 So building on a little bit with what we've been talking about in this chapter, I wanted to clarify a few of our questions. Now some of the homework problems we did in recitation, and so you should be quite familiar with what's going on in problem one, in problem two, and problem three. We did those in recitation. Now there's some other problems in here, but I'm going to jump down here real quick to problem number seven. Because we did part of this in recitation, but we didn't actually get as far as what the problem was asking about. So here we've got our sphere of radius R1, and it's got charge all over it, and then we've got a neutral conducting shell that goes around it that has an inner radius and an outer radius. And that's like problem number six we did in the recitation. And we set this up using the Gauss's law strategy of determining how much charge is inside. What's the area where you're trying to find the electric field? And then using our Gauss's law equation where the electric field is the charge inside divided by epsilon naught and AG. So we did that much, and you'll probably have slightly different numbers, but you can get it set up to that. But see the problem doesn't ask us what's the electric field at the inner surface of the shell. It asks us to find the surface charge density. So here's where we can start to use our conductor equations that we developed in lecture today. And that equation says that I can find the surface charge density if I know the electric field and epsilon naught is my constant. So just like we did on the worksheet today, you should be able to calculate the surface charge density. Be careful though and think about it because in lecture today we saw that depending on whether the electric field was going in or out of the surface, you could build up a positive or a negative distribution. So think about how much charge you have here at the center. And this problem may have different numbers than what we had in our problem we did in recitation. So for example, this one in recitation was a positive charge density, and this randomized one has a negative charge density. That's going to affect the direction of the field. And depending on whether the field is going out or in, when it hits that surface, it's going to depend on whether you get a positive or a negative charge distribution. So I want you to keep that in mind as you're solving the problem. I also want you to keep in mind that when we're looking for surface charge densities, our original formula came back to a charge distributed over an area. So you could also think of what we've learned about the charge that builds up on conductors and the area of the inner and outer surfaces of that conductor and use that to help you figure out the sigma values. And it's going to be different on the inner surface and the outer surface because of the particular geometry we have here. I wanted to give you that much of a tip here as you're working on the homework. So you can see that it connects to this concept of conductors, but without getting you too frustrated because it's just enough different that it looks like what we did in recitation, but it's really not because it goes a step further. Now the next problem I want to highlight to you here on the homework is problem number eight. And part of it is because we haven't done too many of these equations, but also remember that there is a slight difference between the formula we had for a conducting sheet and a non-conducting sheet. So if you've been looking at the conducting lectures, you might be tempted to go in here and put your equation for a conducting sheet in, but this doesn't say it's a conductor. In fact, it's not a conductor. So you're going to have to assume it's not a conductor to put this equation in right. And just as a little bit of a hint here, I'll remind you that when you're setting up an equation, you might need to set up a fraction and then you might need to go in here and grab a Greek letter or two and then you need to figure out what you have to put on the bottom of this equation. If we're measuring the electric field just above a non-conducting sheet. If you've got questions, make sure you ask so that you understand what's going on.