resistors in parrallel

Discussion in 'Homework Help' started by boobear, Jul 1, 2005.

  1. boobear

    Thread Starter New Member

    Jul 1, 2005
    I'm struggling with the following problem.

    I need to find the value of a resistor (R1) that is in parallel with a 60 ohm resistor (R2), the equivaltent value of both resistors is 15ohms

    I have used a online calculator to find the value of R1 to be 20 ohms, but i would like to know the formula to solve the problem.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Calculating the effective resistance of parallel resistors is an important basic skill in circuit analysis. Since you posted this question in the "homework" forum, I have to imagine that you have access to a textbook that contains the formula. If you don't have access to a textbook you should be able to look around the Internet for the formula.

  3. n9xv

    Senior Member

    Jan 18, 2005
    I love this kinda stuff!

    There is a couple of ways to write this formula so as to make it logical. I like to put it this way;

    To find the "unknown" parallel resistance, one known value in parallel with another unknown value producing a known equivalent resistance.

    Unknown value of parallel resistance = Unknown R

    Value of existing resistance = Existing R

    Equivalent total esistance = Equivalent R

    (Existing R * Equivalent R) / (Existing R - Equivalent R)


    60 * 15 = 900

    60 - 15 = 45

    900 / 45 = 20

    To be sure of your result, work again with the normal parallel formula and make sure the equivalent resistance is always < the value of the smallest resistance.