[Coda]

Overall, you're right on the money about how household power works.

More details:

Household power is alternating current. Remember the discussion of spinning a magnet around a coil of wire? Because that's rotary motion, it pushes the current in one direction as it goes over the top, and then pulls it back in the other direction as it goes under the bottom.

This AC power comes into the house into the circuit breaker box, where it's divided up among several circuits through a set of circuit breakers(*). From each circuit breaker, the power is routed to the smaller blade (called the "hot" wire) of the power outlets on that circuit. The larger blade (called the "neutral" wire) is connected back to the circuit breaker, and then to the ground. You can see from this setup that there's not a complete circuit unless something is plugged into both sides of the outlet. The round pin (called the "ground") is connected directly to the ground instead of going back through the breaker, and this pin isn't SUPPOSED to be electrically connected to the other two inside the devices you plug into the outlet -- it's supposed to be connected to the metal housing of the device, which is SUPPOSED to be isolated from electrical power.

Why is it set up this way?

Well, you'll notice that not every power cord has a large blade and a small blade; some of them have two small blades and can be plugged in either way. This works specifically because the ground doesn't care whether you're pushing electrons at it or pulling electrons from it, and electrically speaking a complete circuit can't tell the difference between the two configurations.

Then why have the two different sizes? Well, there are two good reasons for that. First, the larger connector makes contact first, which means that the path that goes to the ground will always be established before power starts to flow into the device from the other side. Second, devices that use this kind of arrangement put their power switch on the hot wire instead of the neutral wire, because that way when the switch is turned off, no electrons are flowing into or out of the device at all -- putting the switch on the neutral wire would allow the circuit to be broken so the device still wouldn't turn on but there would still be a small amount of electron flow going through the device, which could present a hazard.

That hazard is related to the third pin, the ground. The electrical components inside the device aren't supposed to come in contact with any metal parts of the device's body. If it DOES come in contact with it -- and this does unintentionally happen, especially as things start to wear out and shift around -- then that gives the housing an electrical potential. And if you touch the housing in that state while not wearing insulating shoes, YOU become a path for that potential to go to the ground, and you get shocked. By putting the power switch on the hot line, it prevents this potential from forming. The third pin, meanwhile, is connected to the metal parts of the body so that if that unintentional contact DOES happen, there's a better path to the ground than through your body -- it goes through that nice thick electrically-conductive wire connected to the ground.

Some power outlets, especially those in bathrooms and kitchens, will have a feature called a "ground fault interruptor." If the outlet detects current flowing through the ground pin, the interruptor will IMMEDIATELY (we're talking response times of less than 200 milliseconds, preferably less than 40) cut the power to the entire outlet, shutting off the obviously-malfunctioning device. Why kitchens and bathrooms? Because water conducts electricity and is perfectly happy to flow right between the electrical components and the housing. This saves lives -- if you've ever heard of someone dropping a hairdryer into the sink, this is what keeps them from immediately getting killed by electrocution.

(*) The circuit breakers are the blocks with the big heavy switches. These detect if too much current is going through them and disconnect the circuit defensively if there's too much of a load. This is done because the wiring in the house is rated for a certain amount of current, and if you exceed that rating, you run the risk of fire.

"Solar cell" specifically refers to a photoelectric or photovoltaic device, which is what your picture is of. "Solar panel" can also refer to solar heat storage (big tubes full of water set out to absorb sunlight; the hot water then gets stored in an insulated tank and it slowly lets the heat out into the house when it gets colder at night).

That picture is misleading. This one is better:

What happens is that the sunlight displaces an electron from inside the separation layer. The top silicon layer is "doped" with impurities that make it more likely for the displaced electron to flow that direction, while the bottom silicon layer is doped with a different kind of impurities that give it extra electrons to work with. When the electron moves out, it leaves a positively-charged "hole" behind and the bottom layer gives up an electron to fill it in, replacing it with an electron drawn from the circuit. The separation layer keeps the positive and negative sides from having a shortcut route for electrons to flow that isn't going through the circuit.

Equivalently, since the top layer gained an electron and the bottom layer lost one, it creates a potential difference.



Re: Thermal conductivity: I guess I misread things. That graph says I'm wrong for most metals. Like I said before, I actually DON'T know a whole lot about that and I was guessing.

The billiard balls represent the atoms in the object.