1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Voltage question

  1. Jul 16, 2015 #1
    1. The problem statement, all variables and given/known data
    In a circuit consisting of both parallel and series arrangement, how do you find the potential difference of X(in red) and Y(in blue)? and how did you derive at those answers?

    2. Relevant equations

    3. The attempt at a solution
    total resistance=4 ohms +(1/r eff=1/2ohms+1/4ohms)
    =5 1/3 ohms
    current=16 divide by 5 1/3ohms
    =3 ampere
    potential diff for X ------V=RI
    potential diff for Y-------V=RI
    = 3/4

    can someone help me please? please explain to me what went wrong thanks

    Attached Files:

  2. jcsd
  3. Jul 16, 2015 #2


    User Avatar
    Education Advisor
    Gold Member

    If 3 Amps is flowing through the battery, how much is flowing through each resistor in the circuit? Then what does V=RI tell you for each resistor?

    If you know the potential drop across the "other" resistor, then what does that tell you about the potential drop across the combination of X and Y? What total do you have to make?
  4. Jul 16, 2015 #3
    according to i1=i2=i3, 3 amps flowing through batt means 3 amps flowing through the resistors. I=v/R means 3=v/ 1/2 for resistor x and 3= v/ 1/4 for resistor y

    potential drop in other resistor= R/I
    i know of the V1=V2=V3 rule but why is 4/3 volts= potential diff of both X and Y?
  5. Jul 16, 2015 #4


    User Avatar
    Homework Helper

    Electric current behaves much like water flowing in a river.
    That means that part of the 3 A flows through X and the other part through Y,
    but not necessarily the same amount through each. Less will go through Y and
    more through X, but their sum will be 3A. Since V = IR I do not understand why
    you are inverting the resistance?
  6. Jul 16, 2015 #5


    User Avatar
    Gold Member

    I'm not sure where you are getting those current and voltage relationships. Purely resistive circuits can have currents and voltages that equal each other, yes, but only under certain element configurations. You should review the conditions under which such relationships are valid. Now, on to your problem:

    You need to find total current. You were on the right track with using Ohm's Law and the equivalent resistance of the circuit, but you should not be inverting the resistances.

    You can then use current division to solve for the current in the resistors.

    Here is the general form for current division:

    Ix = Ry*ITotal / (Rx + Ry), where ITotal is the current you've already found, and Ix is the current through resistor x.

    After you have found the current through each resistor, you can use Ohm's Law to find the voltage drop across each.
    Last edited: Jul 16, 2015
  7. Jul 17, 2015 #6


    User Avatar
    Homework Helper

    A parallel combination of resistors, R1, R2, R3... can be replaced by a single resistor RP
    where RP should be

    1/RP = 1/R1 + 1/R2 + 1/R3 ....

    this means that a resistor R1 in a parallel circuit can be replaced by a resistor RP where

    1/RP = 1/R1

    so that RP= R1 and not

    RP = 1/R1
  8. Jul 18, 2015 #7


    User Avatar
    Gold Member

    In this case, since there are only two resistors in parallel, one can use the following relationship:

    REquiv = Rx*Ry / (Rx + Ry)

    But yes, when one wants to combine more than two parallel resistances, the methods you've described is required.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - Voltage question Date
Question about Full scale deflection Dec 5, 2017