Aug 19 00:20 2004 from Jesse Mundis right, the humble student approaches and seeks enlightenment. talking about all the solar stuff has re-awakened my interest in things electircal. Would someone care to provide a quick refresher on the really really basics? Essentially, I know that photovoltaics works based on the idea that a photon with enough energy strikes a wire and "knocks" an electron into a higher enrgy state. This is how we get current in the wires. further, I know that energy is the ability to do work, current (amps) is analogous to the water in water-electricity-based analogies, and voltage (potential difference) is analogous to the water "pressure" in such analogies. What I never really got was what, exactly, is "flowing" during current flow? the electrons don't get ripped off their atoms and carried literally along like water. I have this vague mental image of electrons in a wire doing the football stadium "wave", getting excited and standing until the next one does, then sitting down again. the elctrons (people) don't realy move from their place, but the energy wave, the visible wave form, moves along the wire. So, at my light socket or computer or blender, what is being consumed do the "work", and how does this tie in with amps and volts. Bonus question for practical housing folks - if memory serves, most residential house wiring circuits are rated for 15 amps @ 110 (ac) volts. That's 1650 watts (amps x volts, correct?) so 5 computers with 300 watt power supplies (excluding monitors) all running at the same time are about all that circuit would handle (with 150 watts of headroom to spare), correct? Bonus bonus question: difference between AC and DC? Many thanks. If this becomes a longish explanation, it may be worthy of archiving on the halibut lore page too. Aug 19 00:36 2004 from Will The electrons in a wire *do* move, but not very quickly. That's what separates a conductor from a non-conductor -- in a conductor the electrons are (relatively) free to flow, hopping from atom to atom of the material. What does move fast is a "pressure wave" when voltage changes -- this moves at nearly the speed of light, like your stadium-goers. To answer your 15a/110v household question, the answer is essentially, "yes, that's correct, but...". And the "but" is the concept of power factor, which really can't be explained until your AC vs DC question is answered. Aug 19 00:47 2004 from Jesse Mundis cool. good start. we can hold off on power factor and AC vs DC for the moment. I'm still not clear on what the "pressure wave" of voltage change actually *is*, or how it is "consumed" by devices plugged into the wall to make work happen. Going back to analogies, this time rocks on hills, I understand that voltage is "potential" and if there is no potential difference, you can't extract work. Rock on level ground doesn't go anwhere. Rock on top of hill has more potential energy which can be turned into kinetic energy by rolling it down the hill. Likewise, stretched rubber band stores the potential energy until it is released. These are all mechanical examples, like the water flow one. What i fail to grasp is the conceptual idea of what voltage potential really is. I can see the potential difference in a hill, waterfall, or stretched rubber band, and I can grasp the manifestation of matter moving quickly as kinetic energy. The water slows while pushing a paddle, and the water wheel grinds grain as work..got it. But the electrons moving through the conductor don't push my blender in the same way. this voltage "presuure wave" which moves near the speed of light does. I'm still just missing either the "how" or the "what" in my mental model. Thanks for putting up with my fumbling descriptions of what I know to be basic concepts. Aug 19 00:48 2004 from Will AC is just ramping the volage on a conductor up/down in a sine wave. In the US, it goes from -170ish volts to positive 170ish volts in 1/60th of a second, resulting in 120ish RMS AC voltage (see http://www.myhometheater.homestead.com/RMS.html for explanation of peak vs RMS voltage). The main reason to use AC is that you can use transformers to bump the voltage sky-high for transmission. At a higher voltage, you need less current to move the same amount of power (again, because watts=volts*amps) and power loss in a wire is proportional only to the *current* (amps), so the higher voltage the better for long-distance transmission. The trick with AC power measurement is this: Imagine a graph of the voltage going up/down at 60hz (a sine wave). The trick is, some loads won't draw current proportianal with the voltage at any given point. So you end up with a voltage curve that is different from the current curve. This difference gives rise to the "power factor", which is a measurement of how out-of-phase these two curves are. So you end up with "true power" (watts) and "reactive power" (volt-amps), which are only equal when the power factor of the load is 1. For lots of loads (like, you guessed it! switching power supplies like those found in computers!), the power factor is decidedly *less* than one, which means that reactive power > true power. The power company bills you for true power. UPSes are rated in reactive power (VA). And I believe, that since circuit breakers are really current-sensing devices, they too will pop at a certain current, regardless of the true power you're pulling through 'em. Anyway, this is probably more than you wanted to know... Aug 19 00:51 2004 from Will I think it's more like water than you're thinking. The pressure wave moving on the order of the speed of light is what makes the device respond "instantly" when you turn it on (light bulb, blender, etc). But it is the electrons that were sitting there in the wire (just outside the motor or lightbulb filament) that immediately get *moved* through the (motor or lightbulb filament) that do the work. Aug 19 01:03 2004 from Will Or this: you have a valve connected to a pressurized water supply. It opens into a pipe which goes a few feet and there's a little water wheel built into the pipe. Then a "return pipe" comes back to your position from the wheel. The pipe is *always* filled with watter, just no pressure. The moment you turn the valve on, the wheel starts spinning instantly because the pressure wave moved faster than you could perceive. Simultaneously, water started pouring out of your "return pipe" onto the ground -- at *low pressure* because the potential energy has been removed by the water wheel. Substitute electron for water molecule and you've pretty much described a DC circuit... Aug 19 03:51 2004 from Mark Jesse: Will has done a bang-up job at describing this. You're right, it's not the actual electron flow that makes things happen, it's the "preasure" behind the electrons that makes it happen. Just like with water, just having a bucket of water isn't all that exciting, its the movement of that water that makes things happen, the force of that water pushing on something, the kenetic energy that the water holds. It's not the water molecule itself, the ^W it's the energy contained in that molecule's movement that is interesting. Same thing with electrons. The force pushing that electron through the wire is the interesting bit. Aug 19 20:01 2004 from The Other Josh Jesse: You may find this link useful (bunch of PDFs): http://www.enm.com/training/siemensstepcourses.asp Aug 19 21:41 2004 from Jesse Mundis Thanks Will (and Mark and TOJ). I think it is becoming clearer. So, in answer to one of my first questions, what gets consumed by the device doing the work is essentially that "kinetic energy" or "pressure". The electrons are just carriers the way water is a carrier of momentum in physical systems. Part of what I was missing before is that, in Will's words, "the pipe is always full of water" so that the "pressure wave" (potential difference / voltage front) can travel nearly instantly along the length of the conductor to the device, but it's the electrons that were all ready right next to the motor, and not those at that origination point that *deliver* the power to the device. So, I suppose that leads into the question of what voltage, or potential really is. I'm guessing it's just the physical "pull" exerted on charged matter by the electro-magnetic force. Correct? the term "columbs" is surfacing out of my physics past. Would it be correct to say that voltage is a measure of physical "pull" (or push) on the charged particles due to the presence of other highly chraged particles, through electromagnetic repulsion or attraction? If I'm not completely off base here, then I'm still a little baffled by AC. As the voltage varies from positive to negative, it would seem the "pressure wave" would "slosh" back and forth. How do AC powered devices sync up? Do they get motive force from only half the waves (like a ratcheted water wheel?) that seems inefficient. so I'm guessing they make use of the coming and going directions of the current flow somehow. Again, thanks to all of you for indulging me here. Aug 19 22:22 2004 from Will I don't really know what is happening at the atomic level in terms of voltage. If you find out, I'd love to hear it. An ampere is 1 coulomb/sec. All the definitions of a volt I've seen just make reference back to other units (volt = electromotive force at which 1coulomb/sec will dissapate 1 watt in a wire with a resistance with 1 ohm), but they don't explain what the electrons are doing. Aug 19 22:27 2004 from Will As for AC.. it really depends on what the load is. An inductive AC motor can just use the AC directly to create moving magnetic fields and cause the motor to spin. Many electronics devices use a transformer to get low-voltage AC and then rectify it (with diodes) to "bumpy" DC and then smooth it out with capacitors and inductors. Regular incandescent light bulbs can't tell the difference between AC and DC (and their power factor is 1, if you want to think about that). Aug 19 22:29 2004 from Will If youy jave a specific AC-powered device in mind mention it and we can dissect it. Aug 19 23:41 2004 from Mark AC: You're absolutely correct that the electrons just "slush" back and forth. Again, its the kinetic energy of that slushing movement that devices use. You mentioned a ratcheting water wheel. You can think of a diode as a ratcheting water wheel. A diode allows current to flow one way through it, but not the other way. It's a one-way valve. You can feed an AC signal through a diode and it will effectively stop the lower half of the waveform. You can use four diodes in what's called a bridge rectifier to take the bottom half of the wave form and fold it up to the top (this is the "bumpy DC" that Will mentioned.).. This puts all your voltage in the positive side, but it still changes as the absolute value of a sine wave. You can then use a capacitor as a sort of bucket or balloon, a holding tank of current, to even that out. Durring the peaks, you're putting more current into the cap than is being used. Durring the valleys, you're putting less current into the cap than is being used, so the cap starts to empty some of the current you previously put into it. The output is a much more consistant voltage. This is called a power supply. Take AC, turn it to noisy DC with a bridge rectivier, then filter off the noise with a capacitor. You now have clean DC. Add a transformer on the AC side before the rectifier, and you can have any voltage you want. Add a voltage regulator after the filter cap and you can adjust the voltage. (Voltage regulators also clean up the DC some more.) Most "AC" devices do this. Anything with a power supply does this (in one way or another. There are other ways to turn AC into DC; this is just the simplest). Other AC devices use the AC directly. A motor is the best example. It uses the varrying voltage to its advantage. Lets start with an electromagnet. Did you ever wrap a wire around a nail and connect each end of the wire to a battery and pick up nails with it? That's an electromagnet. Now, take a few of those nails, and arrange them radially around a shaft that has a perminant magnet attached to it. (Correction: Take two of those nails, not a few.) Pass your AC signal through one of the coils. Then, pass it through the other coil, but going the other way. Now, as the voltage is at a peak in one coil, it'll be at a valley in the other; the coils are out of phase by 180 deg. As one coil prodices a N magnetic field, the other produces a S magnetic field. This will spin the magnet on the shaft so that its natural S points to the N coil, and that its natural N points to the S coil. As the voltage changes and goes to a valley, the magnetic polls in the coils will reverse, swinging the magnet on the shaft around the other way. Lather, rinse, repeate 60 times a second. Congradulations, you have a 3600rpm motor. There are tricks you can play with the number of coils and magnets on the shaft to change the speed of the motor, and with three-phase power, you can even get constant power transfer into the motor, but these are advances topics. Will's other example is an incandesant bulb. It's just a resistive load. Current passes through it, it gets hot and starts glowing. It doesn't matter which way the current flows, so AC works just as well as DC. A light bulb actually flickers 120 times a second (it glows on both the valleys and peaks, but not the zero crossings.) But, with incandesant bulbs, they can't cool down and heat up quickly enough for you to notice it. But, you can sometimes notice it with a florecant bulb. That's why they flicker. Any other questions? Aug 19 23:45 2004 from Mark ...and a short one. Aug 19 23:49 2004 from TheFinn holy crap that was a nice explanation, it even filled in a few holes for me *) Aug 19 23:50 2004 from rick Oh, a prior post said "In the US, it goes from -170ish volts to positive 170ish volts in 1/60th of a second..." Which should have inserted "and back to -170ish volts" to be correct. Aug 19 23:52 2004 from Will Right, that was me. 60hz AC power means 60 complete 0->positive->0->negative->0 cycles per second. Aug 19 23:53 2004 from rick Otherwise, great explanation! Aug 19 23:57 2004 from Will I started to understand a lot more of this stuff while working towards my ham radio license... After reading about a lot of 'practical' applications and dealing with 3-phase & reactive power issues at work all the stuff from physics classes years ago clicked. Aug 20 00:03 2004 from TheFinn Of course, the 6phase power supply that Gigabyte is using now is interesting Aug 20 00:47 2004 from Jesse Mundis Mark, thanks for the motor example. I have, of course, played with electro magnets, but every time I thought about the motor, it would seem like the next phase of current would undo the work of the previous phase. having them out of phase on opposite sides makes perfect sense to me. Thanks all. So, someone want to capture that excellent discussion for posterity?