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!

Connecting parallel resistors

  1. Sep 13, 2013 #1
    okay i have this somekind of a paradox
    in the circuit below
    voltage around R0 and R2 is supposed to be equal
    now if it is equal , wouldn't that mean that the current will not pass through the middle vertical wire ?
    and if it doesn't then this means that the resistors are now connected in parallel such that the above two are connected in series so are the two below , and thus i will not be able to say that they have the same voltage
    while if we look at it from another point
    such that the current does flow through the wire in the middle , thus the voltage around R2 and R0 is not equal * hence the current passed through the wire * then i can safely say that R2 and R0 are connected in parallel and therefore they have the same voltage across them ?
    please please help me with it , even my teacher could not explain it to me .

    please note that this is not a homework question , i have posted the homework part of this question in the homework forum , but in this case i have difficulty grasping the physical idea of connecting parallel resistors

    Attached Files:

  2. jcsd
  3. Sep 13, 2013 #2


    User Avatar

    Staff: Mentor

    The connections between the resistors are equivalent to connecting the output of R0 and R2 together, then running a single line to the inputs of R1 and R3. Try drawing it that way and it will be clearer.

    That little vertical wire will not carry current if there is no voltage difference, but it can and will carry current if a voltage difference were to start to form; this flow will be just enough to level out the voltage difference and ensure that the voltage at both of its ends will be the same.

    It's like connecting two water tanks with a pipe so they'll both maintain the same water level.
  4. Sep 13, 2013 #3
    if there was a voltage difference
    say R2 has more potential than R0 so electricity flows from R2 to R0 , i can consider the wire * if it has a resistance * to be connected in series with R0 so it consumes the extra potential leveling out the potential right ?
  5. Sep 13, 2013 #4
    According to Ohm's law current doesn't need a potential difference to flow in a zero resistance path.Hence current does flow in the middle wire.
  6. Sep 13, 2013 #5
    oh then screw my physics teacher -_- ...
    it would have been better if he just said * i dont know * -_-
  7. Sep 13, 2013 #6

    Whether the current would flow or not in the middle wire depends on the resistors R0,R1,R2,R3 .

    If R2/R0 =R3/R1 then current doesn't flow ,else current flows .

    Now whether current flows from top to bottom in the middle wire or bottom to top depends on the resistors.

    Does this help ?
  8. Sep 13, 2013 #7
    yes tanya this is very helpful , i have came to this conclusion 3 minutes earlier :p
    any way , what if there is a resistor in the wore , and r2/r0 =/ r3/r1 .
    how do i calculate the current going through the resistor in the wire ?
  9. Sep 13, 2013 #8
    Well...then there is no current in the middle resistor .
  10. Sep 13, 2013 #9


    User Avatar

    Staff: Mentor

    If the wire has a resistance, then you have to draw it as a resistor - and of course there can be a voltage drop across a resistor.

    If you haven't tried redrawing the diagram as I suggested above, do so. It will be a lot clearer (and if you don't see why that rearrangement is legitimate at first glance, figuring out why it is will steer you in the right direction).
  11. Sep 13, 2013 #10
    in this case we treat the the upper resistors as if they were series * so do the lower two * and thus the whole thing is a parallel circuit right ? * just as if we removed the middle line *
  12. Sep 13, 2013 #11
    Yes...you are right.
    Last edited: Sep 13, 2013
  13. Sep 13, 2013 #12
    That is exactly correct.It is a classical example of a balanced wheatstone bridge.
    Last edited: Sep 13, 2013
  14. Sep 13, 2013 #13
    i drew it the way you said
    i think now since it has resistance , the first two resistors are not connected in parallel anymore Alone , the whole circuit is now parallel . not just the first two and the second two
    and the middle wire is useless now
    or i could be wrong and if i am
    then if R2/R0 = R1/R3 then there will be no current going through the resistor
    if R1>R3 , the current will flow more through R3 , thus it will flow R0 to R3 ,
    so we consider R0 and Rmiddle to be in series and both of them connected in parallel to R2 .
  15. Sep 13, 2013 #14
    B4ssHunter...If the middle line has a resistor and R2/R0 ≠ R1/R3 then some current flows through the middle resistor.In that case the resistors are neither in parallel nor in series .

    But if R2/R0 = R1/R3 ,then the observation you made in post#10 is true .
  16. Sep 13, 2013 #15
    then what are they ?
  17. Sep 13, 2013 #16
    It is not necessary for a group of resistors to be classified in either parallel or series .
  18. Sep 13, 2013 #17
    A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take. The current is the same through each resistor.

    A parallel circuit is a circuit in which the resistors are arranged with their heads connected together, and their tails connected together. The current in a parallel circuit breaks up, with some flowing along each parallel branch and re-combining when the branches meet again. The voltage across each resistor in parallel is the same.
  19. Sep 13, 2013 #18
    Interesting observation in case R2/R0 = R3/R1

    1) (R0 || R2 ) IN SERIES WITH (R1 || R3)

  20. Sep 13, 2013 #19
    okay now i understand the situation when there is no resistor in the in the middle wire
    if R0 = 2 ohms R2= 3 ohms R1 = 8 ohms and R3 = 4 ohms and the middle wire has a resistance of 1 ohm
    can somebody calculate the total equivalent resistance ? just to know how i would calculate the middle resistor
  21. Sep 13, 2013 #20
    You can use kirchhoff's laws to calculate equivalent resistance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook