Con someone explain electricity to me...in small words?

Factual Questions
61

The way I’ve always heard the saying is “it’s the volts that jolts but the mills that kills” (mills in this case referring to milliamps).

There is a point to be made here, but it gets a little confusing because voltage and current are related to each other. It gets even more confusing because the human body is not a simple resistor and does in fact have a very complex response to electricity.

As Chronos said earlier, electricity kills you in basically one of two ways. The first is that the current produces heat as it passes through your body and you basically get cooked to death. Here it is the total wattage (voltage times current) that matters. This is how lightning bolts and accidental contact with power lines kills you. It is also how the electric chair kills you.

Generally speaking, the higher the voltage the more current you will get to flow, so the higher the voltage the more dangerous it is. There are other things than the voltage that can affect how much current flows, so it is entirely possible for someone to get struck by a lightning bolt (several million volts) and live, while someone else can get killed by 120 volts from a defective lamp. To make it even more complicated, your body’s effective “resistance” to electricity varies quite a bit depending on the voltage being applied. At a low voltage, your body may seem to have millions of ohms of resistance, but at higher voltages your body has maybe a thousand ohms. Generally speaking, you can safely touch voltages under 50 volts and anything over 50 volts can much more easily kill you. This is why you can grab both terminals of a car battery and not get shocked but touching both conductors in a power outlet is a much less pleasant experience.

The second way that electricity can kill you is that it can screw up your heartbeat. This is related to the amount of current that you can get to flow through your chest (it’s the mills the kills). You can be killed by a surprisingly small amount of current. Most safety standards these days are built around anything under 5 mA being “safe”, though a current of only 5 mA is not at all likely to interfere with your heart’s rhythm. Once you get up around 50 mA though that’s when you are almost guaranteed to run into trouble. To put the numbers in perspective, 5 mA is 0.005 amps, and your house breaker won’t trip until you exceed 15 amps. So you can very easily be killed without tripping your breaker (that’s why they invented AFCIs and GFCIs).

Your heart is kind of a weird thing. It is much more immune to getting its rhythm all thrown out of whack at certain times during its heartbeat cycle than it is at other times, so getting your heartbeat screwed up is very much hit and miss (unlike being cooked to death by high voltage/current, which is much more predictably fatal). Hit your heart with a shock at some times, and nothing happens. Hit it at just the right time though, and your heart goes into fibrillation. This is where your heart gets weird. If you get it into this funky state, it is stable in this state. Instead of beating it will just sit there and shake, and unless someone happens to be standing next to you with a portable defibrillator it’s going to stay in that state and won’t be pumping blood, which is generally a very bad thing for you.

Another weird thing is that as you increase the current you generally increase the risk of your heart going into fibrillation, but at some point later, as you continue to increase the current the risk of fibrillation actually starts to drop. What happens is that instead of screwing up your heartbeat, the current just causes all of your heart muscles to contract. Your heart isn’t pumping blood at that point, so this is still a very bad thing, but usually when you remove the current the heart will go back into a normal rhythm. The important word in that sentence is “usually” thought. It isn’t “always”.

Coincidentally, if you use alternating current, the frequencies that are most likely to throw your heart into fibrillation are right around 50 or 60 Hz. Basically, from a safety standpoint, we couldn’t have picked worse frequencies if we tried. It is fairly easy to see how we ended up with those frequencies though. The lower the frequency, the bigger and heavier you need to make your transformers. Higher frequency transformers can be smaller and lighter (which is why aircraft tend to use a lot of 400 Hz instead of 60 Hz) but then the transformer hum from loose laminations and such gets really freaking annoying. Early power systems ran on a wide variety of frequencies, such as 25, 30, 40, 50, and 60 Hz. 50 Hz beat all of its competition in Europe, and 60 Hz won in the U.S. Exactly how we ended up at 60 is a bit of a mystery though. The story that I heard (that is likely an electrical urban legend) is that one of the early test systems was designed to run at 50 Hz, but they couldn’t get enough power so they cranked up the generators to 60 Hz. Everything after that had to be compatible with it, so the standard stuck.

Most electrical things that a person will encounter tend to be voltage sources, meaning that they will keep a constant voltage and the current will vary depending on conditions. So it may be the mills that kills but keep in mind the mills depends at least partially on the volts. It’s not one or the other.

62

Take a coil of wire. Run current through it. Congratulations, you’ve just made an electromagnet. Yep, it’s that simple (be a bit creative with it and you can create a motor). If you run current through a coil of wire a magnetic field will be created, and if you remove the current that field will collapse, releasing its energy as current. There’s no defying physics here. The energy that comes from the collapsing magnetic field is exactly equal to the energy that was used to create the field in the first place. The coil basically stores energy while current is applied, and releases it once the current is removed.

Home electricity is AC, for alternating current, which is a sine wave. So the electricity is applied, gets bigger, gets smaller, drops to zero, gets small in the opposite direction, then bigger, then smaller, and back to zero, following a sine wave which repeats over and over 60 times a second (50 for you Europe folks). So for part of this cycle, your coils of wire (aka inductors) are being charged. Current is going through them and energy is being stored in a magnetic field. Later on in the cycle, the current has been removed and the magnetic field collapses, and the inductor supplies current.

So basically these coils of wire are sitting there being charged and discharging and being charged and discharging, but the electricity isn’t being used. A light bulb, in contrast, takes the energy and converts it into heat. Coils of wire just store it and release it. They don’t do anything with it.

This is what is called “imaginary” power or “reactive” power. It is measured in “volt-amps reactive” or “vars”.

If you take two plates of metal and put them close to each other, the same sort of thing happens, except energy is stored in an electric field instead of a magnetic field.

The funny thing is, if you have a sine wave, capacitors (plates of metal) are charging while inductors (coils of wire) are discharging, and vice-versa. You can kinda think of it as inductors and capacitors working opposite to each other.

Power companies hate vars, because their generators and wires have to supply all of the extra current during the charging phase of the cycle, even though that energy just gets dumped back into the system during the discharging phase of the cycle.

Residential loads (i.e. houses) tend to be a bit inductive overall, because of things like motors in your fridge, washing machine, vacuum cleaner, etc. Power companies can take advantage of the fact that inductors and capacitors kinda work opposite of each other, and can add big capacitor banks at the substations to balance out all of the inductive homes on the power lines. The power company will switch capacitors on and off of the line as needed to balance things out. Get it perfectly balanced, and the reactive energy will end up just bouncing back and forth between the capacitors and the inductors. The capacitors will discharge while the inductors are charging and the inductors will discharge while the capacitors are charging. The power company’s generators then only have to supply enough current for the “real” power (the stuff that goes into light bulbs and makes heat, for example) which means much less of a load on the generators, and the power company saves themselves quite a bit of fuel.

63

Ahhh, now we’re into my area. V-Fib is what you’re describing. The fix: defibrillation.
mmm

64

Yes, but I wasn’t trying to explain chemistry, I was trying to explain why and how do voltage and current produce different effects: one measures energy, one measures amount. In case you guys weren’t paying attention, the OP is asking about electricity.

Nava, ChemE, MSc in Theoretical Chemistry

65

Ok, back to the simple stuff. What happens to the electrons? Do individual electrons leave the safety of their atoms (creating ions?) and physically travel down the conductor?

Or is there some sort of Newtons Cradle thing going on?

And what about AC? Do the electrons head north then turn around and go south, 50 or 60 times per second?

Cause if this happened, the active and neutral would swap roles at 50 Hz, and I don’t think they do. Never understood how AC works, so I guess my mental model is broken.

66

Oops

67

Free electrons are electrons that have left their respective atoms and are free to move about as determined by the electric field. Current (Amps) is defined as Charge (Coulombs) per unit time (Seconds). A Coulomb is equivalent to the charge of about 10^18 electrons.

Not in the sense that they transfer their momentum to one another. It’s more like a queue where they all want to get to the same place determined by the electric field. Like a series of balls rolling down a ramp, they are simply traveling from a high energy potential to a lower one.

This is essentially correct.

The neutral remains a virtual ground. During the first half of the AC cycle, the potential is positive with respect to the neutral and during the second half, it’s negative (which means that the neutral is actually at a higher instantaneous potential than the line. There’s a pretty good explanation here:

http://www.play-hookey.com/ac_theory/

68

I don’t think this is off topic since it is electric/electronic circuit related. If it is, I’ll try to get in under the 5 min. limit and delete it.

How does a capacitor store charge? The way I picture it is that you herd a bunch of electrons down what is basically a dead end and you keep stuffing them in there like a rush hour train in Tokyo. Then, when current is no longer flowing into the corral, the electrons rush back out into the circuit.

I’ve got a bazillion of these but I’ll try to restrain myself.

Completely OT: I just want to say that as a new member I am really impressed with the people here. Everybody seems to be a genuine, honest-to-Zeus expert on something. And I don’t mean “internet expert” either, but the real deal. Now please excuse me while I go wipe this brown stuff off my nose.

69

In a metal, there are some electrons (usually about one or two per atom) that aren’t particularly attached to any single atom, but just sort of wander around as they please. When you’ve got current running through a wire, these electrons do actually move relative to the nuclei. However, for typical situations, they move extremely slowly, and will travel almost no distance at all before the AC reverses and they start going back the way they came. When you flip a switch, if you had to wait for electrons to get all the way from the switch to the light bulb, you’d be waiting all day. Fortunately, you don’t have to wait for electrons to travel all that distance, since the electrons throughout the wire all start moving at once (well, almost at once: It’s still limited by the speed of light).

70

Sticking with Bill Beaty’s educational articles (which is the only stuff on his website that I’ve ever looked at), I always liked his capacitor analogy.

71

With respect to AC vs. DC circuits, I have an analogy of my own that I used back when I was teaching this material.

Picture a crank-driven driveshaft connected to another shaft via a drive belt. (An example of a drive belt is the rubber belt you have to periodically replace on your car’s engine.) The goal is to turn the second shaft, which is friction-braked. Turning the crank thus produces frictional heat at the far end.

As soon as you start turn the driveshaft with the crank, you start producing heat at the far end. You don’t have to wait until the section of belt at your end actually reaches the far end before you start producing heat at the far end. This is analogous to a DC circuit transferring energy to a light bulb much faster* than the drift speed of electrons (which is only a few centimeters per second).

Alternatively, you could just turn the crank back and forth. This will cause the shaft at the far end to also turn back and forth, and still produce frictional heat. Any given section of drive belt pretty much stays in place, but the work (energy) you put into the driveshaft is still rapidly transferred to the shaft at the far end. This is analogous to an AC circuit, in which the polarity is constantly reversing, but energy is still transferred in one direction from the generator to the load.

Note that the drive belt is not “used up” any more than electrons are. It is simply a method of transferring energy from one end of the circuit to the other. And, of course, in practice you don’t just want to transfer heat, but useful work.

*Note: “much faster” means a significant fraction of the speed of light.

72

That is one of Bill’s best articles. The ‘rubber membrane water analogy’ seems to be spreading through the online explanations, and replacing the old models that showed the ‘Tokyo rush hour’ type of analogy.

One of the simplest analogies is a comparison of a reciprocating saw to a circular saw. In the reciprocating saw, the teeth move back and forth, in the circular saw they travel in a continuous circle. (actually most reciprocating saws don’t cut on the backstroke, which is a good introduction to half wave bridges)

73

That’s a good analogy, too. Too bad I’m no longer teaching…

74

Pretty much. You have a C where the two ends of the letter are the poles of the capacitor, there is a wire between them, a source of voltage (battery or equivalent DC source) somewhere along the wire, and the open space between the ends of the letter is the capacitor’s insulator.

The battery herds your electrons: if there was a complete loop of wire, the electrons would just go 'round and 'round, but because there is that bit of insulator in the capacitor they have to stop and pileup right in front of the insulator, so what happens is that after enough time you end up with the same voltage difference between the poles of the capacitor as the battery was giving. The + side of the capacitor is an empty train wagon, the - side is full, but the electrons can’t jump from one to the other, it’s too far. And the push from the battery means they can’t go back along the wire. When you open the circuit it’s as if you chopped off a piece of the station’s platform: the electrons from the - pole can go back to the bit of wire hanging off the pole, but they can’t jump between the two wagons over the track because it’s too far and they can’t jump from a piece of wire to the other because there is another too-large hole. So, the full wagon stays full and the empty wagon stays empty…
until you close the circuit, but this time with another bit of wire, not a pushy battery. At that point theelectronsrushdownthewirethankyouverymuchI’msurethere’sstillroomthereIcanseeitdamn!

(Well, as Chronos explained, each individual electron doesn’t run all the way, it’s more like relief races, but let’s not crowd our explanation even more)

75

You might want to clearly point out that there aren’t extra electrons being stored in the capacitor as dzero implied. The imbalance of charges between the two conductors is creating an electrical field, which causes the current to flow when the circuit is closed.

As the Bill Beaty article points out, capacitors don’t store charge (i.e. electrons), they create an electrical field (i.e. energy), which pushes the electrons in a closed circuit when there is a voltage difference. The capacitor is like a spring wound pulley. If you loop a ‘clothes line’ of electrons over the pulley, you can wind up the spring by pulling the ‘clothes line’ in one direction, but the pulley is accumulating energy in the spring, not more ‘clothes line’.

76

There are six ways to make electrons move –

Magnetic (your standard generator)
Light (Photovoltaic/Solar cells)
Chemical (Batteries)
Pressure (piezo crystals)
Heat (thermocouple effect)
Friction (static electricity)

Of these, the first one is the only one proven to be able to generate power on a large scale. Solar is coming along, but it’s nowhere near as practical as magnetic generation.

And interesting, but useless factoid, all but one of these methods is directly reversible. That is to say, we can use electricity to directly create
Magnetism
Light (arc sources or LEDs)
Chemical reactions (electroplating)
Pressure/vibration (piezo crystals)
Heat

Friction is the only one we can’t reverse. To create friction, you’d need to use one of the other processes.

77

Is ‘magnetic’ accounting for Lorentz force and/or opposite charges?

78

Similarly:

Volts measure how much effort various posters put into explaining electricity in this thread.

Amps measure how many simul-posts occur in this thread, where people post explanations all at the same time.

Ohms measures how often the SDMB servers crash and people have to retype their entire analogy-ridden post

:smiley:

79

Doesn’t fighting ignorance count?

80

Robby, Nava, Tripolar - thanks for the clarification. It was only my mental image not a statement of fact, but still - thank you.

Would anyone have a good analogy or explanation for what an electric field is? I think most people have an intuitive grasp of what a magnetic field is just from playing with magnets as a kid and middle school expeiments with iron filings and such. But I’ve never really understood what an electric field is.

thanks again.