Alternating energy

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With AC electricity, I understand the energy goes back and forth, even on the high voltage power lines that fulfill the energy needs for factories , community buildings, or homes.

However, some of these powerlinea go long distances, and I’m trying to figure out how the electricity alternates such long distances. It has to be broken up in segments by right? -Such as being received by a unit, and then relaunched for the next segment to alternate-

Or can it truly go hundreds of miles back and forth changing direction rapidly, because of the high voltage and amplitude?
 

russ_watters

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With AC electricity, I understand the energy goes back and forth, even on the high voltage power lines that fulfill the energy needs for factories , community buildings, or homes.

However, some of these powerlinea go long distances, and I’m trying to figure out how the electricity alternates such long distances. It has to be broken up in segments by right? -Such as being received by a unit, and then relaunched for the next segment to alternate-

Or can it truly go hundreds of miles back and forth changing direction rapidly, because of the high voltage and amplitude?
Length of the wire doesn't really have anything to do with anything. The source just creates the oscillations and they travel down the wire. It isn't like the entire group of electrons has to go back and forth as a unit. It's longitudinal, like sound, not transverse, but here's a longitudinal transverse:
 
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Ok, I understand that energy travels in waves around the baseline. However, for conversation purposes, if we were using a single phase power line, then how does your example differ from direct current? Doesn’t the alternating current mean back and forth down the line and back because of a magnetic change at the source?

Pardon my ignorance on the matter
 
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russ_watters

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Ok, I understand that energy travels in waves around the baseline. However, for conversation purposes, if we were using a single phase power line, then how does your example differ from direct current? Doesn’t the alternating current mean back and forth down the line and back because of a magnetic change at the source?
Yes, it does*. So....I'm not sure I see what your question is really getting at.

*Note: Since electricity is really an electromagnetic phenomena, I'm not sure if this is completely accurate at a deeper level, but should suffice for our purposes.
 
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Personally - I really do not like discussion of EM Wave in AC power distribution, IMO it only complicates the discussion and is not needed to review the flow of power.

I'll start with the OP statement, "the energy goes back and forth" - this is not accurate, energy, ideally flows in one direction. ( we will set aside the issue of Power Factor for now)

The Polarity of the Voltage Oscillates, and the Direction of Current Oscillates, power ( energy / time ) is the factor of these two elements. Not unlike the Piston rod of a steam locomotive, the rod pushes/pulls and moves in both directions, but the power flow is from the piston to the wheel - one direction.

The whole question boils down to P = V * I.
 
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Ok maybe I didn’t ask my question with clarity.

And I absolutely agree with keeping things simple. I think my question is pretty elementary, so I don’t want to lose anyone in a deep context.

Einstein said something along the lines of - paraphrasing - “ unless someone can explain something simply, then they truly don’t understand it”

Ultimately, I was taught that direct current goes mainly in one direction when in a circuit.

Then, I was taught that Tesla found alternating current which moved back and forth from the start of the circuit to the end of the circuit , and then back to the start- at very fast speeds , if I remember the education right it was at 60 times per second.

So I was wondering when alternating current is applied as a load on power lines, if it in theory performs the same as on a small circuit. (Because the distances of power lines cover huge territories sometimes such as the Midwest United States)

Basically, is the alternating electricity still alternating when it is applied to the real world grid over large distances and what are the the points to which it alternates to and from?
 
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Ok maybe I didn’t ask my question with clarity.

And I absolutely agree with keeping things simple. I think my question is pretty elementary, so I don’t want to lose anyone in a deep context.

Einstein said something along the lines of - paraphrasing - “ unless someone can explain something simply, then they truly don’t understand it”

Ultimately, I was taught that direct current goes mainly in one direction when in a circuit.

Then, I was taught that Tesla found alternating current which moved back and forth from the start of the circuit to the end of the circuit , and then back to the start- at very fast speeds , if I remember the education right it was at 60 times per second.

So I was wondering when alternating current is applied as a load on power lines, if it in theory performs the same as on a small circuit. (Because the distances of power lines cover huge territories sometimes such as the Midwest United States)

Basically, is the alternating electricity still alternating when it is applied to the real world grid over large distances and what are the the points to which it alternates to and from?
You seem to be thinking that the electrons in the power lines travel from the source, along the lines and back to the source again 60 times a second - they don’t.

When a current is flowing, electrons are travelling very slowly - millimeters per second or less. In fact, the electrons are jiggling randomly, and the current is expressed by a net movement in a certain direction.

The impulse that gets them all travelling runs along the wire at near light speed. For 60Hz AC, that impulse reverses direction at that rate, so the ‘net jiggling’ follows suit.

Imagine a very long hosepipe already full of water - turn on the tap, and the water shoots out the other end almost instantaneously. But the new tap water hasn’t travelled to the end in that short time - the pressure pulse has.
 

Stephen Tashi

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Imagine a very long hosepipe already full of water - turn on the tap, and the water shoots out the other end almost instantaneously. But the new tap water hasn’t travelled to the end in that short time - the pressure pulse has.
To expand on that metaphor, imagine a machine that is pushing water in a pipe in one direction a fraction of an inch and then sucking it back the other way a fraction of an inch. Can you use the output of the pipe to power a device? Yes, but you'd have to be clever. It's easier to design devices that run on a continuous stream of water. Likewise, it's easier to understand devices that are powered by DC current than to understand devices powered by AC current.
 
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Wonderful answers that cleared up the grey areas in my mind. Those were the answers I was looking for. Thank you all for clearing that up
 

jim hardy

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Put your math to work.

Power is volts X amps.

Alternating current indeed reverses direction.
IF:
when voltage is positive current flows left to right,
THEN:
when voltage is negative, current flows right to left..

So during at every half cycle, current and voltage BOTH swap their signs
Now think ---
since a (+) X a (+) = a (+)
and a (-) X a( -) = a (+),

voltage and current might reverse direction but their product is always positive..
So energy DOESN't flow both ways, only from source to load. That its sign doesn't change proves it.

See ? You already knew that..

(CAVEAT - we're ignoring power factor for now, as @Windadct astutely observed)

old jim

(and @ still doesn't autocomplete until second or third try)
 
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davenn

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Basically, is the alternating electricity still alternating when it is applied to the real world grid over large distances and what are the the points to which it alternates to and from?
When a current is flowing, electrons are travelling very slowly - millimeters per second or less. In fact, the electrons are jiggling randomly, and the current is expressed by a net movement in a certain direction.

for DC yes, for AC they don't really go anywhere …. as in the electrons in the tungsten filament of you light globe never leave the light globe, they just oscillate back and forwards about a point with in the filament ( or other part of the circuit)

Personally - I really do not like discussion of EM Wave in AC power distribution, IMO it only complicates the discussion and is not needed to review the flow of power.
well for AC you have to, because it is the EM field around the conductor that is carrying the energy that does the work, not the movement of charges along the circuit, because for all practical purposes they are not moving any significant distance


Dave
 
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With AC electricity, I understand the energy goes back and forth, even on the high voltage power lines that fulfill the energy needs for factories , community buildings, or homes.

However, some of these powerlinea go long distances, and I’m trying to figure out how the electricity alternates such long distances. It has to be broken up in segments by right? -Such as being received by a unit, and then relaunched for the next segment to alternate-

Or can it truly go hundreds of miles back and forth changing direction rapidly, because of the high voltage and amplitude?
Electrons are not tiny packet of energy that deliver power. They will not move quickly at any voltage level used in the power system. You confuse propagation of an electromagnetic wave (which is electricity) with drift velocity of charges. EMag waves travel at a goodly portion of the speed of light through most substances. This is why your lights go off almost instantly if a substation breaker opens many miles/kilometers away from you. The drift velocity of an electron is usually much slower than an infant would crawl.

I'm generally unsympathetic to "think of it like this" explanations wanted by those trying to understand the engineering/physics aspect of things. The math will give you what you need to know.
 
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I'm generally unsympathetic to "think of it like this" explanations wanted by those trying to understand the engineering/physics aspect of things. The math will give you what you need to know.
I agree. See this thread https://www.physicsforums.com/threads/wanted-b-level-explanation-of-conduction-and-resistance.938868/ and these insights articles
https://www.physicsforums.com/threads/ohms-law-mellow-comments.883987/#post-5557225
https://www.physicsforums.com/insights/circuit-analysis-assumptions/
where I make similar arguments (without a lot of success).

I'm afraid that we will never succeed in discouraging the use of analogies to explain electricity. The math needed to explain it the right way is considered too difficult by many people. Many people who are curious about electricity are not curious enough to study the math.
 
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I agree. See this thread https://www.physicsforums.com/threads/wanted-b-level-explanation-of-conduction-and-resistance.938868/ and these insights articles
https://www.physicsforums.com/threads/ohms-law-mellow-comments.883987/#post-5557225
https://www.physicsforums.com/insights/circuit-analysis-assumptions/
where I make similar arguments (without a lot of success).

I'm afraid that we will never succeed in discouraging the use of analogies to explain electricity. The math needed to explain it the right way is considered too difficult by many people. Many people who are curious about electricity are not curious enough to study the math.
Very nice. I've read all of your insights with great interest. Thank you for taking the time to write them. I'd not read B Level explanation of conduction thread. It went off-rails about as quickly as one would expect. Language can only take you so far. Pictures are the key, I think. They are the key only because you have to have the math to create them. Were one to start with the math and let intuition follow it'd save a lot of headache and re-learning in the future.

I don't mind trying to give a lineman a "simple" explanation, but I expect an engineer to understand the detailed, down-and-dirty, real-talk. Engineering professors should get my old QM teacher to give their lectures. Analogies were not allowed in his class. You looked at the math or you looked at nothing. This seems to be a common thread among most physicists. We engineers could learn from them.

Nothing was ever mentioned in any of my rudimentary circuits classes about the short-comings and assumptions of basic circuit theory. For some reason, algebra and basic DiffEq were always the focus. Probably because, although having taken (and passed) those classes, no one ever seemed to know what was going on mathematically. The teacher had to re-teach these absolutely fundamental principles to the detriment of the crucial subjects at hand. I think this is the main issue: People are not learning the math in the first place. Rather than try to ensure proficiency during the math classes, engineering topics are "dumbed-down" to meet the inadequate skill-level. This has an unquestionably disastrous effect on everything thereafter, in my opinion.

One needs two things to be a good engineer: A firm grasp of mathematics of varying degrees of difficulty, and the ability to write masterfully. Both of these subjects are not being adequately learned, in my opinion.
 
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Wow.
 
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I'm generally unsympathetic to "think of it like this" explanations wanted by those trying to understand the engineering/physics aspect of things. The math will give you what you need to know.
Water analogies and suchlike are not perfect but, as long as this is given as a caveat, they are capable of bringing a lightbulb moment necessary for the start of a good understanding, I think. Not everyone has a mind that can work solely from mathematical explanations, especially when starting from scratch.

They give a framework from which to hang future knowledge. Like Wikipedia, they are a good place to start, but not a good place to end or continue, one’s study of electrical theory.

I still find it useful, for example, to think of an inductor in a circuit like a heavy flywheel - initially resisting an increase in current until ‘up to speed’, then in turn resisting a fall. Similarly, the action of the shorted turn in a shaded pole induction motor can be grasped much quicker with a flywheel analogy. The math(s) can come later.
 

jim hardy

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They give a framework from which to hang future knowledge.
they can train your mind to make the thought steps that arrive at the right formula.
that's how i cross check my thinking.

Perhaps some people actually do think in equations - i cannot know .

Here's a longer extract from Einstein's answer:

"(A) The words or the language, as they are written or spoken, do not seem to play any role in my mechanism of thought. The psychical entities which seem to serve as elements in thought are certain signs and more or less clear images which can be "voluntarily" reproduced and combined. There is, of course, a certain connection between those elements and relevant logical concepts. It is also clear that the desire to arrive finally at logically connected concepts is the emotional basis of this rather vague play with the above-mentioned elements. But taken from a psychological viewpoint, this combinatory play seems to be the essential feature in productive thought--before there is any connection with logical construction in words or other kinds of signs which can be communicated to others.

(B) The above-mentioned elements are, in my case, of visual and some of muscular type. Conventional words or other signs have to be sought for laboriously only in a secondary stage, when the mentioned associative play is sufficiently established and can be reproduced at will.

(C) According to what has been said, the play with the mentioned elements is aimed to be analogous to certain logical connections one is searching for.

(D) Visual and motor. In a stage when words intervene at all, they are, in my case, purely auditive, but they interfere only in a secondary stage, as already mentioned.

(E) It seems to me that what you call full consciousness is a limit case which can never be fully accomplished. This seems to me connected with the fact called the narrowness of consciousness (Enge des Bewusstseins)"

From "A Mathematician's Mind, Testimonial for An Essay on the Psychology of Invention in the Mathematical Field by Jacques S. Hadamard, Princeton University Press, 1945." in Ideas and Opinions.
https://www.pitt.edu/~jdnorton/Goodies/Einstein_think/index.html

Before i can believe a formula i have to "feel it" .

old jim
 
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Before i can believe a formula i have to "feel it" .
You’re in good company. Feynman, working at Los Alamos, was said to sit at his calculating machine ‘singing’ the computations. Listeners could tell if he was cogitating a linear function (“whoooooiiiiiip”) or an exponential (“whoiiiiiiipp!!”)
 
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