Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Making the sum of 2 resistors independent of 1 of them

  1. May 23, 2015 #1
    I am really not sure about this ......
    I may be generalizing it ...but anyway...

    suppose we have two resistors in series...connected by ofcourse a voltage source...and a certain amount of total current is flowing through the whole circuit..

    now if we want to make the total current totally independent of the value of one resistance then do we have to make the value of other resistor very high...???

    independent here means...no matter what is the value of that resistance.. it does not effect the total current
  2. jcsd
  3. May 23, 2015 #2


    User Avatar
    Gold Member

    The current is entirely dependent on the sum of the two resistors so the only way to to what you are asking (assuming I understand what you are asking, which I'm not sure about since what you are asking really doesn't make sense) is to devise a double-potentiometer with a single control mechanism so that as one of the two resistances goes up, the other goes down by an equal amount so as to keep the total resistance the same.
  4. May 23, 2015 #3


    User Avatar

    Compare the current through a 1-Mega-Ohm resitor with the current drawn by a series combination of 1 Megohm and 1 Milliohm.
  5. May 23, 2015 #4
    with 1megaohm individual resistor ...current is 2*10^-5A...whereas with series combination it is 1.999999998*10^-5A....

    but what is the conclusion in reference to the problem...?
  6. May 23, 2015 #5


    User Avatar
    Gold Member

    I believe I see what he is getting at, but I think my response more directly addresses your question.
  7. May 23, 2015 #6
    can you please conclude...the problem....i am really stuck...
  8. May 23, 2015 #7


    User Avatar
    Gold Member

    I think I answered it completely already. Did you not understand my first post?
  9. May 23, 2015 #8
    but in your first post current still seem to be dependent on both resistances....i am asking thst would it be possible to make current independent to any one resistance just by increasing the other resistance to a higher value....
  10. May 23, 2015 #9


    User Avatar
    Gold Member

    And my solution shows clearly that the answer to that question is a resounding NO. You really need to study what I said.
  11. May 23, 2015 #10
    OK...then what logic LvW is talking about
  12. May 23, 2015 #11


    User Avatar
    Gold Member

    He is attempting, I believe, somewhat indirectly, go get you to arrive at the same conclusion I presented you with.
  13. May 26, 2015 #12


    User Avatar
    Gold Member

    Yes, it is easy. Just make one of the resistors infinite.

    We are doomed.
  14. May 26, 2015 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    That's the trouble with calculators versus slide rules.

    Mother Nature is analog.

    If you can discern the difference between those two currents with any real measuring device ,,,,

    So -
    As an engineer , to how many significant digits do you wish to push the question ?
    As a mathematician, how would you describe the behavior of the limit as one of the individual resistors approaches infinity ?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Making the sum of 2 resistors independent of 1 of them
  1. 1 resistor (Replies: 2)

  2. 2 in 1 Book (Replies: 4)