[QUOTE]
*In response to post by K43 *
The Classic Column on 7/19/00 is from 7/22/94
Let me start by saying that as a physics teacher, I take offense to your remark about high school physics … perhaps you would never have said it if you actually understood the topic being discussed.
I will now pick apart your post piece by piece (The Straight Dope on the topic then follows):
YOU SAID:
Despite what they may have told you in high school physics, electrons do NOT migrate down the wire from the generating plant to your house and into your toaster, and would not do so even if the supply were DC rather than AC.
I SAY:
It is true that with AC, electrons do not get very far in any half-cycle, and then are put back to essentially where they started in the next half-cycle. In DC however, there is a net flow of electrons. For typical currents in typical wires, this “Drift Velocity” (as it is called) is very slow: about a millimeter per second or slower for the wire attached to your toaster, if your toaster was plugged into a 120 V DC source. Do not confuse the speed of electrons moving with the speed of the electrical signal: the latter is quite fast (details below).
YOU SAID:
That is (putting aside the quantum mechanics question about whether they are physical entities), electrons do not flow down wires, jumping like cattle-prodded frogs from one atom to the next.
I SAY:
I don’t have a clue what you are talking about here (there is no “question about whether they are physical entities” - to make sure, I even contacted a professor of modern physics, who agrees that you seem not to have a clue … perhaps you could clarify what you mean here. Again with the electrons moving though: when we speak of current, we measure it in “Amps” - where an amp is a unit of charge flowing per second; current is charge in motion. To have current and no charge in motion is a contradiction.
YOU SAID:
Rather, it’s the ENERGY that travels down the wire, like the desk toy with the steel balls hanging from wires, where you pull back the ball on one end, it whacks the next ball, and the energy is transmitted down the line of balls to the last one, which flies out. The other balls hardly move at all.
I SAY:
This is a fantastic analogy, but one that does not apply here. This would relate more to a spark jumping from a wire - the electrons at the end of the wire are “kicked out” and move significantly, though other electrons in the wire barely move.
YOU SAID:
Remember how as a kid you would pull a clothes line taut and tap it to send a “loop” down the line, to bounce back and forth. The atoms in the line don’t move horizontally, but only waggle as the energy wave travels.
I SAY:
What you are describing is wave propagation. Electricity flowing down a line is not purely a wave phenomenon.
YOU SAID:
It’s like a wave in the ocean. It’s not particular molecules of water moving along, but energy transmitted from one molecule to the next. A particular molecule of water moves in a closed curve (roughly oval) and ends up where it started.
I SAY:
This is a good analogy for electron movement in AC, but not DC … DC electrons would flow, much as the water molecules do flow in a river or stream.
YOU SAID:
Incidentally, AC or DC would make no difference. Since electric energy moves down a wire at a considerable fraction of the speed of light in a vacuum, if a particular electron is imagined (contrary to fact) to move down the wire, at the AC frequency of 60 Hz, it would have plenty of time to zip from the generating station to your house and back before the polarity changed.
I SAY:
Again, as discussed above there Is a difference. Electrons typically travel relatively slowly within conductors as mentioned above, and so would not go to your house and back before the polarity changed.
The Straight Dope:
Here it goes … electrons themselves travel quite slowly within a conductor that has a current in it. The original question was, however … “How long does it take electricity to travel from the generator to my house?” I think I can do a reasonable job answering this:
The answer is: that it depends on many factors and is quite complicated. However, I am fortunate to have a summer job with a company who builds computer systems that are designed to simulate the intricate workings of a power grid (see http://www.rtds.com if you like). With a little help, I set up a simple simulation of a generator hooked up to 15 km of typical transmission line, and monitored the voltage at the end. According to this simulation, after closing a switch at the generator end, the voltage appeared at the far end about 55 microseconds later, which would give this electrical signal a speed of about 2.73E8 m/s, or about 91% of the speed of light. Knowing this, and the length of conductor that separates your toaster to the generator, you should be able to get a ball-park answer.
