 So now we get the current density. Well, to define the current density in words, it's the current per unit area. In terms of an equation, that's going to be j equals i over a. j is the symbol we use for current density. i is the symbol we use for current. a is the symbol we use for area. Using the j for current density is partly because we just have to reuse a lot of symbols. So j is not being used for very much. We use j for current density. Now if we look at the units for current density, well, current is measured in units of amps. Area is measured in units of meters squared. Now be very careful here because I've got the a for amperes, which is the unit for current, and the a for area, which is the variable symbol. Don't get those confused. So together these mean that my current density, j, has units of amps per meter squared. And we don't have another unit assigned to that, it's just amps per meter squared. Now in some materials called omic materials, the current density is directly proportional to the electric field. The electric field is causing the current density. In these materials, j is the current density, e is our electric field, and they're related according to this equation where my proportionality constant is something called the conductivity. Now that conductivity is the Greek letter sigma. Well, the symbol is a Greek letter sigma, but the name of the variable is still conductivity. Now don't confuse my use of sigma here with the previous use of sigma for charge density. We have to reuse some of our symbols sometimes, but in this case I've got conductivity. And conductivity is a material dependent property, which is a constant for omic materials. So for example, copper has a conductivity sigma of 6 times 10 to the 7th per ohm meters. So 6 times 10 to the 7th is a very large value, meaning copper is a very good conductor. Seawater has a conductivity that's approximately 4.8 inverse ohm meters. And air is approximately 5 times 10 to the minus 15th, so air is not a good conductor at all. Seawater's kind of in the middle, copper's really good. So the higher the conductivity value, the more current density you're going to have flow. Now my units here is 1 over ohm meters, which can either be written as the 1 over ohm meters, or written as this inverse ohm meters. So in summary, I've got two equations here, both of which are an equation for the current density J. One in terms of the current and area, the other one in terms of the conductivity and electric field. So that's our current density.