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Effect of resistance on voltage flow in a wire

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  1. Feb 9, 2017 #1
    Hey guys!

    Any answers will be much appreciated!

    The following is a deep conceptual issue I have. The diagram is a bit crude but will suffice I think.
    Note: Had drawn the circular 'electric field' at the time, to just help myself over my confusion. Feel free to ignore it.

    How voltage increases in an (illustrated) electric field when resistance changes.JPG
    As per Ohm's law V = IR. In the above scenario there is greater resistance at point 1 then there is at point 2 (as shown on the diagram). The charges (q1 and q2) and so the current flowing through both points are the same.
    Voltage is defined as the strength of an electric field to cause current flow. And current flow is nothing but the movement of charges i.e. Movement of electrons.
    So following on, this means that according to Ohm's law of V = IR, there is a greater voltage at point 1 then there is at point 2. Therefore voltage will 'flow' from point 1 to point 2 i.e., From high to low.

    This is where my issue arises. With there being greater resistance for the electrons to flow from point 1 how is it that the electrons will go from point 1 to point 2? Shouldn't it be the other way around. I don't conceptually understand how the greater resistance in this way causes voltage flow and so current flow from point 1 to 2. From my definition of voltage given, the greater resistance at point 1 would actually make it harder for the current to flow from point 1 to 2. Conversely the lower resistance at point 2 would make it easier for the current to flow from point 2 to 1.

    Yet, and as per Ohm's law, voltage is greater at point 1 so current somehow flows down?

    It is kind of a question within another question, so had to explain myself a bit. I am really not knowledgeable in electrical phenomena, so this may very well be a basic matter that I don't know!
     
  2. jcsd
  3. Feb 9, 2017 #2

    DrClaude

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    Staff: Mentor

    This is not clear at all. Could you simply describe the setup you are considering?
     
  4. Feb 9, 2017 #3

    LvW

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    "I don't conceptually understand how the greater resistance in this way causes voltage flow..."

    What is Voltage flow?
     
  5. Feb 9, 2017 #4
    Sorry for it not being clear to you. Basically in a nutshell, my confusion is all about how Ohm's law works in reality, i.e., how does increasing resistance increase the voltage at the level of current and electron flow? The equation is V= IR but I don't see how increasing the resistance actually increases voltage!

    In the above diagram I haven't shown a battery or a power source but say current was flowing through the wire and one end of the wire has more resistance then the other end, which way would the current flow within the wire? (The current being the same throughout the wire) From Ohm's law we know that V = IR so the end of the wire where there is more resistance would have a greater voltage potential, correct?

    Following on simply, voltage flow occurs from a point of high voltage to a lower voltage. If I am right up untill now then what I am conceptually confused is how, despite Ohm's Law of V = IR, can increasing the resistance increase voltage potential. Because, resistance would repel or obstruct the flow of electrons. Yet this somehow increases voltage potential?
    Voltage is the measure of how much electrons can move (current) so I don't see at the atomic scale, how increases resistance can help in moving electrons and increase voltage potential?

    Have tried my best to make it clear, please get back to me.
     
  6. Feb 9, 2017 #5

    berkeman

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    You have some very big misconceptions about how electricity works. We will do our best to help you get things straight.

    What resources have you been using to learn about how basic electricity and Ohm's Law work? Can you give us a couple of links to what you have been reading?

    Here are some basic links:
    https://en.wikipedia.org/wiki/Ohm's_law
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html

    Resistance does not exist at a point. It exists between two separated contact points on a material. The resistance between those points determines how much current flows when you apply a voltage between those two points.

    Could you please read through the links, and post your follow-up questions? Thanks.
     
  7. Feb 10, 2017 #6
    Thanks for your reply. You seem to be a helpful person:)

    Actually, my misconceptions are rather due to ignorance of the matter, its not that I read anything particular on electricity. However now having gained interest I would like like to start. I am thinking of reading a physics book to begin. Perhaps other things like YouTube videos and other websites like the one you shared could do me well.

    You mentioned, "Resistance does not exist at a point. It exists between two separated contact points on a material. The resistance between those points determines how much current flows when you apply a voltage between those two points" - Thanks for this clarification, I now see that if there is more resistance between two points then there will be a greater voltage potential.

    However - I don't understand why resistance doesn't exist at a point. For their to be resistance in between two points there must be resistance at each and every other point (in space) along the way also shouldn't there? Applying this to my situation above I simply see (and now remember from what was taught in high school) that electrons or current follows the path of least resistance. So as there is greater resistance in point 2 compared to point 1 current will flow down. This is fairly logical is it not?

    Thanks for your links provided, I will check them out and get back to you if there is anything!
     
  8. Feb 10, 2017 #7

    berkeman

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    That's not really the best way to think about it. Usually in these Ohm's Law situations, the resistance is constant and set by the geometry and resistivity of the object, and the current is dependent on the voltage applied, V=I*R. You can apply a known current and measure the voltage that results, but that is a less common way to measure the resistance.
    It's better to think of resistance existing across a volume or volume element. In fact, we calculate the resistance of unusual shapes by dividing the shape up into little cubes or thin slices through the shape, and add up the resistance pieces along the path between the two electrical contact points.
     
  9. Feb 10, 2017 #8
    Yes, so you're saying that in a circuit for example, the resistance along the wire is the same. The 'points' of resistance in a circuit would then be where the resistors are!

    Right, so this must be just some multi-integral calculus you are referring to aren't you? Also, where can one learn or come across the foundational derivations for quantities like resistance and voltage etc. Is it through study of Maxwell's equations? This is where even the very common Ohm's Law is derived isn't it!

    Lastly, have just purchased the book "Introductory circuit analysis 10th Ed" by Robert Boylestad. From the reviews given it seems to be a SUPERB book on both electronics and circuit analysis for total beginners. Am stoked about learning from it.

    Thanks again!
     
  10. Feb 10, 2017 #9

    berkeman

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    It can be an integral or a summation. Here is a good starting point:

    http://web.mit.edu/viz/EM/visualizations/coursenotes/modules/guide06.pdf

    Post if you have more questions. We are glad that you are learning so much and so well now. :smile:
     
  11. Feb 10, 2017 #10
    Thank you much, you've been patient! Will do!
     
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