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Ohm's law, voltage

  1. Jan 13, 2014 #1
    At this page http://amasci.com/elect/vwatt1.html it says:

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

    "against the voltage" I don't understand this. Isn't voltage pro-active with regards to the movement of the electrons? How come it says 'against'?

    Thank you
     
  2. jcsd
  3. Jan 13, 2014 #2

    jbriggs444

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    From the point of view of a battery driving charge through a circuit, it is pushing the charge carriers upstream against a potential difference.

    This is similar to, for instance, the water pump at an amusement park pumping water to the top of the log flume ride. You are pumping water from the pond at the bottom to the spout at the top and letting the water flow down the flume and into the pond. Water = electrons. Pond at bottom = low potential. Spout at top = high potential. Flume = rest of the circuit.
     
  4. Jan 13, 2014 #3
    Thank you, this helped on my reflections.

    The thing is that I am translating these articles into Danish.

    He takes a version of ohms law and writes it out into words. And regarding these words: is the explanation off when it says "against the voltage pressure"? I mean: I would've said: it takes energy to push some charge against/through electrical resistance(/ohms).
     
  5. Jan 13, 2014 #4

    sophiecentaur

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    The problem about this statement is that Voltage is not a 'pressure'. It is a difference in energy so why does he bring in a false mechanical analogy? Volts are the Work needed to move charges between points with different Electrical Potential. The sign of the Work / Energy (i.e. put in or taken out) will reflect whether energy is being supplied (from a battery etc.) or used (through a resistor or charging a battery). Everyone has an initial problem in assigning the +- signs to the emfs and PDs in circuits when trying to use Kirchoff's Laws. I think he is adding confusion in this statement.
    That article is not, on the whole, too bad and he is tries quite successfully to make a number of electrical terms and quantities more approachable. (I have read an awful lot worse!) However, you will need to be very careful in translating technical idioms and metaphors between languages and the best way will be to refer constantly to what the Maths of the situation is telling you - so that both verbal explanations fit the Maths accurately.

    It's worth while (as you are getting into the education business) to make clear just what Ohm's Law actually says and not to use the rather sloppy way it is so often used used. Ohm's Law specifies the condition of constant temperature, for instance, and it refers to metals, which behave linearly. There will always her a ratio of Volts and Current for other electronic component but 'Resistance' is only the ratio for a particular combination and can't be relied on any other time.
     
  6. Jan 13, 2014 #5
    The whole purpose of these Electricity Articles (as I understand) is to teach people what is going on in the real world (the physical concepts, and without much math really(and also the math is simple!). And in science it seems to me students will often need to learn simplified concepts (sometimes even leaving out relevant information) to first build a solid picture and later modify the whole thing. So: the goal is to get a gut-level understanding of so called electricity since many (even engineers) is lacking this understanding (although they might master the math perfectly).


    What do you think of the specific phrasing "against the voltage pressure"? Are the charges not flowing AGAINST the resistance/ohms? The energy help the electrons move AGAINST the resistance/ohms? I don't understand why the article says 'AGAINST the voltage pressure'? As if the electrons move along in spite of the voltage pressure-force?
     
  7. Jan 13, 2014 #6
    These articles are probably the best texts on electricity I've ever read, and thousands of people have visited the amasci.com site because of them.

    The reason that I want to translate the material is that a lot of the same misconceptions occur in Danish. There's the same problem with the word electricity in Denmark/Danish. Also, just a couple of weeks ago my teacher tried to teach me that the individual electrons are themselves carrying the energy from the battery to the lightbulb and afterwards they are empty of energy. Explanations like this one are widespread ALSO AMONG TEACHERS! So, I would be glad if we could get rid of these wrong ideas.

    However, you are right that one has to be careful when translating texts like this one. Also a few physicists will go through my Danish versions of the articles to make sure that I haven't misinterpreted any terms.
     
    Last edited: Jan 13, 2014
  8. Jan 13, 2014 #7

    sophiecentaur

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    I understand his intention and, as I said, he does a fair job but why associate voltage with pressure when he could just as easily associate it with work or energy? He has obviously been careful in his use of words in order not to be a hostage to fortune and here is an example where he just hasn't done it so well.
    I must say, I find it tiresome that people expect to get the higher concepts in certain areas of Science with a "Gut Feeling". How can they be so presumptuous? Could you possibly get a clue about higher Maths by 'gut feeling'? (For some reason, that stuff is never undersold; I wonder why?) How about Carbon Chemistry, without learning about bonding? There is a serious risk in dumbing down in these matters. People ask for a "Physical reason" for certain phenomena when what they really mean is a 'mechanical' model that will explain things. there is no good mechanical model for electricity. It is what it is. By all means make it approachable but don't chuck the baby out with the bathwater.

    This is not actually essential in many cases. The so-called child-centred learning can do more harm than good - particularly with able kids who will never be presented with anything 'hard' until long into their education. They are just not used to dealing with 'too difficult'. It is possible to teach nearer the truth as long as you use the right words and avoid the future cognitive dissonance just when students need to make the big steps.
     
  9. Jan 13, 2014 #8

    sophiecentaur

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    'Entry level', I think you mean.
    Yes - but not talking with a quasi water flow analogy and not by talking about pressure.
    You seem surprised about "TEACHERS" not teaching well. You would be amazed at many teachers know anything of Physics but are called upon to deliver it to GCSE level students ( up to 16 years old)
     
  10. Jan 13, 2014 #9
    It seems to me that you talk in abstract terms. If a person doesn't have a clear understanding of electric current and electric energy in a simple circuit (what is physically going on), and he/she gets this concept corrected and then is able to make sense of things - then I would say there is a pretty good chance that he/she will attain a better/more real "gut feeling".

    However, this has nothing to do with my question.

    My question is:
    What do you think of the specific phrasing "against the voltage pressure"? Are the charges not flowing AGAINST the resistance/ohms? The energy help the electrons move AGAINST the resistance/ohms? I don't understand why the article says 'AGAINST the voltage pressure'? As if the electrons move along in spite of the voltage pressure-force?
     
  11. Jan 13, 2014 #10

    sophiecentaur

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    Is there a "clear understanding", without considerably more basic knowledge?
    Could you not be undervaluing the whole thing? That tricky little 'understanding chemical' flows through the bloodstream when we think we've go something sussed can be so misleading. Science, based on gut feelings will let you down very soon.
    To follow your analogy a bit. When you push against a spring, are you "pushing" in the same way as you would "push" against friction? Are the two 'resistance' forces the same? What is happening to the energy in each case? Energy rules, usually and it's not that hard to get familiar with.
    Sorry for so many questions but I am demonstrating how the energy situation is far more relevant than the simple force situation. Likewise for charging a battery or capacitor or causing a current in a resistor. You have more or less made my point for me. A gut feeling can just as easily lead to the wrong conclusion as the right conclusion.

    Electricity is abstract. It is not a good idea to think it isn't.
     
  12. Jan 13, 2014 #11
    If you do not have an answer or comment to the specific question I pointed out, and which this thread is about, then do not comment.
     
  13. Jan 13, 2014 #12
    If the charge is moving against the voltage -> [a (-) charge is moving from + to -] OR [a (+) charge is moving from - to +]. So it takes energy.
     
  14. Jan 13, 2014 #13
    Energy and charge is all you need to define voltage, no matter if you are a 14 y.o. student or a great physicist.
    ¿What do you mean by "more basic knowledge"?
     
  15. Jan 13, 2014 #14

    AlephZero

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    I think the problem is trying to describe this in terms of "cause and effect", and taking voltage as the "cause".

    Certainly there is a correlation between energy, movement of charge, and electrical resistance. That is what Ohm's law says. But as alva said, "voltage" is really defined in terms of an electrostatic field and the work done to move a charge from one place to another. Analogies like "pressure" that somehow make "voltage" an independent quantity are wrong, at a fundamental level, even if they are sometimes useful teaching aids.

    If you move a fixed quantity of charge "through" a resistance, you have to do work. You have to do more work if the resistance is higher.

    So if you move the charge in the same amount of time (constant current), the voltage across the high resistance is higher, and because of the way we measure the quantities, Ohm's law says the voltage is proportional to the resistance, of course.

    Or, if the voltage stays the same, you can only move the charge more slowly (lower current) if the resistance is higher.
     
    Last edited: Jan 13, 2014
  16. Jan 13, 2014 #15

    sophiecentaur

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    Yes, exactly. You need to be familiar with using the concepts of Energy and Charge - both of which are pretty sophisticated and are very far from water flowing / pressure / mythical electrons moving like tiny peas through a tube. This is my whole point. The 'basic knowledge' is not actually included in the things that people assume you need to have before 'understanding Electricity'.
    I have talked to very few 14 yo students who could discuss what charge or energy actually 'are' and even fewer who would know that 1V = 1J/C. That is only served up to them in the first year of Sixth form (17 yrs +)
     
  17. Jan 13, 2014 #16

    sophiecentaur

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    Anyone who thinks Electricity is easy enough for 'anyone' to understand, should do an experiment on the first person they meet in the pub, shop or work place. Draw a simple potential divider circuit and ask them how it works. Even those of you who work with electrical engineers may find some surprising results.
    Electricity is damned hard stuff to understand.
     
  18. Jan 14, 2014 #17
    Reminder: the quoted sentence is one single phrasing out one rather long article.

    Maybe I haven't been clear: it's not that I don't understand ohms law, voltage, energy or current (basically). It's about the phrasing of:
    VOLTS x COULOMBS = JOULES It takes energy to push some charge against the voltage pressure

    I see what you mean, AlephZero, with cause and effect, and obviously the terms are very much interrelated. You NEED ohms and current to have voltage and so forth. To me the phrasing gives the impression that the voltage-pressure/force is holding the electrons back and that they move along in spite of this force? The guy is a physicist and an engineer so I just wondered if I was missing some point.
     
  19. Jan 14, 2014 #18

    sophiecentaur

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    Actually, you only need Potential for Volts to 'exist'. I realise this is a bit chicken and egg but if you start with Volts then they could be generated in any way you like (charges or induced emf; battery or transformer). Then the rest will follow.
    If you start the 'chain of causality' with Charge, then you need a different approach for a changing current. Yes - you could make it work but I don't think it would be very satisfactory.
    The energy situation (or you may like to think of the Field) is the primary thing here. Once you introduce some charges, they will move at a rate determined by the nature of any path you provide. This path may either involve supplying energy (charging a battery) or dissipating energy (through a resistor).

    I think that, unlike in most of the document, he actually doesn't do such a good job at that point. Which is why you were thrown a bit. I wouldn't mind betting that he would be prepared to revisit that passage and re-word it in his usual clear way.
     
  20. Jan 14, 2014 #19
    You're right, the field is the primary thing. I was obviously wrong saying you need both ohms and current (so to speak). But you do need resistance right? To have an e-field? If there were no electrical resistance between the two poles of the e-field it would just neutralize itself, yes?
     
  21. Jan 14, 2014 #20

    sophiecentaur

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    That's a funny way of putting it, I would say. The potential is only dependent upon the value of a resistance in a roundabout sort of way. If you start with an ideal voltage source, the presence of a resistor has no effect on potential at all - there might be a modification of the field in the vicinity of the resistor but only if it's resistance is non-uniform. You are right that a Zero resistance between two points would force the PD across them (from a real power source) to be Zero - but that is an extreme / idealised case, like an irresistible force or an immovable object and should be treated with great care.

    Why would the field 'neutralise itself' without a resistor? You can get substantial fields in a vacuum. You may be putting the cart before the horse here. Read a few google hits on fields, volts, resistance etc. and see the order these things are usually stated in. It may solve it for you.
     
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