Yes. This can be done by attaching a device which measures the current flowing through the wire and the voltage across the lines. The device measures the current and voltage many thousands of times per second, and with each measurement multiplies the current by the voltage to generate a measurement of how much power is flowing (and in which direction) at that moment. The average of this measurement over time will tell you how much power is being delivered, and which way it is flowing.
This works because the current reverses direction every time the voltage does, so by multiplying the current by the voltage you can correct for the reversals and get a true reading of which way the power is flowing.
So if I had super-microscopic X-ray vision and were able to see the actual electrons inside the two wires in that cord coming out of my toaster, they would not be flowing down one wire and up the other, but just sort of vibrating back and forth in place real fast?
Rather than saws, I think it’s better to look at water or sound waves. When I call your name, the molecules in the air are just moving back and forth. But the sound still propagates from me to you. When I drop a rock into the water, the ripples spread out even though the water is just bobbing up and down. It’s the wave that defines the direction of power flow, not the electrons.
To go back to the saw analogy, consider this: it’s not the saw moving back and forth which cuts the tree, but the force of the lumberjacks (they’re OK) pushing it back and forth. Similarly, the work done by electricity is not accomplished by the mere motion of the electrons moving back and forth, but by the generator which pushes them through the wire. If more work is required, the generator has to push harder (literally; load on the circuit exerts a force on the turbine, trying to stop it.)
if you had a small paddle wheel that would spin when water hit the blades then:
you could place this paddle wheel in a sink under the faucet. the paddle wheel would spin as long as water came out of the faucet and went down the drain. the water only goes one direction. this has similarities to DC.
you could take a cup of water and pour it over the paddle wheel causing it to spin. you could catch all this water in a second cup. when the 1st was empty and the 2nd full then you would lower the 1st and elevate the 2nd and pour. keep repeating and the paddle wheel keeps spinning. this has similarities to AC.
Not difficult, probably just impractical to scale up; any small toy motor can be used to generate DC if used as a generator. This can be demonstrated with a couple LEDs wired in reverse parallel across the motor; only one will light when it is spun in a given direction. Also, motors with multiple poles can produce pretty smooth DC by themselves, with no filtering, or just enough capacitance to suppress noise and spikes. Also, if you add a diode, you can shut down a generator without disconnecting it.
Of course, the biggest problem is converting DC to lower/higher voltages; it is pretty easy to step up DC but not the other way since the input voltage of a switch-mode power supply is limited by the voltage ratings of transistors, which don’t go much higher than 2 kV (1.5-2 kV transistors are pretty common, having been used in CRT TVs and monitors for decades, but anything higher is rare; here is a 4000 volt transistor, but with very low power capacity, and expensive for that). This brings up the question of how they convert HVDC (100s of kV) to lower voltages, obviously not with traditional SMPS topology.
It is where it is dissipated. It flows in both directions, but is dissipated only at one end.
Hydro-Quebec’s main transmission lines run at 730,000 volts DC because it is more efficient to do so. AC losses come from electromagnetic radiation as well as resistive losses, while DC has only resistive losses. The cost of changing from AC to DC and back used to far outweigh the cost of the radiative losses, but high power rectification and reconversion to AC for local distribution by solid state electronics have changed the equation. Incidentally a significant part of their output goes to aluminum smelters which require DC power.
What you’re talking about are AC generators that use a commutator or rectifier to produce DC. The only DC generator I know of is a homopolar generator.
Nope. What you’d actually see would be the electrons just sort of vibrating back and forth in place extremely slowly. The energy flows through (or more precisely, around) the wire quite rapidly, but the electrons themselves are moving at less than a snail’s pace.
Technically, they might be the same (of note, a commutator is just a modification of the slip-rings used on AC generators), but they really do output DC so you can call them DC generators; as shown here, which also shows what I mean by multi-pole generators giving smoother DC without filtering (as stated before, I’ve found motors with enough poles so that ripple was insignificant).
Thanks to everyone who took the trouble to reply to my post, but for some reason almost everyone missed the point of my question (or completely ignored it):
Is it possible, by taking measurements at [an intermediate point on the cable], to determine which way the power is flowing?
AndrewL, thanks for this answer. Unfortunately, I don’t understand it.
The voltage on the line is in the form of a sine wave. The current is also a sine wave. They may be in phase, or the voltage may be leading or lagging the current. But for simplicity let’s say they are exactly in phase.
If I measure at high enough frequency (say 1 kHz), I can plot the voltage and current waveforms. As you suggest, I can also multiply them together to show the instantaneous power.
However, my contention is that these measurements will tell me nothing about the direction of flow. A sine wave is symmetrical and looks exactly the same flowing east-west as west-east.
Yes. The further from the power source, the lower the voltage (AC or DC doesn’t matter) between the two conductors due to transmission losses.
ETA: So you’d have to take measurements at two intermediate points on the cable, and the lower the transmission loss the further apart they would have to be for you to draw a certain conclusion.
Bear in mind that “the cable” is actually two conductors, because we need to form a complete circuit (loop) in order for current to flow between the source (a coal-fired power plant 50 miles away) and the destination (your electric dog polisher).
So during the first part of the AC cycle, the alternator at the power plant is producing large positive voltage, i.e. terminal #1 on the alternator is at a much higher voltage than terminal #2. During this part of the cycle, electrical current - composed of electrons each carrying a quantity of energy - flows down wire #A, passes through your dog polisher, and experiences a drop in voltage as it does so. When an electron experiences a drop in voltage, it gives up energy; in other words, the energy that was at the power plant has been shoved down wire #A into your dog polisher. After passing through your dog polisher, these lower-energy electrons continue along wire #B, headed back toward terminal #2 of the alternator at the power plant.
During the second part of the AC cycle, the alternator at the plant is producing large negative voltage, i.e. terminal #2 is at much lower voltage than terminal #1. Now the current flows in the other direction, down wire #B toward your dog polisher. The voltage at the plug of your dog polisher is now reversed (relative to the previous paragraph) because current is being shoved through in the opposite direction by the power plant. So as the electrons flow through your dog polisher, they again experience a drop in voltage, dumping energy into your dog polisher and then heading back toward the power plant.
So power flows down one wire from the power plant, and then it flows down the other wire from the power plant, and then this cycle repeats, 60 times every second.
You can measure which way current is flowing, and you’ll be able to measure power being dissipated (or generated) between any two points of the circuit by measuring (and multiplying) the voltage between those two points and current between those two points. For DC systems, you can use single measurements of voltage and current, but for AC systems, the voltage and current are constantly changing, so we use root mean square values: when we speak of a wall outlet being at 120 volts AC, it’s actually fluctuating between extremes of 170 volts and -170 volts (170 = 120 * sqrt(2)), but we can multiply RMS voltage with RMS current and obtain the time-averaged power flow. Because RMS values don’t have a sign, you can’t tell which way power is flowing with those numbers. If you want to know whether a portion of the circuit is generating or dissipating power, you would have to measure instantaneous voltage at the ends of that portion of the circuit and multiply by instantaneous current between those two points.
So consider the simplified circuit under discussion earlier: the alternator at the power plant is connected through cross-country wires to your dog polisher. Each component - alternator, cross-country wire #A, cross country wire #B, and your dog polisher - has two terminals on it, terminal #1 and terminal #2.
During the first part of the AC cycle, the alternator is at high positive voltage:
V[sub]1[/sub]-V[sub]2[/sub] = V[sub]1-2[/sub] = 170 volts
Current is measured to be flowing from terminal #2 to terminal #1, opposite the direction of voltage drop - so the alternator is producing power.
As current flows in wire #A, there is a slight voltage drop from one end to the other because the wire is not a perfect conductor; energy is being pissed away as heat. The drop always coincides with the direction of current flow: if the plant is producing 170 volts at alternator’s terminal #1, you might see just 169 volts at terminal #1 on your dog polisher. And that’s how you know power is being dissipated in this wire: the voltage drop coincides with the direction of flow of current.
(I’m ignoring for now the fact that a cross-country power grid circuit is more complicated than this, involving extremely high voltages at the power plant and on the cross-country wires, and step-down transformers that ultimately deliver just 120 AC volts to your house. But the important point is there: conductors - and this means cross-country wires, house wiring, and power cords - aren’t perfect, and they will dissipate power when current flows through them, and you can measure this with a volt meter and a current meter.)
The same is true at your dog polisher: voltage drops from terminal #1 to terminal #2 when current flows from #1 to #2. Power is being delivered to your polisher, where it’s used to produce mechanical work that spins the buffer wheel, leaving your dog with a streak-free shine.
Wire #B? Look up two paragraphs at what’s going on in wire #A, and you’ll get the idea.
“vibrating back and forth in place real fast” is the same thing as “flowing down one wire and up the other,” it’s just that they don’t get very far down the wire before they’re told to turn around and go back in the other direction. The actual velocity of electrons traveling down the wire is extremely small - Wikipedia says just a few millimeters per hour, depending on the voltage being applied. So if the AC system is oscillating at 60 cycles per second, then the electrons only manage to move a few microns before they reverse direction.
I know what you’re thinking. If the electrons only wiggle back and forth by a few microns, how the hell does any power make it from the plant 50 miles away to the dog polisher in your hands? Well, when the alternator drives up the voltage at one terminal, it drives up the voltage everywhere along that wire all the way to your dog polisher, so the electrons at your polisher get energized right away. An analogy: imagine a 200-foot-long water hose connected to a spigot on the side of your house, with the nozzle pointed at a paddle wheel. The hose is filled with water, but the pressure is zero. Now you open the spigot, and water immediately begins squirting out the nozzle and driving the paddle wheel. Even though you’ve only just now opened the spigot, and the water that just now passed through the spigot hasn’t reached the nozzle yet, the water at the nozzle is already able to deliver mechanical power to the paddle wheel.
Well, shoot, I ran out of brainpower on that one; someone else will have to call on better explanatory capabilities than I have. But if you can intuitively see that the water hose scenario is able to deliver power down the hose even when the high-energy water at the nozzle hasn’t gotten anywhere near the paddle wheel, then maybe you can see why electrical power can make it from the coal-fired power plant to your electric dog polisher, despite the fact that the electrons only move back and forth the tiniest bit.
