 Hey everybody, I'm going to teach you how to obey Coulomb's law and we should obey the laws, right? That's always a good idea. The first thing I want to just kind of hit you with here is what the actual formula is and you're going to notice it's interestingly similar to another formula that we dealt with in physics 20, which was written by another pretty important physicist. This is Newton's universal law of gravitation. So one thing you should bone up on is the fact that there is a relationship with these two and maybe there's some sort of relationship between electromagnetism and gravity too. That's one of those things we can talk about when we get back in class. But when you look at the formula, I've got it written out here. I actually got a little lazy. If you look in your formula sheet, they put a little something extra in, which is two parallel lines around the force of electricity. Those lines, if you've taken a bit of math, means that this is an absolute function, which means basically when I put numbers in for my Coulomb's constant, the charge, the charge of the other object and the distance, the answer I get out will always be a positive. It will never be a negative number. Now why is that? Well, it isn't that you won't get a negative force, i.e. a force that's going to the left out of this equation. It just means that the equation can't tell you whether the force is negative or whether the force is positive. It doesn't have the ability. You have to figure that out on your own. So we're going to look at a problem here where we're going to actually do that. This is determine the magnitude and direction of the electrostatic force between two objects. And we've got a negative charge here and we've got a positive charge here. They've got a separation of 3.3 centimeters and we've got the charge of each of them. They're measured in microcoulombs. So the first step is actually pretty straightforward and most students don't have much trouble with this. It's just finding the magnitude, first of all, of this electrostatic force. So for that we're going to use Coulomb's law. The value K, this is that value that was calculated with a torsion balance that Coulomb used, is 8.99 times 10 to the power of 9. Now don't get lazy with the units here, okay? Put them in there. Newton meters squared per Coulomb squared. I want to see you writing those in every time you do this equation because they're important. They're going to help us make sure that we don't make a mistake later on. Next, the charges. It doesn't matter which one you call 1 and which one you call 2. And it also does not matter whether or not you put the negative sign in there. In fact, I don't want you to put the negative sign in. Think of these as the absolute values of the charge when I substitute that in as well. The other thing that I've got to do is I'm going to convert my units. This is a microcoulomb and if you look on your data sheet, a micro is the same as 10 to the negative 6. So the charges we're dealing with here are really quite small. And so I have to show that substitution as well. And this is divided by my separation distance which is 3.3 centimeters. Let's make sure we convert our units here. Can I just call that times 10 to the negative 2? Is that okay? You're not here, so you don't care. You don't get any say. I think it's okay. So now we're going to type this into the calculator. Before we do that, maybe we'll just take a second to make sure all our units are in place. So we have coulombs squared in the bottom here and coulombs times coulombs. Check it out. Coulombs, coulombs, coulombs, coulombs. It cancels out. Meter squared in the top. Meter squared in the bottom. So my final answer will come out to some amount of newtons which is great. So doing that little step there where we're just cancelling out our units, it's going to help us to make sure that we haven't made a silly unit conversion mistake which always sucks. Here's the super interesting part of the video where I type things into my calculator and you get to watch. I guess you can be throwing it into your calculator right now too. Or, you know, check your email or something like that. 3.3 to the negative 2. And here's the step that most students forget. We've got to square that. Okay, we've got to square it. So the force that we're getting is, what do we got? Two sig gigs, 4.5 times 10 to the 13 newtons. So 4.5 times 10 to the 13 newtons. And we're almost home. We want the magnitude. We've got that. So that's good. And the next thing we want is the direction. So to figure out the direction, we've got to do a little bit of work in thinking about positives and negatives. So if I'm talking about the direction of the force acting on the negative object here, well, it's going to be moving to the left. And it's moving to the left because a positive and a negative charge will repel on another, like we talked about in the very first day of the unit. Whereas the force acting on the positive charge is going to be moving to the right. Again, because they're going to repel. So we're getting one force. In fact, this isn't even really a vector at this point because it doesn't have direction. This could be plus or minus. But when it comes to actually labeling in the question, it could be either positive or negative depending on the direction and depending on the charge. And that seems kind of silly right now. But when we get into some 2D problems, and maybe I'll do a video on one of those later, you'll see that that's going to be really important. So hopefully that sets you off on the right track with Kuhlm's law. Make sure you're taking a look at that stuff in your textbook and on the websites that you got all the ideas down. Okay, good luck.