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!

B Is there a relationship between voltage and resistance?

  1. Jun 10, 2018 #1
    I was getting a bit confused with Ohm's law, and the relationship between the current, resistance and voltage/potential difference?
    From my understanding, current and voltage both increase or decrease together, while current and resistance do the opposite?
    I was then wondering if there was any relationship between the voltage and resistance, or did I get everything wrong?
    Thanks!
     
  2. jcsd
  3. Jun 10, 2018 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    there is: it's called Ohms law. We write V = I R as if it were a 'law'. In reality it is a (generally very good) first approximation.

    For a given resistance, if you double the voltage, the current doubles too.
    For a given resistance, if you double the current, the voltage drop over the resistance doubles too.

    For a given voltage, if you double the resistance, the current halves.
    For a given voltage, if the current doubles, the resistance has halved.

    For a given current, if you double the resistance, the voltage doubles too.
    For a given current, if the voltage doubles, the resistance must have doubled.

    [edit] nice of you to like it, but I still must iron out some mistakions...
     
  4. Jun 10, 2018 #3

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    when the resistance is constant, yes.
    Yes: when the voltage is constant, the product of current and resistance is constant too.
     
  5. Jun 10, 2018 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    etc. etc.
    All this is just effectively re-arranging a mathematical equation. "Ohm's Law" is not so much a "Law" of Nature (as with Newton's Laws of Motion, which apply usefully in all cases until the conditions get extreme). We can measure the V and I of a component (something in a black box) and assign a value for R but there's absolutely no reason to assume that the R value will stay the same for another value of supply V. That would have to assume that the component actually followed Ohm's Law. We usually assume that a "resistor" out of the drawer will have a constant R (given by the coloured bands along the side) but it is only an assumption.
    A Light Bulb Filament is Metal (Tungsten) and would follow Ohm's Law if you could find a way to keep the filament at a constant temperature. But it can get hot (by design, of course) and its Resistance will vary by a factor of about Ten between cold (off) and hot (full volts applied). Using the term Ohm's Law is, to my mind, just confusing but it is perfectly valid to measure the Volts and Current and to give it an instantaneous value of Resistance. Light bulbs and many other components are referred to as non-Ohmic conductors but they can all be assigned an instantaneous value of R, according to the volts applied.

    Despite what is written in many text books and even in Wikipedia, I strongly feel that the relationship R=V/I and Ohm's Law should be treated as two distinct ideas.
    Do we have a "Speed Law" to describe Distance / time = speed? No; that's just a relationship between two variables.
     
  6. Jun 10, 2018 #5

    Nugatory

    User Avatar

    Staff: Mentor

    Any two of them determine the third.
     
  7. Jun 10, 2018 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    That's the Maths of the situation. The Resistance is not readily changeable for the same physical entity, though. Changing the resistance would actually involve changing, say, the length of a rheostat wire so it is no longer the same entity.
     
  8. Jun 11, 2018 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    So, Cheese2, did this help or did it just leave more nagging doubts ?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted