 Here's one more video about the current density and some example problems, but in this case we're going to start off with the equation for the current density, which relates the conductivity to the electric field. Now if we start off with a value for the conductivity, similar to what we saw for seawater in the last one, and a large electric field like we might have seen in our electrostatics chapter, we'd see that we're going to get a fairly decent current density as we work through our equation. And again here, you're going to take the conductivity and multiply it by the electric field in order to find your current density. It's a little harder to see what's happening with the units here in terms of the inverse ohm meters, the volt meters, which was one of the alternate electric field units, and the amps per meter squared. We'll understand a little bit more when we get to Ohm's law that volts, ohms, and amps are related in such a way that these equations work out. Now as some contrasting examples, when I first introduced the conductivity, we had shown that for air you have a very very low conductivity. So with the same electric field, but this low conductivity, you have a very very small current density that would form under those conditions. And in contrast in the other direction, if you've got a metal like copper, which has a high conductivity, you would end up having a really really high current density form under those circumstances. So this form of the equation is going to tell you what kind of current density you would find if you know the conductivity and the electric field. Now obviously the equation could be rearranged and one way you can rearrange that equation is to solve for the electric field. And this is an example calculation that works it in that way, where you're given some value for the current density, maybe you found it by taking an actual current and dividing it by the area, and then you divide it by the conductivity of the material and you can find out what sort of electric field would be necessary to create that current density. And again the amp-ohm per meter is going to be the same unit as a voltmeter. The last way that we can solve this equation would be to use it to find the conductivity. So this would be the case, for example, if you were given a value for the current density and a value for the electric field, and you want to find out what was the conductivity of the material that this whole experiment took place in. And this is how they determine conductivity values, is if they can measure the electric field and they measure the result in current density that lets them know what the conductivity of the material was. So again as we're working through these equations, the units are a little bit more complicated than some of the other equations, but just plugging the numbers in you can solve for the current density, the electric field, or the conductivity.