Question about electric generators

Bararontok

Quote by HassanEE

You need to go back to first principles, and you'll see why everything you've proposed becomes awful complicated.

Charge exists everywhere, but for practical purposes, it isn't useful whatsoever unless it is moving nicely in a predictable manner, with amplitude and direction. Your typical copper wire has 8.5×10²⁸ free e^-/m³--that's a lot. You can harness them all you want using a static generator but what for? You want them moving down the cable that's what powers your light bulb.

The point of electric power generation is to get electrons moving in a closed loop. It's true on any scale. That's what your battery does, small/large generator, etc. So to say let's replace the battery with a charge accumulator, then create some mechanism to make that charge flow nicely is, in my opinion, as strange as saying let's suck all the fresh water from a stagnant river, pump it onto an uphill pond (at a higher potential), then let it flow again. But I assure you, it will stagnate again. It's a lot smarter (& efficient) to use the water pump to give it that lil nudge to move in its preferred direction.

This is what I mentioned in this post:

http://physicsforums.com/showpost.ph...1&postcount=16

It turns out to produce so many complications and inefficiencies that it is just not practical. Things would actually work more efficiently with the current electrical generation methods. Besides an induction alternator has an efficiency of up to 97% and a transformer has an efficiency of >90% so the grid is already very efficient as it is.

In conclusion, what you said about using the conventional induction electromagnetic generator to give the electrons a steady nudge is better and more controlled than forcing the charge distributions to change and letting the distribution equalize then repeating the process which is inefficient and produces an output that is unstable and difficult to control.

jim hardy

What's in a name?

The point of electric power generation is to get electrons moving in a closed loop.

That's why, when first starting out with beginners i stress that "E" for Voltage is short form of "EMF",
which is the abbreviation for Electro Motive Force", a good name for it
which should be thought of backwards: Force that Motivates Electrons (to move).

Then one can introduce the concept of potential, absolute and differential, with finger on the table,
and from that it follows naturally that work = Force X Distance...
Some people need that kind of visual aid in order to gain confidence in the equations.

Not lecturing you guys, just i've seen this method help struggling newbies. Hope it helps you help someone.

Windadct

It now seems we have 2 threads in one ....

Back to the OP to clarify "Generators in Power Stations" - to be large scale > 100MVA, this is where the issue of a step up transformer is most relevant.

The issue of practical energy harvesting from static - is a completely different topic. -- really should not be in the same discussion.

Bararontok

Alright, my questions have been completely satisfied. Thank you very much for the advice. If ever I have more questions about this, I will post it here or in a different thread.

cabraham

Quote by jim hardy

What's in a name?

That's why, when first starting out with beginners i stress that "E" for Voltage is short form of "EMF",
which is the abbreviation for Electro Motive Force", a good name for it
which should be thought of backwards: Force that Motivates Electrons (to move).

Then one can introduce the concept of potential, absolute and differential, with finger on the table,
and from that it follows naturally that work = Force X Distance...
Some people need that kind of visual aid in order to gain confidence in the equations.

Not lecturing you guys, just i've seen this method help struggling newbies. Hope it helps you help someone.

But the force the gets electrons moving would have to be Lorentz force. How could it be otherwise? F = F_e + F_m = q(E + uXB).

I am not positive about the origin of the term "emf", but it is not at all a real force. In the early days before Lorentz force was known about, "emf" was coined and it stuck since then although we now know that the force acting on electrons is indeed Lorentz force. Just thought it should be mentioned. BR.

Claude

jim hardy

Thanks Claude

i was never quite sure whether Lorentz included both magnetic and electric fields - so just now i looked at hyperphysics and it appears that it does, just as you said.
http://hyperphysics.phy-astr.gsu.edu...ic/magfor.html

From a thread earlier this year:
http://www.physicsforums.com/showthread.php?t=588037

I've got to where i try to understand why things got the name they were given.

To that end it's sometimes helpful to parse acronyms backward like we do for RMS.

Term EMF is acronym for Electro-Motive-Force , that is a Force that Moves Electric Charge. (in some circles, that'd be electrons)

It is not implied by the name of that term whether the origin of the Force is elecrostatic, magnetic , thermoelectric , or electrochemical.

And that's how i answer that question.

GM's "Electromotive Division" makes those beautiful railway locomotives.
Every time i see one my thoughts leap to the huge generators and traction motors inside, and QV cross B. Now THERE"S some torque !
Wouldn't it be fun to be see the charge getting shoved sideways inside those conductors?

old jim

cabraham

Hi Jim,

I would say that just as "emf" is not an actual force, it is equally true that "mmf" is also not an actual "force". The term "mmf" is "magneto-motive force". As I said, neither is actually a force, but generators, motors, & transformers were examined prior to 1890-91, which is when Lorentz published his force equation. Induction was measured & the terms "emf" & "mmf" were coined. Based on known mAcroscopic laws, it was postulated at the mIcroscopic level, that induction must involve forces acting on charged particles. So the terms were coined. We still use emf & mmf when describing xfmrs, motors, & generators, without a problem since it is understood that the "f"'s in emf/mmf are not true "forces".

Anyway, this is an interesting discussion. Thanks Jim, for your contributions.

Claude

sophiecentaur

Isn't all this a bit 'speculative'?

Magnetic induction is a very good way (tried and tested) to generate electric power at useful AC voltages which can be conveniently transformed up and down, to suit - and at enormous powers. Electric induction can be used to pump charge into a circuit but the system is not "electrostatic" if it is supplying 'current' electricity. The area involved in making what would be, in effect, a massive Whimshurst Machine would be in the order of Football pitch size if you wanted to produce any significant power. Something like the London Eye perhaps?

jim hardy

Hmmm i always stumble a bit when learning something new. Please forgive.

An electron moving in a magnetic field assumes a curved trajectory, as in a CRT with magnetic yoke,

Does not its acceleration away from the straight line result from one term of "Lorentz Force Law" , QV cross B ?

How should i conceive of it if not a force? Is Lorentz Force Law misnamed?
http://hyperphysics.phy-astr.gsu.edu...magfor.html#c2

not being argumentative here, just i'd like to step up to next level of understanding.
Guess i'm stuck in the(18)90's. Been reading Sylvanus P Thompson...

old jim

cabraham

Quote by jim hardy

Hmmm i always stumble a bit when learning something new. Please forgive.

An electron moving in a magnetic field assumes a curved trajectory, as in a CRT with magnetic yoke,

Does not its acceleration away from the straight line result from one term of "Lorentz Force Law" , QV cross B ?

How should i conceive of it if not a force? Is Lorentz Force Law misnamed?
http://hyperphysics.phy-astr.gsu.edu...magfor.html#c2

not being argumentative here, just i'd like to step up to next level of understanding.
Guess i'm stuck in the(18)90's. Been reading Sylvanus P Thompson...

old jim

Your conception of Lorentz being a force is quite correct. It is a true force in the full sense of the word. The "forces" that I referred to as not being true "forces" are the "electro-motive force", & the "magneto-motive force" as well. Lorentz force is rightly named, but "emf/mmf" are not. But there is no harm in using the emf/mmf terms when describing induction because it is understood that these terms were coined while there was still more to learn about the subject.

This happens elsewhere as well. We use the term "centrifugal force". Is it really a true "force". Inside the rotating reference frame it is valid. It's inclusion maintains Newton's laws in that rotating frame. But in an inertial reference frame, if we draw free body diagrams of all forces acting on the body in circular motion, there is no "centrifugal force" at all. It is merely a math construct employed when viewing circular motion while inside the rotating frame of reference.

We use the phrase "centrifugal force" knowing how it should be construed. There is no need to acknowledge the nature of said quantity every time it is mentioned, so we don't. Likewise, emf & mmf are understood as being related to volts/turn & ampere-turns. When summing forces acting on charges, we use Lorentz forces due to E & B fields with velocity taken into account. A free body diagram of the forces acting on a charge carrier does not include "emf or mmf", because they are not actually forces.

I hope this helped. Best regards.

Claude

jim hardy

Claude - it helped far more than you know.

Thank you.

old jim

"Science is but language well arranged" - Lavoisier

sophiecentaur

Shame about the term 'emf'. It's really hard to think in terms of fields (volts per metre) producing force on a charge when the wires connecting the two terminals of a source of emf are taken round the room and back again. Where's yer volts per meter then? This hypothetical field is shared out all over the geometry of the circuit.

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Windadct	It now seems we have 2 threads in one .... Back to the OP to clarify "Generators in Power Stations" - to be large scale > 100MVA, this is where the issue of a step up transformer is most relevant. The issue of practical energy harvesting from static - is a completely different topic. -- really should not be in the same discussion.

Bararontok	Alright, my questions have been completely satisfied. Thank you very much for the advice. If ever I have more questions about this, I will post it here or in a different thread.

jim hardy Recognitions: Gold Member Science Advisor	Claude - it helped far more than you know. Thank you. old jim "Science is but language well arranged" - Lavoisier

sophiecentaur Recognitions: Gold Member Science Advisor	Shame about the term 'emf'. It's really hard to think in terms of fields (volts per metre) producing force on a charge when the wires connecting the two terminals of a source of emf are taken round the room and back again. Where's yer volts per meter then? This hypothetical field is shared out all over the geometry of the circuit.