 So now let's look at Ohm's law. For Ohmic materials, the voltage is directly proportional to the current. That's how we define what an Ohmic material is. It's a material that follows this relationship. Now we can write this out as an equation by looking at the voltage and the current being directly proportional, along with a proportionality constant called the resistance. As a matter of fact, the resistance is defined according to the Ohm's law equation, where our capital R resistance is going to be equal to the ratio of the voltage over the current. So capital R is a device-specific quantity if it's an Ohmic resistance. And if it does follow Ohm's law, then I've got a material-dependent constant and some geometry that are going to come into that. And we're going to cover that equation in a bit more details. But coming back to just this resistance equation, I can start looking at the units. We know that voltage has a unit of volt and current has a unit of amp. So that means resistance, according to Ohm's law, should be equal to a volt per amp. And we define a volt per amp as a new quantity called an Ohm. Again, this resistance is only true if it's an Ohmic material that follows Ohm's law. So that's our basic Ohm's law. We're going to use it in lots of different situations this semester.