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Anderson Video - Motion of Charge Between Plates

Professor Anderson
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Okay, so the problem says an electron moves between plate A and B And it gets energy when it does that. It gets kinetic energy. Delta K of six point six times ten to the minus sixteen joules. This is how much energy the electron acquires when it moves between those two plates. And we need to find Delta V of the plates. Okay, we need to find the potential difference in those two plates. All right so first off let's see if we can just draw a picture of this thing, okay? Plate A is right there. Plate B is right there. I'm gonna put an electron at Plate A and it's going to accelerate towards plate B and it's going to pick up energy and it's picking up six point six times ten to the minus 16 joules of energy. So We need to find Delta V of the plates. Let's ask a question off the, off the bat. This plate has some charge on it and this plate has some charge on it. Does this plate over here have positive charge on it or negative charge on it? What do you guys think? Plate A. Is it full of positive charge or is it full of negative charge? Well, the electron is gonna move away from it. Is the electron positively charged or negatively charged? The electron is negatively charged right? Negative one point six times ten to the minus nineteen coulombs. So if this electron is a little negative charge what must be on Plate A? Negative charge. Exactly right. Likes repel, there must be a whole bunch of negative charge on that plate to push that electron away. And therefore, the other plate it's got to be filled with positive charge because that's going to pull the electron in. Okay? All right. How do we do this? Well, what we said was any work in the system is Q Delta V and all of that work is going to go into changing the kinetic energy of the electron. And so we can write down a very simple relationship here. Delta K is just Q Delta V. That doesn't look too bad. Let's see if it makes sense. Change in kinetic energy has got to be equal to the work that goes into it. And we're not worried about signs here we'll worry about that in Part B. So we put some magnitudes on that thing and therefore we don't have to worry about signs here, okay. All right, Delta K equals Q Delta V we're solving for Delta V so Delta V equals change in kinetic energy divided by Q. We know this number. Delta K is six point six times ten to the minus sixteen joules. We know what Q is because it's one electron and again we're not worrying about signs here so we'll just put in the magnitude. One point six times ten to the minus nineteen coulombs. Those are SI units. The units of V are volts or joules per coulomb. And in fact one volt equals one joule per coulomb. Okay, so now we can plug in all these numbers right? Delta V, I'll approximate it here if you want to punch into your calculator, you can try it. I get six point six over one point six and I have a ten to the minus sixteen I have a ten to the minus nineteen in the denominator so that gets me a ten to the three. Six point six over one point six is that's got to be pretty close to four. I'm gonna say that's about four. And we have a ten to the three. And did anybody punch it into their calculator and get a real number there? Okay, 4.1 times 10 to the three and the units are joules per coulomb or volts. Which you can just write with a capital V when you plug it in to your homework. 4.1 times 10 to the 3 volts Okay. The second part of that is related to the very first thing that we did. Which of these is at higher potential? The one that is at higher potential is the one with the positive charge. And so the potential here at plate B is going to be bigger than the potential at plate A. VB is greater than VA. And so this is a good exercise when you're doing these problems. Go back to a picture put in a charge see if you can figure out where the other charges must be. And to do that all you need to remember is likes repel opposites attract. Okay. Questions on this one? Everybody okay?