Ohm's law, voltage

  • Thread starter kjeldsmark
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  • #51
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Like sophie said , the argument you brought in is used in a purely electrostatic situation , charged objects repulsion etc, in a circuit PD moves charges and forms current
 
  • #52
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"Why would the charges need to be pushed agaisnt the very own field which made their flow possible in the first place?"

This is what i'm asking!

"VOLTS x COULOMBS = JOULES It takes energy to push some charge against the voltage pressure"

So, how do read this sentence (I'm not talking about ohms law in general but this specific sentence(which of cause is about ohms law)?

The sentence (FOR ME) says: it takes energy to push som charge (IN DIRECTION ONE) A G A I N S T the voltage pressure (DIRECTION TWO). In other words: it says that the same forcefield is pushing in two opposite directions. This makes NO sense to me!!
 
  • #53
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I need to be absolutely sure that I don't misinterpret the English language with regards to the word "against" in this specific context.
 
  • #54
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just throw out that sentence , in the context were talking here it is wrong.
sophie already said that.
in a circuit there is no field agaisnt a field.There is a PD and a electric field which causes the charges to align and move in a given direction.there is no field against them.

in a transformer and other inductive devices there is a phenomenon called back EMF which acts against the change in current.that could be something closer , imagine in an inductor or a transformer when you first apply a PD , say turn on a switch imediately current rushes through the windings inducing magnetic flux in the core the induced flux creates a field opposite to that which induced it in the first place called back EMF.And even then your description doesnt apply in the way you think of it.
So I would say just let go that statement and rewrite it the way people explained it to you here.

Under normal conditions when you apply a PD to a circuit or whatever wire or etc there is a field and that field moves charges in a direction , there aren't two fields opposing each other , if there were there would be no charge movement,m the field would cancel out and it would be a static situation.Charges flow from higher potential to lower , the actual electrons if you care about them flow the other way but they both flow from one place towards another and there is a difference in potential between those two places , voltage doesn't go against voltage , that's like putting two battery + together, if they are at the same potential , guess what? Nothing happens.
 
  • #55
sophiecentaur
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Why do you insist on using the phrase "voltage pressure" when it is such a bad one? Also, you are making a pointless stand about the particular sentence which I have already said, doesn't make proper sense. It never will. Just because you read it in your favourite article in the whole world, it is not 'Gospel' but it is a fair article..
Just have the confidence to look at the whole thing another way round.
I don't think you can have read my last post because I did put the whole thing another way round to sort out your problem. To find which way a charge will flow, you just need to know which way is 'downhill'. Take a 3V battery, connect the neg terminal to Earth. Take a 6V battery, do the same. Connect the two + terminals together with a 1 Ohm resistor. Current will flow into the + terminal of the 3V battery against the 3V because there is a higher potential on the + terminal of the 6V battery. There will be (6-3)/1 Amps flowing (I=V/R)
The 3V acts against the 6V and the actual current flowing is regulated by the resistor.

You will notice that I never once mentioned Fields because they just do not help in any way to solve this problem. When you have taken things on board my way, and got some working knowledge of circuit calculations, then it might be a good time to investigate the Fields thing but you can spend y our whole life as an Electrical Engineer and only use the Field approach once or twice. :smile:
 
  • #56
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Thank you for trying to help me.

The reason I'm so opsessed with this expression 'against voltage pressure/force' is that it's used by so many professionals and people who I considder true experts. That's why I was/am thinking that the most reasonable explaination is that they mean something by this use of words which I don't see.
 
  • #57
sophiecentaur
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"so many"???
In what context?
I know a lot of seriously 'real' professionals and it's not the sort of thing they would be likely to say because it doesn't describe an electrical situation except in electrostatics, when other forces than directly electric ones come into play..
Are you sure this isn't a bit of displacement activity - putting off getting down to the real stuff? Huh?

'Professionals' talk in the correct way about Potential Difference, in any case. Voltages (emfs) can oppose one another, as Kirchoffs Second Law demonstrates but that is not what you are worrying about, I think.
 
  • #58
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I also said in earlier posts that there are situations in circuits where a situation similar to that you describe comes into play but first of all it's not defined the way you say it here and secondly the situation you are describing which is a fairly simple one , not even the sophies mentioned batteries rather a simple wire and some voltage on it and current running through it in such a situation there isn't anything opposing the original PD that creates the current in the wire.
So I would say in the situation you are talking about which is a basic current through a wire in a circuit situation nothing like you describe happens.


2+2=4 no matter is it said by a approved professional with a phd or a random stranger on the street you must be smart enough to see that sometimes truth can be said by anyone , I believe this is the case as I see no reason to disbelieve in sophies and other forum members arguments.
 
  • #59
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but the charge moves BECAUSE of the voltage force. When the sentence say the opposite - how can it be correct?
Well ok I agree then. When reading a larger chunk of that text, then everything seems fine, except that one line sticks out, and there's a feeling that that one line is wrong.
 
  • #60
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And regarding 'learning style' I feel the same way as Mr. WJB:

"When I went into engineering school, I found it extremely odd that there were still no good explanations of bipolar transistors. Sure, there were detailed mathematical treatments. Just multiply the Base current by "hfe" to obtain the Collector current. Or, treat the transistor as a two-port network with a system of equations inside. Ebers-Moll and all that. But these were similar to black-box circuits, and none of them said HOW a transistor works, how can a small current have any effect on a larger one???? And nobody else seemed curious. Everyone else in the class seemed to think that to memorize the equations was the same as learning concepts and gaining understanding of the device. (R. Feynman calls this the Euclidean or "Greek viewpoint;" the love of mathematics, as opposed to the physicists' "Babylonian viewpoint" where concepts are far more important than equations.) I'm a total Babylonian. For me, math is useless at the start, equations are like those black box Spice programs which might work great, but they don't tell you any details of what's happening inside a device in the real world. I can learn the math, but that just means I can run a "mental spice program" without needing any computer, and I still don't know how transistors work. First tell me what "Transistor Action" is all about. Show me animated pictures, use analogies. Only after I've attained a visual and gut-level understanding of something, only then is the math useful to me for refining it and adding all the details. However, for me the math alone is not a genuine explanation. Math is just a tool or a recipe, a crutch for those who want nothing except the final numerical result, and it certainly does not confer expert knowledge."

(http://amasci.com/amateur/transis.html)
 
  • #61
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Ok I understand your concern , I too am not a fan of mathematics mainly because I'm not that good at it.
Just to start this I want to say that the maths and the images and schematics in your head of how it works go hand in hand.I agree that it is good and even necessary to understand how does it work, but when you will need to make a physical device you will need some accurate measurements and sizes and dimensions and then mathematics will become involved.
Einstein realized that the atomic bomb is possible because he was a man of great mind yet it took much more physiscists and Oppenheimer as the leading one to calculate all the details to make a real physical example of that imagined bomb.

Second of all when we get down to the very small , say quantum level it becomes really really troubling to imagine and have a picture in your head about how it works , mainly because you can't see it , and every analogy every story about how atoms look or behave isn't a perfectly accurate picture it just goas as far as it can , and here maths also isn't the full answer it describes the interaction in detail but that doesn't make you suddenly an observer of atomic stuff.
I would say that in certain physics areas due to the fundamental limitatios of this world we live in no way is good enough to have this " gut" feeling you are talking about.

Why do you think there is something magical inside a transistor ? It's nt so much about smaller current controling larger one in a sense that you would think that you can move a big rock with a tiny little stick , its rather applying that small current into the right place.
let me make an analogy , maybe not a perfect one but from what i ca tell you like them.
imagine the BJT as a dam on a small river made of sand and soil.the damn keeps the river at a certain height so the water before the dam has a pretty high potential and that would be our collector current. Now the water even though could destroy the dam and wash it away doest do that it just sits there.Now take a little side stream and apply it to the right place inside them dam so that it washes away the soil and makes it wet , now the large water mass is able to push the dam aside and run free.

in a transistor bjt, the small base current isn't physically pushing the large collector current, it just makes the transistor conducting from a non conducting state and so the larger current can flow.
 
  • #62
sophiecentaur
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And regarding 'learning style' I feel the same way as Mr. WJB:

"When I went into engineering school, I found it extremely odd that there were still no good explanations of bipolar transistors. Sure, there were detailed mathematical treatments. Just multiply the Base current by "hfe" to obtain the Collector current. Or, treat the transistor as a two-port network with a system of equations inside. Ebers-Moll and all that. But these were similar to black-box circuits, and none of them said HOW a transistor works, how can a small current have any effect on a larger one???? And nobody else seemed curious. Everyone else in the class seemed to think that to memorize the equations was the same as learning concepts and gaining understanding of the device. (R. Feynman calls this the Euclidean or "Greek viewpoint;" the love of mathematics, as opposed to the physicists' "Babylonian viewpoint" where concepts are far more important than equations.) I'm a total Babylonian. For me, math is useless at the start, equations are like those black box Spice programs which might work great, but they don't tell you any details of what's happening inside a device in the real world. I can learn the math, but that just means I can run a "mental spice program" without needing any computer, and I still don't know how transistors work. First tell me what "Transistor Action" is all about. Show me animated pictures, use analogies. Only after I've attained a visual and gut-level understanding of something, only then is the math useful to me for refining it and adding all the details. However, for me the math alone is not a genuine explanation. Math is just a tool or a recipe, a crutch for those who want nothing except the final numerical result, and it certainly does not confer expert knowledge."

(http://amasci.com/amateur/transis.html)
I have to say that William Beatty and / or his tutors must just have been out of their depth about the mechanism which makes a transistor work. Of course there is a 'good' (accurate) description in many text books. The point is that the description cannot be reduced to a trivial level whilst remaining accurate. How is that 'unsatisfactory'? Life is not full of simple solutions to all problems. If someone can't understand the full description then they are either limited by the level of their basic ability or by how much they are prepared to work at it. (Like weight lifting and triathlons).
That statement reflects either a sloppy attitude or an over estimate of his understanding of the topics he is applying to. It's not a matter of 'the Maths of the Ideas'. You need both, if you want to understand advanced Science to a level where you can predict and synthesise usefully. If you didn't need the Maths, then you could get a higher Physics degree on the strength of what they tell you on the TV Science programmes.
Do you really believe that this computer you are using, at the moment, was made to work by people who just waved their arms about and had a 'comfortable' feeling that they grasped how solid state devices work? Maths is not "just a crutch". It is a vital extension to everyday language which allows people to manipulate and communicate ideas and relationships that are way beyond the limits imposed by English. A picture speaks a thousand words - very true - but there is no satisfactory verbal description that can deal with even the simple operation of changing the subject of a linear algebraic equation to get a numerical answer to a simple electrical problem. Try describing, concisely, the Fourier transform in simple words.
 
  • #63
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another thing people are not ready to wait these days, eberyone needs everything on the spot.understanding and knowledge including , but the thing is understanding and " gut" feeling comes only after a while , after many hours spent learning and trying to undersand stuff.
I may hate that myself but sadly it's the truth.

So paraphrasing Jack Nicholson, I want to ask you , Can you handle the truth? :D
 
  • #64
ZapperZ
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This thread has degenerated into something completely different and is no longer on the physics. It is now done.

Zz.
 

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