going to decide the direction of the electric field the direction of the electric field is the direction that a small positive test charge would want to go if it could how small a positive test charge Jordan so small that it doesn't have its own electric field because that would change the whole question almost imaginarily small but it's positive so the direction of an electric field is which way would a positive want to move if it could it's always from negative to positive example five says this find the direction of the electric field at some point where this negative 0.5 micro coulomb test charge experiences a force towards the cell first let's find the magnitude of the electric field did they give me the force in this equation then i'm going to use f divided by q it's going to be 5.2 times 10 to the negative 4 divided by 0.5 what's the magnitude of the electric field oh thank you times 10 to the negative 6 micro coulombs i was looking at this and going this seems awfully small i was trying to do it in my head now what's the magnitude of this electric field it should be in the thousands i think just same number that we did but now we're going to add a direction so which way would a positive charge here want to move if it could well which way is a negative charge moving south so which way would a positive want to move north we get the direction by asking which way would a positive charge want to move if it could and we have to kind of think and use some logic and reasoning says find the direction of the electric field 0.45 meters to the right of a 2.3 micro coulomb charge so right there we already calculated the magnitude on the previous page what was the magnitude on the previous page someone turn back and tell me what the magnitude was the previous page anyone turn back right now and tell what was the met huh yeah a little quicker next time boys and girls times 10 to the fifth on the same page whatever it's on the same piece of paper but it is on the previous page right i just want a little faster reaction than that what's the direction all right put a tiny positive charge right here which way would it want to move if it could well this charge here is it negative or positive according to this question positive so which way would a positive right here want to move if it could depending on what they said oh well they said to the right in the question so to the right what would the direction of the electric field be right there which way would a positive want to move if it could for the left see draw the electric field around the positive charge the electric field is millions and millions and millions of electric field lines that go out in every direction what we draw is a model or a representation we're going to use arrows to represent electric field lines the direction is going to be the electric field direction so don't write this down just yet but which way would a positive want to move right there if it could what about right here what about right there okay here is an example of how we would draw electric field lines big deal oh no actually a very useful model sadly how many lines are there grand total count eight that doesn't mean the electric field is eight but if in this same diagram you saw another charge with 16 lines what that would mean is it's twice as big a charge okay or if you saw another charge with only four lines that would mean it's half as big secondly we say this the closer the lines are together the stronger the field so don't write this down but right here the lines are only that far apart fairly strong field what about right here are they further apart weaker field use your imagination what about here they'd be about that far apart even weaker a field so Dylan what this is is kind of a visual model of what's going on with electric field the further apart the lines are the weaker the field and the lines are proportional to the strength of each other I am not saying the electric field is eight what I'd be saying is if you saw another one on there with 16 lines it's twice as big or if you saw another one with eight lines you'd be saying same magnitude of charge okay what would the electric field around a negative be well which way would a positive right here want to move if it could what about right here what about right here what about right here now what could you say about the magnitude of this bottom charge compared to the top charge not the same absolutely not the same huh no I want the magnitude not the polarity the magnitude pat half as big why four lines that's the significance of the number of lines it has nothing to do with the I'm not saying this has a charge of four and eight what I'm saying is if this was 10 microcoulombs this would have to be negative five microcoulombs if I've done this diagram correctly what if I wanted to show these two charges were the same gourd how many lines would I put on here eight what if I wanted to show this was twice as big so the arrow tells you the polarity negative or positive the fact that the arrows are pointing inwards that's a negative but the number of lines tells you the magnitude compared to something else oh and once again the further apart the lines the weaker the field at that location so right here the field's pretty weak because these lines are pretty far apart right here it feels pretty strong these lines are pretty close together and again Justin this is a terrible model in real life there aren't eight lines in real life it sends out billions of in fact an infinite number of lines oh and it's three-dimensional as well there should be lines coming out of the page towards us and going into the page away from us but we can't draw that so here's our model so the direction of the electric field we ask which way would uh tiny my abbreviation for positive is the plus sign and a ve next to it what do you think my abbreviation for negative is a minus sign and a ve why don't you just lose a plus and a minus because anytime I see a plus and minus by itself I think it's an equation so this is what I came up with in university use whatever you want to but you're gonna be writing the words positives and negatives quite a bit this unit I suggest you find an abbreviation you can also use pause and negative but I that to me looks a little cleaner anyways which way a positive test charge would want to move if it could capitalize that or from negative to positive because any positive charge wants to sorry I said that exactly backwards from positive to negative because any positive charge wants to move away from positives and towards negatives so if you see a line that's going from a positive charge to a negative charge that's the direction of the electric field how tiny a positive test charge so tiny that it doesn't have its own electric field otherwise that would change the whole question it's mythical you really can't have a test charge that small but we can mathematically so let's start doing some questions what if we have two charges and I want to find the electric field we use what we call the principle of superposition what we do is we find the vector sum of the field of the individual charges so Dylan look at example six mr. McDermott how many charges are there in this question see him in example six two and they want us to find the net overall combined electric field where that black dot field point is there is no equation to find the electric field from two charges what I'm going to do don't write this down what I'm going to do temporarily is ignore that guy in fact I'm going to call this charge a Dylan what do you think I'm going to call this charge okay so I'm going to find the electric field at a first now which equation am I going to use if you look up for a second when you're done writing this I think these are both planets I think earth and moon and that's the tiny satellite because this is I don't know the charge at this location I'm going to use the one that has the big planetary charge in it which equation was that out of the two k q I'm going to use k q a all over r a squared it's going to be nine times ten to the ninth one times ten to the negative six all over one squared I think you get nine thousand try it but this has lots of ones in it I can do the math in my head is it nine thousand but from now on that's not enough now we know how to get the direction electric field is a vector so we're going to say this nine thousand newtons per coulomb and if I put a tiny positive charge right there which way would this guy make it want to move if it could right I'm going to repeat this procedure for electric field b and then if they're both to the right I'll add them up if ones to the left and ones to the right I'll go bigger minus loser vector math so on your own try finding the electric field for b I'm going to freeze the screen by the way what's the radius between the field point and b not four three see it you get one thousand newtons per coulomb left so how would I add do you think nine thousand right plus one thousand left what's the answer what is one thousand right sorry nine thousand right plus one thousand left eight thousand to the right okay so we're going to say this the electric field is equal to and I'll do it this way nine thousand take away one thousand what I'm really doing for the vector math and saying all up to the right be naked or to the left be negative whatever I just go bigger minus smaller equals eight thousand newtons per coulomb to the right I like that question I like that question I like that question I like that question I'm going to ask you to find on your test the electric field between two points except you know between two charges but I'm going to make them different numbers than ones you'll have to do a bit of arithmetic example seven draw approximately sketch the resultant electric field at the indicated field points both of these charges are positive one micro coulomb so they're the same size so ignore this one temporarily which way would this guy want to move if he could based on this charge up so I'm going to just kind of temporarily don't write this down I'm going to add a line we're going to change our diagram so don't write this down but this is pushing that way ignore don't write this down don't write this down we're going to change the diagram don't write this down this guy here we're going to temporarily ignore which way is this guy pushing or pulling this guy which way would he want to move based on this guy I think this way but not anywhere near as much because there's a bigger distance I think the electric field right there though is a combination of those two I think and this is what we're going to write down I think the electric field probably about like that ish that makes sense it's getting pushed up and it's getting pushed to the left what about right here dead center it's getting pushed this way by this charge it's getting pushed this way by this charge what do you think the net or the resultant electric field right there is there's an equilibrium point nothing what about right above it right here I think it's getting pushed this way and it's getting pushed that way if I add those two together what do you think the electric field is Evan I heard you say it I think straight up what about right here well this guy is pushing to the right really strong this guy is pushing to the left but not as strong because it's further away you know what I think my net resultant electric field to be right there to the right but not as strong as if that to the right but not as strong as if he wasn't there what about like that let's say I don't know we're sketching what direction do you think the electric field to be right there pull up and this is going to bring us to electric field diagrams and I will be asking you to do these on your test it says draw the electric field pattern for equal like charges so Gordon if the charges are equal what will you say about the number of lines here compared to the number of lines here the same put your pencils down and watch here's how we draw this okay like charges do what repel so electric field lines would look don't write this down until I'm done trust me this would start to head that way but it would get repelled this would start to head that way but it would get repelled this would start to head that way but it would get gradually repelled gradually repelled a line would start to head this way it would also they'll get repelled by this guy, it would gradually get repelled. A line would start to head straight up, but it would get kind of repelled. And I always do them in pairs to make sure, because if I know the charges are the same, this way I'm guaranteed to get the same number of lines. Remember I said to you the number of lines tells me nothing about how big the charge is, it's just a comparison. So even if I end up with a weird number like seven lines, I would still be good with your diagram as long as you had seven on each. Here would head this way and it would get repelled. Here would head this way and it would get repelled. What about right here? It would head this way, it would get repelled. It would get repelled. It would head and get repelled even further and even further. Now we would have one line go straight that way because it's getting repelled by both and we would have, oh boy, try that again. We would have one line go straight that way because it's getting repelled by both. Now what about right here? Don't write this down yet. What about right here? It would get forced down but I think Tyler, dead center, there would be an equilibrium point where a point could sit and be stable, which is why I've kind of left this area blank. I'll do one more line maybe like that and like that. So go ahead. You can copy this or just try and recreate it the way I did looking up to see if you get something similar. Sally, if your diagram is a different number of lines than mine, I won't freak out as long as both of your points have the same number of lines. So do the lines in pairs. Now on your test, frequently what they'll do is they'll give you an electric field diagram and they'll ask you to describe the charges. In other words, on the test ebb and what they'll do is this. They'll call this charge 1 and charge 2 and they'll say first of all, is charge 1 negative or positive? How do I know that charge 1 is positive? The arrows, right? Which way does an electric field point from? Which way would a positive want to? So you look at the direction of the arrows that tells you the polarity and then they might say are the charges like or unlike? Well I can tell that they're unlike because they're repelling each other. Let's try doing another one where we have equal and opposite charges. And I'm going to change colors because mine's going to end up overlapping with this blue, but pretend these charges are gone. Okay, I think starting right here, if I start moving, which way would I want to move if I could if I'm a positive test charge? In fact, I think you're going to have a straight like that. What about if I start right here? I think I would get pulled into the boom negative charge like that. What about if I started here? I think I would get pulled into there like that. What about if I started going this way? I think that I would gradually, ran out of this page, gradually get pulled in like that. You could also argue the same line exists up here, but that's going to clutter up this diagram too much. But your lines, yeah it looks kind of like a happy face. But really what you should have is this too. A line like that, a line like that. I'm going to get rid of them though because they clutter up this diagram way too much. By the way, now that you see this diagram, how can you tell which one's positive and which one's negative? Arrows. These are electric field diagrams. Also again, the further apart the lines, the weaker the electric field. Are these two lines close together? Strong electric field. Are these two lines far apart? Weak electric field because you're far away. So it's a nice diagram system that gives us a lot of information. Turn the page. So the idea of the electric field originated with Michael Faraday. I'll go on a rant about Michael Faraday, one of my all-time favorite scientists, but on another day. But he used a different way of representing the field. He used lines of force, which showed the direction which a positive test charge would tend to move if placed anywhere in the field. There is the electric field diagram for a positive all by itself. There is the electric field diagram for a negative all by itself. I notice these two have the same number of lines, the same magnitude. Here is positive and negative charge, same polarity. How can I tell they're the same polarity? Same number of lines here and here. Here's two positive charges and these are better diagrams than the one that we free-handed. Someone's done this with computer graphics, but the free-hand ones work just fine too. Here is your area of neutrals. It doesn't mean there's no electric field here or here, by the way. It just makes the diagram too cluttered. So let's summarize. The electric field can be found using the definition if we're told the force on a test charge. Electric field is the force per Coulomb, which is the definition of electric field. And Justin, I always use a lowercase q here to remind myself this is like the satellite in orbit around the earth. It can be found using the point charge equation if we know the big planetary charge that's causing the electric field. Recall the electric field definition is found by considering, sorry, the electric field direction is found by considering a positive test charge and it's away from a positive and towards a negative. And you can see that I didn't type that line because I would have put a VE after the plus and a VE after the minus, because a plus and a minus there looks too much to me like an equation. It's just a bad habit I have. So that means we can find the force two ways. The force is either, if I know two charges, k, q1, q2 over r squared, that was last class. Can I not find the force from here? Get the f by itself. Okay. The force is also, if you know the electric field at a location and you know how big the satellite charge at that location is, that's also the force that that little charge is experiencing. Recall that the direction of electric force is figured out. So electric field, which way would a positive want to move if it could? Electric force, like charges, what? Unlike charges, what? Like charges, repel, unlike charges, attract. That, you got to memorize. Like if you're on test day going, I can't remember, you're going to flunk the test. And I'm going to laugh at you. Homework, number one, three. Do you guys have these questions on your sheet, by the way? Did 12 and 13 work because it's the last time some stuff didn't photocopy. You did for you? Okay. I'm going to go 13. 13 is between two charges. 17. And at the bottom here it says to Google something. Does it say that on yours? That's actually going to be the bonus video game. So I'll send that out as a link. Okay. We're going to temporarily stop there, but I get to show you a very cool toy.