[Coda]

Doesn't make much difference. Whichever form is easier to manipulate with whatever technique you happen to be using at the time.

[Potironette]

Is there anything preventing objects from gaining even more electrons? (In class we're learning about positives and negatives and stuff making other things positive and negative. I noticed the more I rubbed plastic pens with fabric, the better it got at picking up pieces of paper. What keeps someone from charging something until, say, the pen could stick to a wall made of paper or something?)

[Coda]

It might be possible to charge something as light as a pen enough that it can hold its own weight electrostatically against a wall. It's certainly possible to do that with an inflated balloon. (Try it! It's fun.)

A general principle in physics, that holds true at all scales, from subatomic to universal, is that processes by default proceed towards lower-energy states or more stable states. (Usually these are equivalent, but there are some cases where a stable state has more energy than an unstable state; the most common examples are endothermic chemical reactions, which absorb heat from the environment to make a final compound that's more stable than the reactants that went into it.) A ball at the top of a hill has more potential energy than a ball at the bottom of a hill, and it doesn't take much of a bump to cause that ball to proceed to a more stable, lower-energy state. To make a system move the other direction, you have to put energy in.

It takes work to separate charges. The greater the difference already is, the more effort it takes to separate them further. You know that like charges repel, so as you get more negative charge gathered on one object, those charges try to push each other (and further negative charge) away. The same is true of positive charge. As you keep forcing charge in, the equilibrium will be such that even the air molecules bouncing off the surface of the object will have an easier time holding that charge than the object itself, so in effect the object bleeds off excess charge into the atmosphere.

And you already know what happens when the potential difference between two surfaces gets too great -- the electrons will actually JUMP from one side to the other through whatever path they can find: sparks and lightning.

[Potironette]

I don't think I have the patience to try to see if a pen can stick to the wall, but in class we did stick a balloon to the wall. It wouldn't stick after rubbing it on cloth, but did stick with hair. I vaguely recall it was because hair let go of electrons more easily...? I stuck a bunch of balloons to the ceiling after they stopped flying by rubbing them on my shirt though. Is it because the balloons were lighter? Because maybe the rubber material was different?

So..eventually the electrons will just have too hard a time moving into an object or out of one and that's what decides the limit?

...if I had a cloth and was using it to stick helium balloons to a wall, would the cloth get less and less effective?

[Coda]

Your body is a good electrical conductor, while the rubber of the balloon or the plastic of the pen is a good electrical insulator. Your own body ends up providing a connection between the cloth in your hand and the ground, so the cloth is able to drain off its excess charge THROUGH you.

In isolation -- say, if you used a pair of rubber tongs to manipulate the cloth... You might be right.

EDIT: Your other questions!

Yes, the buoyancy of balloons in the air helps. The pen doesn't have a lot of surface area to collect charge on, and it has more weight.

And yes, that's the limit. It's not STRICTLY a limit, mind you; it's not some fixed measurable cap. It's just a question of how much work you want to apply to the task. Rubbing the objects together by hand within the atmosphere may simply not impart enough energy to go beyond the small effect you've seen. On the other hand, van der Graff generators can build up HUGE potentials because their parts are very well isolated from grounding.

[Potironette]

Ohh I didn't realize the cloth would regain electrons just by holding it with bare hands :/.

Are electrical conductors and heat conductors completely unrelated?

So..a "ground" is anything that acts as a source of electrons..?
Does a "potential" mean an object is more negative, or does it mean it's far from neutral?

[Coda]

Originally Posted by Potironette

Ohh I didn't realize the cloth would regain electrons just by holding it with bare hands :/.

Yup. Always have to think about the whole system, because no experiment is ever TRULY an isolated, closed system.

Are electrical conductors and heat conductors completely unrelated?

Not COMPLETELY unrelated, but not completely related, either.

Heat is the kinetic energy of the particles within an object. Electrons are particles, so electron motion contributes to heat motion.

For metals, there's a relatively straightforward relationship between electrical and thermal conductivity. At a given temperature, the thermal conductivity is directly proportional to the electrical conductivity (that is, metals that are good electrical conductors are also good heat conductors), and raising the temperature causes electrical conductivity to go down and thermal conductivity to go up, and vice versa (that is, all metals transfer heat better when they're hot and transfer electricity better when they're cold). This is called the Wiedemann-Franz law, although I didn't actually KNOW that name until I went and looked it up just now.

For nonmetals, the electrons still contribute to heat transfer, but the molecules as a whole have a greater impact on how fast or slow the heat is transferred, so the specific relationship isn't so cleanly predictable. It's still possible to figure out the relationship in a given substance, but there's a lot more that you have to take into account.

So..a "ground" is anything that acts as a source of electrons..?

A ground is considered to have an infinite capacity to give and take electrons. Usually it's considered to be an electron sink instead of an electron source, but that's because usually when you're working with electricity you're pushing electrons instead of pulling them, since it's easier that way. But it works fine both ways.

It's called a ground because the Earth itself is the biggest, most convenient ground available, so when you're wiring a house for electricity, you shove a big metal spike into the ground (or you use a big buried metal pipe) and connect stuff to it so that it's easier for the current to go through that route instead of through your body to the ground THAT way if you touch a bare piece of electrified metal by accident.

Does a "potential" mean an object is more negative, or does it mean it's far from neutral?

Neither. It's the difference in relative charge. You could have a weakly negatively charged object and a strongly negatively charged object and there would still be an electric potential between them. You could have two objects with equally strong negative charge and despite the magnitude of the charge there would be no potential.

[Potironette]

Pushing electrons? As in moving them into the ground/earth..?

Why do colder metals transfer electricity better when cold? Because the protons are moving around and attracting electrons :/?

Why do metals transfer heat better when hotter? Does that mean a hot metal pot gets hotter more easily than a cold metal pot :o?

[Coda]

I mean that when you're dealing with chemical batteries or solar cells, you're getting a bunch of free electrons on one end of the circuit, and since like charges repel, they get pushed out as soon as there's a place where they can actually go. And they're not specifically looking for a positively-charged place to go; they're just looking for a less-negative place to go (because being less negative means it's going to push the electrons away less), and the Earth is so massive that you can add a whole lot of electrons to it without making a significant difference to its net charge, so it stays overall pretty neutral no matter how much you pump into it.

You CAN do it the other way. You CAN have a reaction that binds up electrons and leaves a positively-charged void that's desperately trying to attract electrons. That's a negative electric potential. It'll happily draw those electrons from the ground (because the Earth has so many electrons that it'll never miss a few) if there's no place better to get them from.

Chemical batteries do this at the positive terminal, so given the choice to travel from the negative terminal to the ground or to the positive terminal, the positive terminal is twice as attractive of a destination. Solar cells are comparatively weak because the only force pulling electrons through the circuit load is the fact that, after a photon forces an electron to start moving down the wire, there's not an electron there anymore so there's room for another one to come in from the other side.

You're not too far off when it comes to why colder metals conduct better. A hotter material has more random motion in it, the atoms in it are moving faster relative to each other, so electrons have to be moving faster in order to make the jump from atom to atom. A colder material keeps the atoms closer together.

I don't completely understand the thermal conductivity thing myself. I know it because i read it. However, no, it doesn't mean that a hot metal pot will get hotter more easily. It means that the heat will spread out more evenly once it's gotten warmed up compared to at the beginning. Instead of a pot, think about a long metal pipe with a heat source at constant temperature at one end. If you draw a graph of the temperature of the other end of the pipe, then it'll be a curve going upward instead of a straight line -- the hotter the far end already is, the more rapidly it approaches the temperature of the heat source.

If I had to take a guess as to why this is true: Imagine a billiards table. In a cold material, the balls are all close together, with some space between them. If you shoot the cue ball into the cluster of balls, the energy is going to get spread out pretty fast because each ball will bump into more balls as they start moving. But if the balls are more spread out, then when you shoot the cue ball, whatever it hits is going to immediately go zipping off because there's not as much in its way, and so it can transfer its energy to a place farther away on the table sooner.

[Potironette]

Err, so in a house or apartment, electricity comes from some..plant(?). Then that is continuously(?) running through the place and..then gets pushed down to the ground? And when someone plugs something into a power socket..they get access to that electricity and use its energy(?) to convert it into whatever the thing they plugged in does(?) (ex: to light up a light).
^I'm pretty much guessing most of that. How do power outlets work and where does the electricity come from to get pushed into the ground in the first place..?


So...solar cells are parts of the solar panels?
I found this little picture:

Is it saying that normally, electrons are stuck around atoms, then the sunlight hits it --> an electron is freed from the atom --> electron attracted to bottom layer so it moves through a wire making it accessible to household appliances.
I guess the layer separating them makes the electron just "know" the wire lets it get to that more electron attracting(?) layer?
Furthermore, where how the electron get back after it reaches the bottom layer o_o? Isn't the top layer less attractive than the top one? What are all the middle layers supposed to do?


Chemical batteries...so terminals are, I'm guessing, the + and the - labeled ends of a battery? Err is this about recharging batteries..? I'm not really sure how to picture what's happening.


I think I didn't know the definition of conductivity o_o. So conductivity is how easily something spreads(?) throughout an object? Like heat throughout the whole length of a wooden spoon or electricity through a whole length of wire? And when metals are warmer they move more incoming heat more easily possibly because..uhh "electrons contribute to heat conduction" and the billiards analogy is that electrons can go zipping away faster..? But that might not be it because electron movement is impeded with heat, so it's something else? But atoms don't move?

...Oh, actually, does thermal conductivity in metals decrease with temperatures normally?

stainless steel is different though.

But for nonmetals, conductivity does increase with temperature.

But why?
And what are the billiards balls representing?

[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.

[Potironette]

Err, do you mean the bottom layer has electrons that can move, and when a photon makes an electron move away, the electron in the bottom layer moves up--and because it moves it, it is now positive, attracting the electron that got knocked out by a photon? Aand somehow the middle layers only allow electron movement from the bottom to the top..?

And thank-you very much for that explanation on household power!

[Coda]

Yep, that's a pretty good explanation of the phenomenon.

The middle layer is suuuuper interesting, actually. It's a pure (undoped) wafer of silicon crystal. Silicon has some WEIRD properties. Silicon is not exactly a metal, and it's not exactly a non-metal. It's a metalloid. It's not exactly an electrical conductor, but it's not exactly an electrical insulator. It's a semiconductor. You've probably heard that term before when talking about computers; well, you're about to learn what that actually means.

Pure silicon crystal, with no impurities whatsoever, has a very neat arrangement of electrons. Every atom has exactly the number of electrons it wants to have, and it's energetically stable, so the electrons don't have much motivation to go anywhere. It acts as a weak insulator.

But that's assuming that all of the electrons ARE where they belong.

It takes more energy than a normal electric current to dislodge those electrons from their happy place of stability, but it doesn't take THAT much -- it takes less than an insulator would take. And conveniently, photons near the visible light spectrum have enough energy to do the job. And when the electron is forced out of place, if it has somewhere it can go -- such as into a circuit -- then it leaves a positively-charged hole behind. That hole is unstable. There's a pressure to pull another electron from nearby into the hole. But that means THAT electron leaves a hole behind that needs to be filled. So eventually that hole has to get filled by an electron from outside.

But that's the behavior of pure silicon. If you mix in just a few atoms of phosphorus, then you have extra electrons. The region gains a small net positive charge, because those extra electrons will readily leave. If you mix in a few atoms of boron, then you have extra holes. The region gains a small net negative charge, because those holes will accept extra electrons readily. (Yes, it sounds a little bit counterintuitive that adding atoms with too many electrons will cause it to have a positive charge, and vice versa. This is because the phosphorus atom is neutrally-charged normally but when you mix it in, it loses the extra electron, thereby becoming positively-charged.)

So by sandwiching undoped silicon between a negatively-doped ("n-type") region on top and a positively-doped ("p-type") region on the bottom, all of the mobile electrons or holes in the undoped silicon that MIGHT allow conducting electricity are pulled away into the doped regions -- the silicon is "depleted" (hence "depletion zone" in the picture) of charge carriers, leaving a potential difference that further resists electrons passing through. And if a loose electron DOES get in there (perhaps because it was hit by a photon, eh?), it follows the bias of the electric field. So the current can only flow one way through it.

[Potironette]

Err what does a semiconductor have to do with computers?
What's the difference between a semiconductor and a not-so-bad conductor? Or, why is it that being a semi-conductor is special?
On second thought, what is the definition of a metal exactly? Of a metalloid o_o?

What is the "bias of the electric field"? That current can flow only in one direction at a time..?

Undoped silicon doesn't take much to give up electrons, and so its electrons get pulled into..either layer o_o? But maybe electrons can only flow one way(?) so it just goes to the top as the electrons on the n-layer move to the p-layer...?

[Coda]

Computer chips are the most well-known use of semiconductors.

The thing that makes a semiconductor special is the ability to control WHEN it's a conductor. Metals just always conduct, even if the conductivity is low.

Most elements (91 out of the 118 known elements) are metals. Metals characteristically lose their outermost electrons very easily, which means there's a free-flowing cloud of electrons inside the metal instead of a rigid electron structure. This is why they conduct heat and electricity so well. Metals are usually malleable (able to be pressed into shapes without breaking), ductile (able to be stretched into wires), and fusible (can be melted together). A nonmetal is anything that doesn't do these things. Metalloids have properties in between, though which elements are metalloids and which aren't isn't completely agreed upon (for example, some classifications say aluminum is a metalloid, most say it's a metal; some say carbon is a metalloid, most say it's a nonmetal).

I'm not using "bias" in a jargon sense here. I just mean that having an electric field means that electrons will have a strong preference to move in one direction and not the other.

I said "charge carriers," not "electrons." You can treat the absence of an electron as a mobile positive charge. THOSE have all gotten pulled out, too -- that is to say, all of the holes got filled up with electrons and there are no excess electrons hanging around either.

[Potironette]

Oh woops, I missed the "charge carriers." Basically that middle part is content to not have electrons moving through?

Back to this diagram:

Is it that the sunlight hits the undoped silicon --> an electron now leaves that depletion zone and a hole is created there(?) (this part I'm confused about) and somehow it's the bottom p-type layer that now has a hole, maybe because it moved an electron up to the depletion zone because electrons prefer to move in one direction as the electrons from the N-type layer are attracted to the P-type layer?
Why is it that there are two arrows coming out of the "Photon Absorbed in Depletion Zone Electron-hole Creation"? And..the photon just bypasses the N-layer and goes to the Depletion zone :o?

Metals characteristically lose their outermost electrons very easily, which means there's a free-flowing cloud of electrons inside the metal instead of a rigid electron structure. This is why they conduct heat and electricity so well. Metals are usually malleable (able to be pressed into shapes without breaking), ductile (able to be stretched into wires), and fusible (can be melted together). A nonmetal is anything that doesn't do these things. Metalloids have properties in between, though which elements are metalloids and which aren't isn't completely agreed upon (for example, some classifications say aluminum is a metalloid, most say it's a metal; some say carbon is a metalloid, most say it's a nonmetal).

Ah, woops. I completely forgot that metals were characterized by those traits! I didn't know they were characterized by being fusible though.

Computer chips are the most well-known use of semiconductors.

Unfortunately, I don't even know what a computer chip does nor where it is :/. Um, what is a "chip"?