 Now that you've figured out the first couple of questions based on some of the information in the other videos, it's time to move on to our calculation problem. Now we've covered this, but I just want to be really clear here on what we're doing and the equations that are involved, because there's some similar equations in a couple different sections here. So since we're dealing with a conductor and we're looking at the result from Gauss's law, we've got an equation from Gauss's law that says that the electric field is related to the surface charge density and our electrostatic constant epsilon naught. Now this would be, if I have a surface charge distribution on a conductor, what electric field does it create? But it turns out we can use the same equation for the surface charge created on a conductor when it's placed in electric field and we can rearrange that equation and we can solve for sigma and that's going to be the electric field times our epsilon naught constant. So we can use the same equation both for if I have a surface charge density on a conductor, what electric field is going to be right outside of it? Or if I've got an electric field right outside a conductor, what is the surface charge density on that conductor? So the two go hand in hand. Well for this particular problem it gives me my electric field and so it's got 20,000 Newtons per Coulomb as my electric field. And my epsilon naught, which is also called the permittivity of free space or my electrostatic constant, that's our 8.85 e to the minus 12. So if I take all this information I can put in my value for the electric field and my value for that epsilon naught constant and multiply those two things together to give me my surface charge density. Now because this is sigma, a surface charge density, I want you to think about what units you should use when you actually finish your calculation. Now you can do all the algebra on these units to cancel things out, but just to remind you here, we originally defined surface charge density in terms of this equation. It's a charge spread out over an area. We're not using this equation to calculate our surface charge density, but it can help guide us in terms of what we expect for our units. Because you can think what kind of units do I expect for charge? What kind of unit do I expect for area? So the unit for my surface charge density has got to be that charge per area type of unit. Not Q and A, but the units for Q and the units for A. If you go back and look at some of your earlier class notes you should have written down what kind of unit you expect here. And it should match here and it should also match the algebra you're doing here. So for your worksheet that you're turning in, make sure you actually multiply this out and put the correct units. This is not part of your calculation, it's just a guide for what we would expect for our units. So you'll fill this in on your worksheet and have an actual answer with units in here.