Another circuit objects to talk about is battery. Let's define the battery. A battery is a device that holds two conducting terminals at a constant potential difference through the power of chemistry. That's all that's going on. Standard battery, this is a nine-volt battery. You don't see these quite as often as like the AA and the AAA cylinder batteries. But in this case, the nine-volt battery, here's the two terminals. Chemical reactions happen in here to hold a difference in this case of nine volts between the terminals. One looks like this and they're shaped difference so you can tell them apart and so that they can clip together. One looks like this. One is the positive terminal, one is the negative terminal. We know that it's a nine-volt battery. That means the potential difference it holds is nine volts. When we're thinking about batteries and circuits, we have to think about where's the potential zero? So for point charges, we said all the potential zero will always be at infinity. When you're doing field theory, the potential zero at infinity. When you're making circuits in devices, you can put the potential zero wherever you want. It's more convenient to pick one of your terminals or one of your electrodes, make the potential zero there. In this case, we're going to say the negative side will be just called zero volts. If that's zero volts, then this is plus nine volts. So potential's always a difference but if you define a zero, then you can start defining absolute potentials relative to that zero. Let's think what's happening really in this region if we have nine volts here and zero volts there. We actually have an electric field in here. So every time you hold a nine-volt battery, you're wielding a little electric field between its terminals. You can say roughly what it is. Well, its magnitude of about nine volts per centimeter, right along that line. They're not parallel plates so it doesn't make a uniform field, but it doesn't make some sort of a field. We can also think about the potential. Well, these are conducting electrodes, so they must have equipotential surfaces all along their surface. So here is the zero-volt equipotential surface, It's the surface of that terminal, and here is the nine-volt equipotential surface. It's the surface of that terminal. We know then also that the potential is going to go down as we go along the field. So the eight-volt line is probably something like that, 7, 6, 5, 4, 3, 2 and then the one. We can actually do field theory inside of a battery if you really want to. So that is sort of a law of physics and chemistry that the chemical reactions in here must hold these at a potential difference of nine volts. What we're going to do now, is we're going to break the laws of physics now. It's a little bit risky and scary to break the laws of physics. You never know what's going to happen. But we're going to break them somehow because we're going to set something up where two laws can't be true at the same time. The laws of physics and chemistry require that the terminals of the battery be at the different potential. But we also talked about how in a conductor the surface has to be all at the same potential because all the electrons will move to make the surface, albeit a constant potential. That's in fact why each terminal on the battery has to be completely at nine volts and completely at zero volts. We're going to do this with a little bit larger batteries so you can see it better. This is a 12-volt battery. Here's its two terminals, so this one is negative, we can call that zero, this one is positive, we can call that plus 12 volts and this is our metal. So this is iron wire. It's 75 microns in diameter, so it's about the diameter of a human hair. You can't quite see it very well in the video, but I promise you that there's wire there. How else would I do this? There is wire there. I'm going to take this metal wire and I'm going to place it across the terminals of the battery. We're somehow breaking the laws of physics. We can't have this be at nine volts or 12 volts, this will be at zero volts, and this will be at the same because we're going to make one essentially large conductor. Let's see what happens when we do it. It's very exciting. Here we go. What happens when you break the laws of physics? The universe gets very upset when you break its laws of physics. Let's try again. Maybe the universe was just having a bad day. But let's make sure that's really the outcome of this and we bring it down and is it going to be 0 volts, 9 volts, 12 volts, what's it going to be? The universe has no, can't exist, has to go away. So the answer is no. Apparently, you just can't do this. So it's impossible. So we have to figure out what's going on. Well, as I alluded to in our discussion of what's happening in the metal plates in a capacitor, whenever something looks wrong in physics, there's always an answer. You can always figure it out. Almost every time the answer is, you deviated from your model. There's always some condition. Anytime you say something, there's always a condition, there's always a model. Well, here we have once again deviated from our model and the way we've deviated is that we're no longer in electrostatics. Remember the surface of a metal has to be at a constant potential in electrostatic equilibrium. In this experiment, we are no longer in electrostatic equilibrium.
