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Misconceptions About Electricity

Uncayimmy

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A while back I stumbled upon this site about what is electricity (the site has large sections of white space for some reason, so don't think it's over before it's over). Being a layman, there were always nagging questions. This site addresses them.

For example, I was taught something along these lines:
Science books in elementary school correctly teach us that electrons are particles of electricity, and that electric current is a flow of electricity. In other words, they teach that electricity is like that chain we yanked upon [example in previous paragraph]. But then the books contradict themselves... they also tell us that electricity is... a form of energy that travels almost instantly along the wires! WHAT?! In other words, electricity the electrons themselves, and ALSO electricity is like the wave that moved along the chain of electrons? Well, which is it? If "electricity" is the wave, it can't be the electrons in the chain.
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That always bothered me. I also always wondered where all them damn electrons went. This site goes on to explain it as follows:

Here's a way to understand how electric circuits work. Get a long chain and hook its ends together to form a loop. Wrap this chain around two distant pulleys so the chain is like a conveyor belt. Now if you turn one pulley, what happens? The other pulley turns almost at the same time.

The chain is like the electrons inside a wire. The chain flows slowly in a circle. That's how electrons flow too. However, energy flows very fast. When you turn one pulley, the links of the chain yank on their neighbors, and waves of energy flow down both halves of the chain. The distant pulley turns almost instantly. And, (Ta Dah!) the first pulley is like a DC generator, while the distant pulley is like a DC motor. The circle of chain is like an electric circuit. The links of the chain are like the electrons inside a wire.
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That makes a lot more sense to me. I thought others might appreciate what this site has to offer. And, as usual, if the experts here see some problems, please point them out.
 
Yeah, the biggest point is that there always has to be a circle, a circuit, just like a bicycle chain. What goes out one end has to come in the other.

This sounds like a fairly good starting point to me. If you wanted to go further, next steps in various directions might be:

- coils and capacitors, filters and resonance (AC circuits)
- motors, generators and electromagnetic devices
- diodes as rectifiers, power supplies, inverters, regulators
- tube circuits (old-fashioned, but still good for understanding CRTs, for example. The practice might be obsolete, but the theory still works.)
- diodes, transistors as switches - digital circuitry
- digital design, computers
- transistors as amplifiers - analog circuitry
- radio transmission, receiving, modulation, antennas, microwaves
 
Could you have AC applied to a single line, at the other end you have a capacitor and arrangement of diodes that would charge each plate of the capacitor as the AC cycles?
 
Science books in elementary school correctly teach us that electrons are particles of electricity, and that electric current is a flow of electricity. In other words, they teach that electricity is like that chain we yanked upon [example in previous paragraph]. But then the books contradict themselves... they also tell us that electricity is... a form of energy that travels almost instantly along the wires! WHAT?! In other words, electricity the electrons themselves, and ALSO electricity is like the wave that moved along the chain of electrons? Well, which is it? If "electricity" is the wave, it can't be the electrons in the chain.
Click to expand...
Its both which is confusing. If you are dealing with current flows through your computer engineers treat the flow of current as an electromagnetic wave than the traditional concept of electrons flowing. If we are dealing with a refrigerator then in fact you treat it as a the flow of electrons.
 
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alexi_drago said:
Could you have AC applied to a single line, at the other end you have a capacitor and arrangement of diodes that would charge each plate of the capacitor as the AC cycles?
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Ummmm, you have to have the other end of the capacitor connected to something. That would normally be ground, which implies a return loop through the ground. The capacitors charge when there is a potential difference between the two ends; with only one wire you can't get a potential difference, except perhaps for very small transient times. Except for the use of AC, it sounds like you seek Van de GraaffWP generator.

Grounding is a whole discipline in itself. It is usually just a shorthand for the return wire; in large power systems it may actually be the real earth, which can be both a sink and a source of electrons. Grounding also involves the concept of making sure that some part of a circuit is always very close to ground potential, so you don't get knocked on your ass, or worse.
 
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technoextreme said:
Its both which is confusing. If you are dealing with current flows through your computer engineers treat the flow of current as an electromagnetic wave than the traditional concept of electrons flowing. If we are dealing with a refrigerator then in fact you treat it as a the flow of electrons.
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Of course, when the frequencies are high enough to have to worry about radiation and/or antenna interference in a signal. But they are, of course, equivalent, part of the wave/particle duality. Which one you use depends on which one is more useful to understanding or predicting what will happen.
 
Okay, the pulleys and chain is better than the water hose analogy.

But, using either, or another, can somebody explain 'draw'? Why does the 'motor' pulley want to pull my chain?
 
casebro said:
Okay, the pulleys and chain is better than the water hose analogy.

But, using either, or another, can somebody explain 'draw'? Why does the 'motor' pulley want to pull my chain?
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Friction? f = n * kf

Friction f is the force pulling the chain, causing it to follow the pulley. n is the force pulling the chain into contact with the pulley, and related to the tension on the chain, and k is a constant that depends on the materials of the chain and the pulley and whether the chin meets the pulley or is sliding along it; the amount of striction between them.

The water hose works as well, as long as the water gets returned to the source, or that there are sufficient sources/sinks of the water at the appropriate ends - these are analogous to an earth ground return..
 
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A good analogy for electricity is a conga line wrapped around into a circle. Each of the conga dancers represents an electron. If you push one of the dancers forward then he will push the next person who pushes the next and so on. You can think of that push as an electric signal and the movement of conga dancers as the flow of individual electrons. Obviously, the signal to start moving the conga line around can travel much faster than any individual dancer. Likewise, an electric signal travels much faster than any individual charge carrier. Of course, real electrons bounce around at random like Brownian motion. So really a better analogy would be pushing a mosh pit around a circular hallway.
 
casebro said:
But, using either, or another, can somebody explain 'draw'? Why does the 'motor' pulley want to pull my chain?
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I'll bite. Well, hope I understood the question.

Once an electrical circuit (loop) is closed, the electrons are urged to pass through the circuit because of the potential difference between the ends of the generator. The motor has no intentionality, it doesn't "want to pull the chain"; rather, we could say it's the moving chain (flow of electrons between the ends of the generator) that wants to make the motor move. The motor is sort of a passive entity here, just waiting for a flow of electrons under certain operational ranges to go through it. It will just respond to that flow given how it was built.
 
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rsaavedra said:
.... It will just respond to that flow given how it was built.
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You might as well have said "It's that way because that's the way it is".

I guess analogies have their limits. Leave it at Ohm's Law.
 
Ah, I see I was being obtuse; you meant what causes the electrons in the wires to act like a chain.

The metal atoms in the wire are in a stable equilibrium, with the electrons in the wire closely matching the count of protons in the material's nuclei. The electrons in the outermost shell in each atom are loosely bound, but the atom still resists becoming an ion by holding too many or too few electrons. When electrons are forced into the end of the wire, they force other electrons out, which in turn move down the wire forcing still more electrons out by taking their places. This propagation of electrons takes place at some fraction of the speed of light, and thus the current flows at that speed, while individual electrons move from atom to atom. The force that propagates the wave, keeping the chian together, is, of course, the electrostatic force, the force of like charges of the electrons repelling each other; it also involves the quantum exclusion forces that decree how electrons can fit in their shells of probability around the atoms nucleii. In an AC circuit, the electrons actually reverse direction in each cycle. Work is done through the movement of electrons regardless of their direction; a light bulb doesn't care which way they flow through, or even that they reverse that direction 60 or more time a second.

You could think of the wire as a rigid tube full of small, hard rubber balls. When another one is forced into one end, a different one pops out the other. The propagation is very fast, although the individual balls moved very little. The ease with which electrons can bounce each other through the atoms' outer shells is a measure of that material's intrinsic resistance.
 
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casebro said:
You might as well have said "It's that way because that's the way it is".
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Certainly, no objection to your comment.

To paraphrase my vague comment: if the motor wasn't properly built, let's say just had a lightbulb inside, that lightbulb would just light up when the circuit got closed, but no moving part would move, and the motor therefore wouldn't work as expected. So the intentionality is not in the motor, and is not even in the electricity. In any case, the intentionality was in the builder of the motor. Everything else indeeds works just following the rules of physics; just because indeed, that's the way it is.
 
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UncaYimmy said:
A while back I stumbled upon this site about what is electricity (the site has large sections of white space for some reason, so don't think it's over before it's over). Being a layman, there were always nagging questions. This site addresses them.

For example, I was taught something along these lines:


That always bothered me. I also always wondered where all them damn electrons went. This site goes on to explain it as follows:



That makes a lot more sense to me. I thought others might appreciate what this site has to offer. And, as usual, if the experts here see some problems, please point them out.
Click to expand...

Electrons in electricity do not move at the speed of light or anywhere near it, they move extremely slowly, on the order of a few inches PER HOUR! In a DC circuit it would take days for electrons to flow the entire circuit. In AC electricity the electrons don't flow at all, they vibrate ever so slightly in place (a distance of nanometers). It is the electrical field that propogates near the speed of light. For example if one electron is pushed at one end of the wire, even a tiny bit, its field is effecting the next electron at a great distance, pushing it also, and so on and so on. So as soon as you push on one electron (via an electric field) the one at the other end of the wire moves almost instantly. The movement of the fields happens at almost the speed of light.

Its important to realise that electricity works by something pushing electrons at one point and something resisting the electrons at another point (drawing power). Think of it like a bike wheel with an electric dynamo on it. If you power the dynamo the wheel spins. If you apply the break, the break heats up by taking power from the wheel and resisting the flow of the wheel. Many text books instead promote a stupid idea that electrons flow out of the positive and then dump energy off at a load and flow back to the negetive. This is completely wrong.

I would strongly recommend you read this site: http://www.eskimo.com/~billb/miscon/eleca.html.
 
DC loss of power?

In a straight DC circuit, with 3 light bulbs connected serially, is there any even theoretical drop in the relative brightness of the 3 bulbs? This puzzle was put to me by a friend who was coaching his stepson in maths and physics as the boy was coming up to some exams.
The reason I ask is mainly that I vaguely recall the original attempts at installing domestic electricity - using DC - failed because of power loss over large distances. That might be a separate issue from my question, I realise.

cheers
 
GlennB said:
In a straight DC circuit, with 3 light bulbs connected serially, is there any even theoretical drop in the relative brightness of the 3 bulbs? This puzzle was put to me by a friend who was coaching his stepson in maths and physics as the boy was coming up to some exams.
The reason I ask is mainly that I vaguely recall the original attempts at installing domestic electricity - using DC - failed because of power loss over large distances. That might be a separate issue from my question, I realise.

cheers
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There is not enough information to pose such a question. Compared to what?


DC power transmission suffers from excess ohmic losses at useable voltages. This is compensated for by jacking up the voltage. Unfortunately high voltage DC is difficult to convert back to a reasonable voltage at the point of use. This method is used for dedicated point-to-point long distance transmission.

AC is easy to convert, but suffers from excess reactance losses over long distances. An irritatingly low frequency must also be used to keep these losses low.
 
G O R T said:
There is not enough information to pose such a question. Compared to what?
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Compared to each other. Will 3 identical bulbs burn equally bright no matter what the length of the intervening wires?
 
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