Please clarify Voltage/Current relationship

Discussion in 'General Electronics Chat' started by levydee, Dec 7, 2016.

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  1. WBahn

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    Again, you can choose to view things however you want. Ohm's Law is very simple (for most people) to comprehend. It states that the current through the conductor is proportional to the voltage across the conductor. Look it up. Thus, if you double the voltage across the conductor, the current through the conductor doubles. This is what proportional means. Look it up. Mathematically, this means that

    current = k * voltage

    where k is a constant of proportionality. Notice the word "constant". If k is NOT a constant, then the relationship between the current and the voltage is no longer proportional and Ohm's Law does not apply. In practice, k is never a perfect constant over all conditions, but if the changes in k over the range of interest are small enough, then k is close enough to being a constant for that material to be considered ohmic (for that purpose over that range of conditions). If it is NOT close enough to being a constant, then you can call the ratio of V/I to be anything you want, including R, but it is NOT the R that is the proportionality constant in Ohm's Law (where R = 1/k in the above equation). You are perfectly free to pretend otherwise, but that doesn't make it so.
     
  2. Tonyr1084

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    Sep 24, 2015
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    Here's what I'm thinking may be your tripping point:

    A transformer drawing 1 amp at 120 volts is 120 watts. It's secondary (for example) may be putting out 12 volts at 120 watts. That's 10 amps.

    Now, transformers are not perfect devices, and much more complex than simple resistances. You have to learn about inductance to get a better understanding of how resistance comes into play. Nevertheless, you've not created more power, you've merely altered its state. 120 watts at 120 volts is 1 amp. 120 watts at 12 volts is 10 amps. Power (measured in watts) is the energy you know you can't create or destroy. Energy can be converted, not created. Since all things are not perfect, some of that energy is lost as heat. So in reality, you'll never get as much power OUT of a transformer as you put IN. I said "POWER".

    Depending on how efficient your transformer is - that's how much power you'll get out. Actually, I'm wrong. You'll get the same exact amount of power OUT as you put in. Trouble is some of it will be lost as heat. So if you put 100 watts of power into a transformer and, oh, lets say 15% of that is converted to heat, then the "ELECTRICAL" power you get will be 85% of what you put in. And heat is not the only enemy. You lose some of that energy as radiated energy - magnetic waves that go off and interact with the environment in some unknown way. That's why they say you're not supposed to live under high voltage lines. Radiated magnetic energy can be blamed for ill effects your body may suffer.

    I'm just sayin'.
     
  3. EM Fields

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    You know very well that I know what I'm talking about, so I can't imagine why you'd want to pick a fight by proffering belittling comments instead of just sticking to the facts.
     
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  4. WBahn

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    There is little evidence that you know what you are talking about when your claims regarding what Ohm's Law is contradict what Ohm's Law is -- namely that the current is proportional to the voltage. Again, feel free to continue believing whatever you want -- the real world will continue to get along just fine.
     
  5. EM Fields

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  6. WBahn

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    So, your comment was directed at saying that my comment, which specifically included the observation "to a good approximation" was correct?

    As for saying that "R=E/I is only true at a slice in time when it is measured" makes the whole concept of resistance virtually useless. More pointedly, that "Ohm's law can always be used to determine the voltage across, the current through, or the resistance of a chunk of material at any instant in time if two of the three quantities are known," that is patently NOT true. It's one thing to say that Ohm's Law applies to a material to a sufficient degree to call it an ohmic material if the current increases by a factor of 2.07 when the voltage is doubled. That almost certainly would be "a good approximation" in real world for most applications. But trying to claim that Ohm's Law applies to a material or device for which the current increases by many orders of magnitude when the voltage is doubled is patently absurd. You can only make such a claim if you either (1) do not know that Ohm's Law is that the current and voltage are proportional, or (2) do not understand what proportional means.
     
  7. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    In the limit, ohms law probably fails. Just take a 1/8 W and a 100 W resistor. If you exceed the power rating, the resistor could do a number of things depending on it's type: puddle, change value and/or break. Do, the same for a fuse. Resistors do have temperature coefficients. See http://riedon.com/technical/understanding-temperature/

    The devil is hiding in the details.

    A wire wiggling in the Earth's magnetic field generates a current. Most of the time we don't care. I had to tape down wires because the current was interfering with measurements in the pA range.

    We think of paper as an insulator, but with the right equipment, it can be ohmic. For fun, I probed a piece of paper and I got a measurable resistance. The equipment I used had a 2 pA range and up to +-100 V of bias could be applied. I might call that piece of paper ohmic and everyone else might call it an insulator. I was dependent on the humidity and air impurities.

    When I was in school, I challenged the teacher about an answer in an exam and won. His response was "Your not supposed to know that yet".
    I don't remember the actual question. In grade school, I learned about circular orbits of electrons and later learned it an orbit based on probability of finding an electron.

    Harder to grasp, pre-Internet, was the notion of electron flow and conventional current when your an EE studying Chemistry. Ben Franklin made a mistake, but we never went back and changed history. We just deal with it. When it's important we changed our view or our conceptions. In semiconductors we have the concept of holes and electrons.

    Another notion that caused me grief was expecting colors of wires to denote polarity. Red in the world of thermocouples is always negative. Black/Red would generally be +/- in the DC world. Black is Hot in the US for AC circuits and White is Neutral. Then you have black/white/green in power cords vs blue/brown and green with yellow stripes. This could be L1, N and ground or L1, L2 and ground depending on the application.
    I was at an Boy Scout explorer's post in 1972 at HP. I was the second person to complete the clock in groups of about 4 and the first working. In these groups of 4, we basically did PCB layout, troubleshooting and assembly in groups of 3 or 4. The component placement was done for us. We each got to keep our own clock. In that group, it came up as to whether the white or black went to the fuse. I challenged the HP employee who was wrong.

    Conventions are juts that, conventions. There are DC barrel power plus where the center is negative. Don't assume it's positive.
     
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  8. dannyf

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    Sep 13, 2015
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    They are like married couples: you cannot have one without the other.
     
  9. EM Fields

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  10. Sinus23

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    I know what you mean but can't potential difference be without current though?
     
  11. WBahn

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    Why the need for the big, bold font?

    So you can't have a voltage without a current? Tell that to a battery.

    So you can't have a current without a voltage? Tell that to a superconducting magnet in persistent mode.

    Now, if you consider 0 V to be "a voltage" and 0 A to be "a current", then the claim is vacuously true -- and just about as valuable.
     
  12. WBahn

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    That is NOT what Ohm's Law states. Look it up! You keep insisting that everyone must accept YOUR fantasy world concepts. I've told you repeatedly that you are free to believe whatever you want. But that's not good enough for you.

    Ohm's Law states that the voltage across a conductor is proportional to the current through the conductor. That is ALL Ohm's Law states. I am not making this up.

    https://en.wikipedia.org/wiki/Ohm's_law

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html

    https://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html

    http://www.thefreedictionary.com/Ohm's+law

    https://www.merriam-webster.com/dictionary/Ohm's law

    http://en-us.fluke.com/training/training-library/measurements/electricity/what-is-ohms-law.html

    https://micro.magnet.fsu.edu/electromag/java/ohmslaw/

    https://www.nde-ed.org/EducationResources/HighSchool/Electricity/ohmslaw.htm

    http://www.rapidtables.com/electric/ohms-law.htm

    http://www.electronics-tutorials.ws/dccircuits/dcp_2.html

    It would be trivial to continue finding resources that clearly state the same thing -- that Ohm's Law says that current is proportional to voltage.

    At the time it was empirically determined Ohm did not know that it did not apply in all situations for all materials. In fact, he had his hands full against the status-quo just for using empirical data to develop a mathematical description in the first place since, at the time, it was maintained that scientific investigation involved only theoretical reasoning and that physical measurements had no place in such pursuits.

    As for the example you asked for where Ohm's Law does not apply, I already gave you one. But let me be more explicit. Look at the data sheet for the 1N4148 diode:

    http://www.vishay.com/docs/81857/1n4148.pdf

    At a forward voltage of about 0.5 V the diode conducts about 0.1 mA of current. If it obeyed Ohm's Law, then at 1.0 V it should conduct about 0.2 mA of current. Anything in this ballpark and we could say that the device, in this region, is at least approximately ohmic. But the expected current at 1.0 V forward voltage is somewhere between 100 mA and 200 mA, to doubling the voltage results in an increase in current of more than three orders of magnitude. You might consider that to be in accordance with Ohm's Law, but good luck finding very many other people that do.
     
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  13. EM Fields

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    Of course it isn't. You claim to be a scientist, and yet you go apoplectic when anyone confronts you with incontrovertible evidence with which you disagree and can't counter except with rancor and bullying. Why should that kind of behavior not be rebuffed by anyone not afraid of you?

    You're right, and if that's all it states, the voltage across the conductor will vary as a function of the current through the conductor or the resistance of the conductor when the measurement is made.

    Indeed, but by virtue of your vituperative reprisals you seem to be unable to grasp the fundamental truth of Ohm's law, which is that the derived resistance of a material is only valid at the instant the voltage across, and the current through the sample, is measured.

    Total nonsense since, after a time, geocentricity was rejected as being valid by physical measurements proving the theory wrong.

    Just more nonsense, since Ohm's law can be used to determine the dynamic resistance of the diode if the voltage dropped across it and the current through it are known.
     
  14. BobTPH

    Senior Member

    Jun 5, 2013
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    Read this line from the Wiki article:

    I had this same argument in another forum, where people insisted that Ohms law applies to any device, including negative resistance devices, because you can always take the voltage divided by the current and come up with a number that you call the resistance. Sorry, but that interpretation of Ohm's Law makes it totally meaningless.

    Bob
     
  15. WBahn

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    Mar 31, 2012
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    You claim that I am the narcissist when it is YOU that demands that everyone must agree with YOUR views, even when you provide no evidence that your views are shared by anyone else at all.

    I provided you with not one, but TEN references that clearly indicate that YOUR view is rubbish. You can't provide a single one, but merely restate what you choose to believe in your fantasy world. I've suggested several times that you might actually look something up. You won't, because apparently you suspect that the real world and your fantasy world will collide.

    You clearly don't even understand the concept of dynamic resistance. For instance, the diode in that example starts out with 0.5 V across it at 0.1 mA through it. You claim that Ohm's Law can be used to determine the dynamic resistance of the diode if those two things are known. So, what is the dynamic resistance of that diode (using Ohm's Law and the known voltage and current)?

    Notice that I am NOT demanding that you accept my view, despite providing significant evidence that my view is shared by many. You are free to hold on to your views in your fantasy world.
     
  16. ScottWang

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    Aug 23, 2012
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    Please try to using quoteStart.gif and quoteEnd.gif to quote all the sentence which you want to reply, or move the cursor to the position where you want to place the reply sentence at the message box, and Anti-White all the sentence which you want to reply, and the system will show up two words as "quote" and "reply" and then you can click the "reply" item, these two words were not the buttons shows on the bottom of the message box, the sentence you wrote already modified completed.

    And be polite to our members, including mods, thank you.
     
  17. EM Fields

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    Last edited by a moderator: Dec 11, 2016
  18. ScottWang

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    @EM Fields.
    If you want to join the forum, please using the right way and it's a very common method, the method already told you, next time your reply could be deleted as useless post.
     
  19. MrChips

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    Oct 2, 2009
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    AAC is an educational website.

    The teaching and understanding of Ohm's Law is fundamental to all things electrical and electronic. It is paramount that the correct meaning of Ohm's Law be defined with accuracy and consistency.

    Ohm's Law states that the current through a resistor is directly proportional to the voltage applied across the resistor and inversely proportional to the resistance.

    The outcome of this proportionality is that a graph of current versus voltage is a straight line.
    Ohm's Law does not apply to non-linear devices.

    The thread starter has acknowledged the clarification provided to the original question.
    There is no further need to continue discussing this matter.

    This thread is now closed.
     
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