CookingWithGas, how is the water analogy flawed at the level of detail we’re using in this thread?
lobotomy, in your Apollo 13 example, a full bucket of water would be analagous to a capacitor, as it’s storing potential.
A Coulomb is strictly a unit of chrage, it’s 6.25 x 10^18 electrons.
Put it like this: a static charge has a huge potential difference (between, say, your fingers and the doorknob you’re about to touch), but not many (relatively speaking) actual “free” electrons built up in your fingertips, ready to fly away.
So when your fingers get within a certain distance of the doorknob, those (relatively few) “free” electrons, that have a huge potential difference behind them trying to push them off of your fingertips, “jump” (arc) to the doorknob. That is current; the air between your fingertips and the doorknob is acting as a diaelectric (has a resistive/insulating effect), which is overcome at the moment of arcing.
A “static charge” is called such because of the excess or absence of free electrons (valence band) in conductive material (and most material is conductive to some degree or another) that have no where to go or any (easy) way to “equalize” with surrounding material.
The topic of electric shock has been discussed ad nauseum on the SDMB. Briefly, a person is electrocuted when there’s too much current through his/her body. (Frequency also plays a part, as does the particular path through the body. But I won’t get into those subjects.)
Anything above 10 mA is “dangerous territory.” In order to get 10 mA or more to flow through the body, you need to impress a *voltage * between two points on the body (e.g. a voltage between the left foot and right hand). The minimum voltage required to get 10 mA flow through the body will depend on the body’s resistance between those two points. So what is a body’s resistance, you ask? There’s no good answer for this, as it depends on a lot of factors.
Generally speaking, any voltage above 50 V is considered “dangerous.” Tests have shown that, when you get above 50 V, you begin to get into the “dangerous territory” where more than 10 mA can flow through a body.
No, this is because one line to the outlet is neutral (has no voltage present*) and the other line is hot (that is, it has a voltage up to 120 VAC WRT ground). By polarizing the outlet and having electricians follow a standard practice of wiring, you can guarantee which part of an appliance is connected to neutral and which to hot. The hot parts are located so as to be inaccessible to people, while the neutral is connected to parts that people might contact.
- The exception is a shared neutral, which may have energized circuits upstream. There’s no (actually some, but comparatively little) voltage as long as the neutrals are all connected to the neutral bus in the load center, but as soon as they are separated, the shared neutrals can present a nasty surprise unless you’re aware of the danger.
Don’t make me bring up the human body model!
Here’s what happens in the winter when you walk across a carpet, and get zapped on a doorknob. Basically, you are a small capacitor charged to a high voltage. This charge is then discharged through a resistance of around 1.5kΩ. If you had your body capacitor charged to 50,000v, you would get peak currents in the 50A range, but only for nanoseconds, as the capacitor discharged. The reason people don’t drop dead every winter from this is two-fold: 1) The current spike is very narrow (T = RC implies 1.5nS to 37% of the initial current), and 2) Most of this charge is stored on the outside of your body (all those little electrons are trying to get as far away from each other as possible, why your hair stands on end when you touch a Van de Graff generator), so there is no path through your heart.
I’d rather thought that your average non-electically-inclined person would be more worried about touching wires, than touching a concept.
You broached it, so you get to explain “Thevinin-In-25-Words-Or-Less-For-Beginners.”
I was trying to keep it relatively simple for the OP. Your basic sine wave is a classic example of your fairly basic AC voltage. Keep this up, and you’ll be trying to introduce phasors and tank circuits to the discussion.
Yes, I alluded to that already.
No. A plug is usually polarized when…
- There is a SPST “line side” power switch in the appliance.
- There is a line side fuse in the appliance.
- There is a line-side circuit breaker in the appliance.
- A finger or other body part is more likely to make contact with one side of the line voltage than the other.
In the first three cases, you always want to place the switch (or fuse or whatever) on the hot side of the line voltage, not the neutral side. Hence the polarized plug. In the fourth case, if a finger or other body part is more likely to make contact with one side of the circuit than the other, then you would rather it make contact with the neutral side (as opposed to the hot side). In a light bulb socket, for example, you would want to connect the threaded part to neutral, and the button at the bottom to hot. This is because a finger is much more likely to touch the former vs. the latter.
I’m sure there are other reasons for polarized plugs. But these come to mind first.
Sorry about the nitpicking, ExTank. But when it comes to electrical discussions, I have an annoying habit of being über-precise. 
Not to get too off-topic, but this is the reason that the FAA has now banned loose Lithium batteries in checked luggage: http://techthoughts.org/2007/12/31/new-faa-lithium-ion-battery-rules-jan-1-2008/#more-215 (you knew about this, right?).
They determined that it was possible for these batteries to cause a fire that the fire-suppression systems couldn’t deal with.
OK so IIRC when you’re dealing with DC current (which, in my brain, is a battery), the positive end is sending electrons out to do work. Having completed that work, they return to the negative end. They must return as negatives, I think, or else you’d have that burning sensation I got in my pants which have made for an amusing dance to any onlookers.
This is AC we’re talking about, though. The stream isn’t +++++, but ±±± etc. and doesn’t return to the source (and which cuts the number of power lines needed in half). So what is the purpose of the neutral? Does it go to ground?
No, I hadn’t seen that; I can’t remember the last time I flew, but thanks for posting it.
I fully believe what they’re saying here (dopers should watch the youtube demo). I’ve never handled anything so hot in my life.
Hm. All of the systems I’ve worked on take 120v AC from the wall, and then using low voltage and high voltage transformers, step it up to somewhere between 60 kvp and 120 kvp (AC) depending on what body part we’re xraying.
Still, we’re usually between 4 milliamp/sec (small extremeties) and 200+ mAs (lumbar spine or large abdomen) at exposure.
I know this sounds weird, but in reality the electrons don’t really “come out” or “go back into” the battery. In fact, electrons don’t even flow through the battery. Well, a few might. But the flow of charge in a battery is mostly due to ions.
To *really * understand what’s going on, you have to whip out Maxwell’s Equations. Even for DC circuits. But there’s no way in hell I’m going to do that.
But if you would like a conceptual (albeit technically wrong) model, you can think of positive charges “coming out” of the battery’s positive terminal, going through the wires and load, and then “going back into” the battery’s negative terminal. Once inside the battery, these positive charges are moved to the battery’s positive terminal against their will (via a chemical reaction), and then flow out the battery’s positive terminal again.
Again, this is not what’s going on. But this model will allow you to analyze DC circuits with more clarity.
The neutral is connected to earth ground back at the circuit breaker panel. But the circuit will still work even if it were not connected to earth ground. Just as a car battery does not need its negative terminal to be connected to earth ground in order to start the car.
But… we always connect the neutral to earth ground. The primary reason is to prevent the 120 VAC from floating up to a dangerous voltage.
That’s the unfortunate result of an incorrect guess (they had a 50/50 chance) and what we today call “conventional current”. In actuality, electrons flow from the negative pole to the positive one. It’s not important to this discussion, however; you have the fundamental idea behind DC right.
This is a bit more complicated. Ultimately, both ground and neutral wind up the the Earth, but the neutral also serves as the return path you allude to and is considered to be at zero volts. The hot line, on the other hand, alternates (hence AC or alternating current) between 170 volts above zero (+170 V) and 170 volts below zero (-170). Since the voltage changes sign, the current it drives changes direction and it does so sinusoidally (because the generators which produce it rotate in circles). Throughout this cycle, the neutral remains at zero volts, but carries the return current fist on one direction and then in the other.
In the Semiconductor manufacturing biz, there are ion implanters that use nearly 1/2 million volts! They have very, very long plastic poles with a metal strap on one end to discharge them.
The tube itself requires only DC because the electrons which produce the x-rays through “braking radiation” (for which electrical geek types like myself use the German term bremsstrahlung) need to flow in one direction from the hot filament emitter to the tungsten target. I have a tube sitting here on my desk, in fact. It’s a small tube used in a dental x-ray rated at 90 kVDC.
I suppose you could could drive a tube with AC, but you’d be essentially throwing away half your power and x-ray tubes are very inefficient as it is–only about 1% of the energy consumed is actually turned into useful x-rays.
@Crafter: what are the ions? I remember ions from Chem but what do you mean? These must be ions in the metal contacts to the battery?
@QED: OK, it makes sense that the flow is from negative (it’s an electron) to positive.
So AC provides a return path of sorts (some positive, some negative but otherwise like DC)?
PS to other posters: I can pretty much track on the sine wave stuff, having studied trig in high school.
Ah, OK, just the tube itself.
I guess you’re right about that part. The rectifiers “straighten out” the flow of the AC in order to achieve the cathode —> anode jump (at least I think that’s where it’s done) but the rest of the system runs on 3-phase 120V AC stepped up to whatever kvp we need.
(Next we can start talking about Compton scatter and the photoelectric effect. :))
This is correct when all voltages are compared to earth ground. But IMO this explanation may run the risk of over-complicating the issue for the uninitiated, since it makes it seem earth ground is an integral part of the circuit.
In my opinion only, the best way of explaining a home’s AC system is to begin by ignoring the fact that the neutral is connected to earth ground. Instead, simply show a toaster (or whatever) connected to an “AC battery,” and then analyze the current and voltages in the circuit. It should be noted that neither conductor is “hot” or “neutral” in such a circuit, and the voltage between either conductor and earth ground is undefined.
Once this is understood, replace the AC battery with the secondary of a transformer (which can be modeled as an AC battery in the first-order) and note that the toaster still works fine. But there is a potential safety problem… a transformer is not an ideal device, and there’s some capacitance and leakage resistance between the transformer’s primary winding (which is at a very high AC voltage relative to earth ground) and the transformer’s secondary winding. So while the AC voltage between the two secondary wires has a peak of 170 V (which is called the secondary’s “normal mode” voltage), there’s a chance the voltage between the secondary winding and *earth ground * could “float up” to a pretty high voltage. This voltage is the called the “common mode” voltage of the secondary. In the worst case, the common mode voltage might “float up” to the primary voltage. This is dangerous, because people are often touching earth ground. If they were to make contact with either secondary conductor they might get zapped.
So how do we fix this? Simple - connect one of the secondary conductors to earth ground. It doesn’t matter which one. It is then noted that - theoretically - only a tiny bit of “leakage current” flows in this earth connection, and that the only reason you do it is to keep the common-mode voltage from “floating up” to the primary voltage.
Note that the above is an explanation for a two-wire/single-phase secondary. The transformer used for homes is actually a “single phase with center tap.” Some people call this “two phase.” With this transformer, the center is connected to earth ground, and there is 120 VAC between Hot 1 and the center tap, and 120 VAC between Hot 2 and the center tap. These are 180 degrees out of phase, thus there is 240 VAC between Hot 1 and Hot 2. The center tap is also called “neutral.”
Heck if I know. I curl up in a ball, sit in the corner, and suck my thumb when I’m asked any chemistry-related question. Googling “battery chemistry” would probably provided the info, but I’m afraid to do it.