Anderson Video - Static Electricity

Professor Anderson
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About one of your homework problems. Okay, and the homework problem says the following, you are going to walk across the carpet, here's the carpet, and when you walk across the carpet you pick up charge and now you have a net charge on you. And we talked about this in class, when you walk across the carpet what kind of charge do you pick up? Do you pick up protons or do you pick up electrons? You pick up electrons. Electrons are hanging out on the tips of the carpet and they jump from the carpet to your shoes, the same way that the electrons on your hair jump from your hair to the balloon when you rub the balloon on your head, you charge up the balloon with electrons. So you have a net negative charge. You are negatively charged when you walk across the carpet. Okay, let's figure out how many electrons you actually picked up and let's say that you acquired a charge of the following. 50 micro Coulombs. 50 micro Coulombs, thats probably a reasonable number for walking across the carpet. How many electrons is that? All right. We're just trying to convert from one unit to another. The unit that we're in is micro-coulombs, the unit that we want to get to is electrons. Electrons of course have charge, we need to figure out what that conversion is. Let's start with the easy one though, 50 microcoulombs is equal to 50 times 10 to the minus 6, that's what a micro means. Right? Micro is a millionth, which is 10 to the minus 6, and now I want to multiply this by 1. Anytime you're converting units you just want to multiply by 1, and you look at the units that you need to cancel. You need to cancel coulombs so we had better put coulombs down on the bottom. We want to get electrons, so let's put electrons up in the top, and what do we know? Well, one electron has how much charge in coulombs? >> (student speaking) 1.6? >> 1.6 times 10 to the minus 19. So this is really the number one, it's in some funky units but it's the number one. One electron equals one point six times ten to the minus nineteen coulombs. That's it. And now we just have numbers to multiply, the units here cancel and we get 50 times 10 to the minus 6 divided by one point six times 10 to the minus 19 and we're left with electrons. And now you can run this number in your calculator, I'll approximate it here. 50 over one point six, that's the same as five times 10 to the minus 7, or sorry, five right? Five times 10 to the minus five divided by one point six times ten to the minus nineteen and five over one point six, it's got to be really close to three, right? Maybe a little bit more, three point one, times 10 to the what? We've got a minus five up there, we have a minus nineteen, so we're going to add 19 to a negative five and so we get 14. And did anybody punch it into a calculator? What'd you get? Same thing? Okay, good. Same thing. Okay, this is the beauty of doing math in your head and also the beauty of doing the lecture twice. All right. So we've got three point one times 10 to the 14 and this is a number of electrons. That's how many electrons you pick up, and these we said we're probably realistic numbers, so that's sort of amazing, right? This is three hundred and ten trillion electrons that you pick up just walking across the carpet, so there's little fibers with tons of those electrons just ready to jump off and land on you. Okay, any questions about that one? There's a second part to it which is how much mass does that correspond to and you can do that conversion yourself, all you need to know is the mass of the electron, which we know is nine point one times ten to the minus 31 kilograms.
About one of your homework problems. Okay, and the homework problem says the following, you are going to walk across the carpet, here's the carpet, and when you walk across the carpet you pick up charge and now you have a net charge on you. And we talked about this in class, when you walk across the carpet what kind of charge do you pick up? Do you pick up protons or do you pick up electrons? You pick up electrons. Electrons are hanging out on the tips of the carpet and they jump from the carpet to your shoes, the same way that the electrons on your hair jump from your hair to the balloon when you rub the balloon on your head, you charge up the balloon with electrons. So you have a net negative charge. You are negatively charged when you walk across the carpet. Okay, let's figure out how many electrons you actually picked up and let's say that you acquired a charge of the following. 50 micro Coulombs. 50 micro Coulombs, thats probably a reasonable number for walking across the carpet. How many electrons is that? All right. We're just trying to convert from one unit to another. The unit that we're in is micro-coulombs, the unit that we want to get to is electrons. Electrons of course have charge, we need to figure out what that conversion is. Let's start with the easy one though, 50 microcoulombs is equal to 50 times 10 to the minus 6, that's what a micro means. Right? Micro is a millionth, which is 10 to the minus 6, and now I want to multiply this by 1. Anytime you're converting units you just want to multiply by 1, and you look at the units that you need to cancel. You need to cancel coulombs so we had better put coulombs down on the bottom. We want to get electrons, so let's put electrons up in the top, and what do we know? Well, one electron has how much charge in coulombs? >> (student speaking) 1.6? >> 1.6 times 10 to the minus 19. So this is really the number one, it's in some funky units but it's the number one. One electron equals one point six times ten to the minus nineteen coulombs. That's it. And now we just have numbers to multiply, the units here cancel and we get 50 times 10 to the minus 6 divided by one point six times 10 to the minus 19 and we're left with electrons. And now you can run this number in your calculator, I'll approximate it here. 50 over one point six, that's the same as five times 10 to the minus 7, or sorry, five right? Five times 10 to the minus five divided by one point six times ten to the minus nineteen and five over one point six, it's got to be really close to three, right? Maybe a little bit more, three point one, times 10 to the what? We've got a minus five up there, we have a minus nineteen, so we're going to add 19 to a negative five and so we get 14. And did anybody punch it into a calculator? What'd you get? Same thing? Okay, good. Same thing. Okay, this is the beauty of doing math in your head and also the beauty of doing the lecture twice. All right. So we've got three point one times 10 to the 14 and this is a number of electrons. That's how many electrons you pick up, and these we said we're probably realistic numbers, so that's sort of amazing, right? This is three hundred and ten trillion electrons that you pick up just walking across the carpet, so there's little fibers with tons of those electrons just ready to jump off and land on you. Okay, any questions about that one? There's a second part to it which is how much mass does that correspond to and you can do that conversion yourself, all you need to know is the mass of the electron, which we know is nine point one times ten to the minus 31 kilograms.