It is hard to figure out what your real problem is. Knowing that would have really helped in crafting a good answer for you.
Your question is, "How do these things work"? But that question is premature. Here are some questions, in order of how they should be asked:
- What do they claim the problem is?
- What do they claim that their product does?
- Are their claims plausible?
- Are there any known uses for electronics that can make their product legitimate?
Since you don't link to the device, I have to guess on the possible answers to these.
Most claims for #1 is that noise on the power line can cause: health problems, noise in your home stereo, or wasted energy.
The claims for #2 is that they filter out the noise. The source of the noise is not always clear, and how it filters it out is also not clear.
EMR causing health problems is not plausible. There are a lot of scientific papers done on this subject, but there is a trend: The better the research (placebo controlled, double blind, large sample size, etc) the less likely EMR will cause health problems. Good research shows no correlation between EMR and health problems, but bad research shows a link. This is a common trend for other things too (acupuncture, homeopathy, etc.).
EMR causing wasted energy is plausible, but still stupid. EMR in your power lines will be a low level. Much less than a watt for your whole house. But appliances, computers, etc consume hundreds of watts. So even if you could fix this (assuming it even needs fixing) it would only result in much less than a 1% energy savings. You can then do the calculations to figure out how long it would take to break even: where the amount of money you save matches the amount you spent on the device. Of the devices I have seen, it would take years to break even.
EMR in your power lines causing noise in your home stereo is the most plausible, but still not reasonable. The power supplies in these devices is designed to filter out noise. If EMR in your power lines is actually putting noise into your stereo then either the amount of power line noise is unusually high (unlikely), or your stereo is terrible. I design professional audio equipment for a living and having power line noise get into the audio is simply unacceptable. If there is a product that makes it to market where power line noise is an issue then the guy who designed that product should be fired.
So, their claims are either not plausible or not important.
Finally, there are no known things that you can plug into a normal outlet that will eliminate EMR from the power lines. I am specifically talking about devices that just plug into an outlet (with no pass-thru connector) and somehow remove EMR from the entire room or entire house. If the device had a pass-thru connector, or if the structure of the house wiring was known, then it could be possible.
But let's say that we were doing a pass-thru. The the filtering would be done using a mix of inductors and capacitors. These are called LC filters. These are fairly inexpensive devices. It might add US$2.00 to the cost of a power strip. Certainly not enough to justify the cost that these people tend to charge.
Conclusion:
These devices are not solving a real problem. It is unlikely that they do anything at all. They are expensive for what they claim to do. In short, don't bother with them. I would go so far as to call a lot of them fraudulent.
The car analogy isn't such a good one, since electrons don't actually flow from one end of the wire to the other (well they do but extremely slowly) and it implies there is some space between the cars, whereas it would be more like a traffic jam whatever the width of the highway.
It's more like a line of billiard balls, and force is applied to the first one, and the energy is transferred to the last one through all the intermediate balls (a bit like newtons cradle, although the balls don't really bounce into each other). The free electrons bounce around, occasionally being impeded (see below) with the potential difference causing an average inclination to the direction of current.
A water analogy is better - the pipe is always full of water, and for the same pump (battery), the pressure (voltage) is always lower the wider the pipe, which equates to more flow and a lower resistance.
This quote from the Wiki page on resistivity explains reasonably well:
In metals - A metal consists of a lattice of atoms, each with an
outer shell of electrons which freely dissociate from their parent
atoms and travel through the lattice. This is also known as a positive
ionic lattice.4
This 'sea' of dissociable electrons allows the metal
to conduct electric current. When an electrical potential difference
(a voltage) is applied across the metal, the resulting electric field
causes electrons to move from one end of the conductor to the other.
Near room temperatures, metals have resistance. The primary cause of
this resistance is the thermal motion of ions. This acts to scatter
electrons (due to destructive interference of free electron waves on
non-correlating potentials of ions)[citation needed]. Also
contributing to resistance in metals with impurities are the resulting
imperfections in the lattice. In pure metals this source is
negligible[citation needed].
The larger the cross-sectional area of
the conductor, the more electrons per unit length are available to
carry the current. As a result, the resistance is lower in larger
cross-section conductors. The number of scattering events encountered
by an electron passing through a material is proportional to the
length of the conductor. The longer the conductor, therefore, the
higher the resistance. Different materials also affect the
resistance.
Best Answer
How fast does electricity flow? This is a good question, because it seems like a simple enough question, but usually it indicates some underlying misconceptions. The first difficulty in answering the question is knowing, what is electricity? Do you mean:
Usually, people asking this question actually care about the former, but are thinking about the latter. However, not having a clear understanding of the difference, their underlying concern actually can't be addressed without stepping back and addressing the underlying misconceptions which lead to the question.
Understand is this: there are forces, and there are things that transmit forces, and they are not the same thing. Here's an example: I'm holding one end of a rope, and you are holding the other end. When I want to get your attention, I tug the rope. There is the rope, and there is the tug. The tug travels as a wave of force down the rope at the speed of sound in the rope. The rope itself will move at some other speed.
Say I have two lookout towers, and when I see the approaching invaders, I shout to the other tower. Sound will travel as waves in the air at the speed of sound. How fast are the molecules in air moving? Do you care?
Some people won't let this go until the motion of the molecules is actually explained, even though it's usually not relevant to their concerns. So here's the answer: the molecules are flying around in all random directions, all the time. They fly around because they have non-zero temperature. Some are very fast. Some are very slow. They bump into each other all the time. It's very random.
When you shout, your vocal tract compresses (and rarefies, as your vocal cords vibrate) some of the air. The molecules in this compressed region want to move to a region with less pressure, so they do. But now this nearby region has too much air, and is a little more compressed than the air around it, so the compressed region expands outward a little more. This wave of compression moves through the air at the speed of sound.
All of this happens superimposed on the random motion of the molecules previously mentioned. It's unlikely that the same molecules that were in your vocal tract will be the ones that vibrate in the listener's ear. If you watch individual molecules, you will observe them going in all directions. Only if you observe a lot of them will you notice that slightly more went in one direction versus another. It is true for all things we would call "sound" that the random motion of the molecules due to thermal noise is much more than their motion due to sound. When the "sound" becomes the more relevant motion, we tend to call it not "sound" but rather an "explosion".
The situation with electricity is not much different. A metal conductor is full of electrons that are free to wander around the entire circuit in random directions, and they do, simply because they are warm. Things in our circuits make waves in this sea of electrons, and these waves propagate at the speed of light1. At the currents we typically encounter in circuits, most of the electron motion is due to thermal noise.
So now we can answer the questions:
How fast do changes in electrical fields propagate? At the speed of light in the medium in which they are propagating. For most cables, this is in the neighborhood of 60% to 90% of the speed of light in a vacuum.
How fast do electrical charge carriers move? The velocities of individual charge carriers are random. If you take the average of all these velocities, you can get some velocity that depends on the charge carrier density, and the current, and the conductor's cross-sectional area, and it's typically less than a few millimetres per second in a copper wire. Above that, resistive losses become high in ordinary metals and people tend to make the wires bigger instead of forcing the charges to move faster.
Further reading: Speed of Electricity Flow by Bill Beaty
1: The speed of light depends on the material in which the light is propagating, just as with sound. See Wave propagation speed.