1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Verifying ohm's law

  1. Mar 7, 2013 #1
    How was Ohm's law ever verified?
    To verify it you need to measure the current, voltage and resistance over a range of values and show that ohms law satisfies their solution. The catch is that typical ohmmeters do not measure resistance, they measure current and voltage and use ohm's law to find the resistance. If you use an ohmmeter to verify ohms law then you are assuming ohms law is true in order to show that it is true which is logically flawed.
  2. jcsd
  3. Mar 7, 2013 #2


    User Avatar
    Science Advisor
    Gold Member

    Firstly, Ohm's law isn't a law as such: it is a relation that is approximately applicable in certain situations.
    That said, if you make sure that a number of conditions are met is should still hold with extremely high accuracy, and it is in fact already used to calibrate the current standards.

    Resistance, voltage and current form the corners of what is known as the metrological triangle. This triangle has never actually been tested with metrological accuracy (say one part in 10^8), so you are partly correct (albeit not for the reason you mention)

    The reason is that we don't have a method for realizing the Ampere that is accurate enough. The ohm is realized using the quantum hall effect and the volt using the Josephson effect and these can be measured with very high accuracy. If we had a way of generating a known current that did not depend on neither knowning the voltage nor the resistance(but say a frequency), we could "close" the triangle because we would be able to use two realizations to test the third.

    Better current sources are becoming available, and the triangle should be closed within a few years,
  4. Mar 7, 2013 #3


    User Avatar

    Staff: Mentor

    The essence of Ohm's "law" is that the current through a resistor or other object is proportional to the voltage across it. We basically define R to be the proportionality constant. (Actually it's 1/R if we treat V as the independent variable and I as the dependent variable).

    So, simply measure various combinations of V and I, make a graph, and verify that it's linear.
  5. Mar 7, 2013 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    I would argue that Ohm's law is what defines an ideal resistor. There are no ideal resistors in the world, but it's still a useful approximation.
  6. Mar 7, 2013 #5
  7. Mar 7, 2013 #6


    User Avatar
    Science Advisor
    Gold Member

    True, but it would nevertheless be good if we were able to verify that the triangle is indeed true for the realizations of volt, ohm and Ampere. There is currently a LOT of effort going into making experiments of this type.

    That said, even if if the triangle turned out to be incorrect (unlikely, but possible) that would of course still not invalidate ohms law as such.
  8. Mar 7, 2013 #7
    To the Ineffable All,

    Are you aware what some very good text books say about Ohm's law? They say that the formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current, impedance, or voltage. Read what the physics books say about this.

    "We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--I curve is linear, that is, if R is independent of V and I. The relationship R = V/I remains as the general definition of the resistance of a conductor whether or not the conductor obeys Ohm's law. ..... Ohm's law is a specific property of certain materials and is not a general law of electromagnetism, for example like Gauss's law."
    The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

    And the following.
    "Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
    Materials that obey Ohm's law, and hence demonstrate this linear behavior are said to be ohmic. The electrical behavior of most materials is quite linear for very small changes in the current. Experimentally, one finds that not all materials have this property. Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is not a fundamental law of nature, but an emperical relationship valid only for certain materials."
    The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.



  9. Mar 8, 2013 #8
    I must point out that just because you can plot a straight line on a graph of voltage and current does not mean that the slope of the line is R. The slope could be 2R+5 or cos(R).
    The previous poster raises a good point that Ohm's law is a worded statement not originally stated in a formula.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook