Now, I consider myself an intelligent person. I have a science degree (albeit in chemistry, not physics or electrical engineering), but I have often thought I have a fundamental problem understanding how electricity works.
I know what it is, at a fundamental level - a flow of electrons carrying charge through a conductor - and I know about the relationship between current, resistance and voltage/potential difference, but there are lots of things I don’t get. Because I don’t get them, I’m not even sure of the best way to ask the questions, but here goes:
What exactly is potential difference. A difference in potential between two points - well, duh - but what exactly is the potential at a given point? And how is this potential generated at a power plant?
What am I actually being billed for by the power company? Electrons come into my house, they go through my wiring and light bulbs, power the TV, boil my kettle etc etc, and then they continue through the circuit. Obviously they are doing work, but how is the energy for this work actually “extracted”? It’s not like the electrons are “depleted” after they’ve done the job, so what is going on, and what are the electric co measuring to determine how much power I am using?
AC. How does that work? DC I can understand more easily - current flows from a battery, round a circuit via a light bulb or whatever, and back to the battery. AC changes direction 50 times a second. Huh? How does the circuit therefore do useful work?
More to come if I think of them.
OK, I’ve clearly got some major misconceptions going on here. I know how electricity works in practice - I got A-grades in physics at A-level and could interpret circuit diagrams and calculate currents and resistances and so forth until the cows come ohm (sorry), but I need help with the fundamentals. Educate me please!
You can’t state the potential difference at a specific point without reference to another point. By convention, one of these points is usually at or assumed to be at ground potential, or 0 V, but any two points will do. In a conductor, the potential difference between two given points is equal to the resistance between them multiplied by the current through them.
The power company bills you for electrical energy usage. Electrons do not carry electromagnetic energy, photons do. When you use electrical energy you’re using photons.
AC is fairly complicated, and I haven’t time to go into it in depth at the moment. Think of a light bulb, however. It doesn’t matter which way the current is flowing, since in each direction, the current does work, (e.g. heating the filament).
Q.E.D. will straighten me out but it helps to think of voltage as water pressure, and current as flow rate.
Also, what you’re extracting is the energy within an electromagnetic wave, not specific electrons. The electrons typically move fairly slowly within the conductor in comparison to the EM wave.
I’m not sure how to describe exactly what a potential energy difference is myself. Some things, buy virtue of their position in a field, impart energy to that field. An easier example is lifting a stone. When you lift the stone, it now has gravitational potential energy.
Likewise, some positions in a circuit have more electrical potential energy.
At a power plant, this energy is usually created by using steam to rotate one magnet through a coil of wire. ( or the reverse). This is called a generator.The movement of the wire through the feild causes it to gain electrical potential energy and causes the movement of electrons.
While electrons aren’t depleted, the resources used to get the generator to spin are. Either something was burned, or something was reacted, but whatever it was cost them money. Not to mention upkeep of the machinery. The potential energy of the fuel was converted to work in your house.
I’m not sure how they measure the power you use, but I think it’s just a wheel caused to spin by the current that flows through a box hooked up to some gears with numbers on them.
3.The key thing to understand here is that while the * electrical potential* of the circuit changes 50 times a second, the electrons themselves don’t make it all the way through the circuit itself each switch. The actual electrons travel pretty slowly, and would take hundreds of years to make it to your house from the power company even if they used DC and the current only went one way.
Think of it like a pulley attached to a rachet. The feild flips back and forth too fast for the rope to go a full revolution, but the rope moving back and forth at the rachet is all that is needed to transfer energy.
It’s less efficient than just having the rope go one direction, but DC current cannot use transformers, which makes it inefficient. Transformers make AC current more efficient by allowing the power company to transmit power at extremely high voltages, but also extremely low current. The less current you have, the less wire you need to transmit it without undue resistance losses. The low current, high voltage power is then transformed back down to low voltage, high current power at local substations and smaller transformers.
OK, but what is potential? I know how you calculate it, but what is it about point A that makes it a higher potential than point B? Saying that it is because current flows from A to B sounds to me like a circular argument - current flows because there is a p.d., and there’s a p.d. because current flows…
The power company bills you for electrical energy usage. Electrons do not carry electromagnetic energy, photons do. When you use electrical energy you’re using photons.
Huh? Photons? I really don’t get this part. Say I plug my electric kettle in and switch it on. Current flows through the element, which gets hot due to the resistance and heats the water. Where are the photons? And Thaumaturge, I know the power company is burning coal or whatever to drive turbines, generators, etc. I think what I meant to ask was, how am I depleting the power? Obviously there is a finite amount of power generated, so what happens if millions of houses are hooked up to the power station and try to use more power than the station is producing? Why can’t the current that’s gone through my house go on to power more stuff in the next house, and so on?
AC is fairly complicated, and I haven’t time to go into it in depth at the moment. Think of a light bulb, however. It doesn’t matter which way the current is flowing, since in each direction, the current does work, (e.g. heating the filament).
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A difference in electrical charge. If I move a bunch of electrons from point A to point B, I’ve created a potential diffence between those two points. The “potential” is simply the voltage. As has been explained, it’s relative. Like distance. How far is Detriot? Depends on where you measure it from.
This is somewhat semantic, so I see why you’re confused. Elctrical charge and Electromagnetic Energy are not the same thing. Electrons carry charge, not electricomagnetic energy. I think he’s just correcting your question. The answer to your question is that it takes energy to get the electrons to your house, and you are paying for that energy.
The water analogy works really well for electricity. Imagine a paddle wheel in a current of water. The wheel gets move regardless of the direction of the flow.
The potential of a point is the amount of work done in bringing a unit charge from infinity to the point in question. Or the amount of work done by the charge in moving that distance. For example. If I have a positive charge and I bring a unit positive charge from an infinite distance to a point in the vicinity of the charge I have to do work. If I start with a negative charge then the unit positive charge will do work on me in the same trip. But the numerical value in both cases is the same.
So if I have two different points in the vicinty of the charge different amounts of work will be done in bringing a unit charge to each point. The difference in work is the difference in potentioal. The basic unit of the Volt is Joules/coulomb.
I tend to ignore photons in this case. The current into your house goes through things like incandescent light bulbs, cooking stove heating elements and the like. These are resistive loads and the heat and light that are generated by the current banging around in the material is the power that you are using. If you run electric motors, and we all do, the current produces magnetic fields so the electrical energy is changed to magnetic energy and this is this, with due account of the rate at which the magnetic energy is produced, is the power you are using for that purpose.
Suppose you applied a DC voltage across a light bulb and you watched it light up. Then you reversed the polarity. The light bulb would still be lit, no? Now do this 50 (or 60) times per second. No difference.
As far as AC vs. DC, in this thread I wrote the following:
In the analogy above, 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 c (speed of light).
Minor nitpick, and I hope this helps Colophon understand one point. In an AC-lit bulb, there are times when there is no current at all (zero crossing points) and other times when the current and voltage are not peak (as it is increasing or decreasing). In contrast, a DC current is steady, and in one direction.
This fluctuation of AC is what makes flourescent lights flicker, and it is the persistance of human vision that makes it mostly unnoticable. Incandescent bulbs smooth out the peaks and troughs a little better, and their output looks even evener to us.
In actual fact,as the brits say it,Electricity and all it’s forms has never been reduced to explanation beyond what all studies of the subject are-----Theories.
It still has all of the qualities of the perfect puzzle,like—“How long is a board” or “how high is up”.
Magnetism,e;ectricity,electronics,Etc—all measurable----all explainable--------but all still theoretical
I wonder if the OP’s confusion about AC vs DC is more a question of “why” than of 'how". To the casual observer, it is not at all obvious WHY electrticity is delivered to us in AC form rather than DC. I don’t remember all the details, but there was quite a controversy in the early days of electricity with Edison heading up the DC camp and Westinghouse (IIRC) heading up the AC camp. Ultimately, I believe it came down to issues of safety and efficiency with AC winning out.
You may be confusing a theory with a hypothesis. In science, a well tested theory is accepted as fact and does not need to be couched in caveats. This is a common mistake of the anti-evolution crowd when they cry “But it’s only a THEORY”. Science never gets any better than the theory level.
OK, about that there AC. As has been mentioned, a resistor doesn’t care, to be anthropomorphic about it, which way the current flows. The same heat is generated in each case. So ac is specified by a value of current that will give the same heating for the ac as would the same current as dc. In other words, 10 amp. ac will heat a given resistor the same as 10 amp. dc.
How is this done. This little diagram about RMS ac should clear it up. If it doesn’t, come back with questions. Unfortunately it is a liitle small and you might have to download it (I’ll assure you it has no viruses or bad things in it) and view it larger. Or use a magnifier. The example in the cite is for sine waves. However, the concept of RMS value is quite general. A certain value of RMS current will do the same amount of work as would that value of dc. However finding the RMS value of complex waveforms can be quite a chore. In fact, a resistor having a linear temperature vs. resistance characteristic is the basis of a lot of true RMS meters.
Using the concept of RMS values, this cite explains power in ac circuits. At least as well as I can do it. As before, if there are further questions, we are always here.
I believe that it was efficiency alone that made AC win out over DC. If I recall correctly, you can use transformers on an AC current, but not on a DC current. Put simply, transformers take one signal, and generate a separate signal that has the same power. Since power equals current times voltage, it can take one signal and turn it into a signal with a much smaller current but a much higher voltage. Since the energy lost over a given wire was related to the current, but not to the voltage, you would therefore lose less money in transfering the second signal across the wires. You would then retransform that signal to your final voltage/current mix at the destination.
If you want to deliver billions of watts over long distances you must do it using as little current as possible. If you don’t do that the resistive losses in the transmission lines will eat you up. AC is convenient in that you can use a simple device to increase the voltage while at the same time reducing the current while maintaining the same power. It’s a transormer and won’t work with DC because it functions by changing the magnetic flux in a coil of wire.
For example, many transmission lines are 1000 megawatt (actually volt-amperes) lines. If this was generated and transmitted at a voltage that was reasonably safe for the average homeowwner, say 500 volts, the current would be 10[sup]9[/sup]/5*10[sup]2[/sup] for 20 million amperes. As it is, it is transmitted at 550000 volts which requires a mere 1800 amps. which takes cables of a reasonable size.
An electric potential difference is the electric potential in a field at a given point in comparison to a reference point, usually ground. The potential difference is generated by generating an electric field at the power plant. Steam is used to drive turbines, generating kinetic energy. The kinetic energy is used to rotate a conductive material in the presence of a magnetic field. Faraday’s Law states that if a magnetic field changes relative to a conductor, an electric field will be generated in the conductor.
At the power plant, the electric potential causes the electrons in the power line to experience a force. The electrons don’t move down the line very fast; they have a very low drift velocity. What they can do is move about locally very quickly, thereby passing on that force to neighboring electrons by repelling them. These repelled electrons in turn repel others down the line. Electrical energy is thus transmitted down the line rapidly. When the energy arrives at your house, devices in your home use this energy to do useful work (how each device does this depends on the device, of course).
As others have said, the light bulb (for instance) doesn’t care which way the electricity is moving. The electrons will still oscillate in the tungsten filament, heating it up and causing it to glow.
