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Two resistors and total resistance

  1. Feb 8, 2009 #1
    1. The problem statement, all variables and given/known data

    Two resistors, R1 and R2, have an equivalent resistance of 710 ohms when they are connected in series and an equivalent resistance of 200 ohms when they are connected in parallel.

    a) what are R1 and R2
    b) Determine the current traveling through the resistors when they are connected in series and parallel to a 12 v battery.

    2. Relevant equations

    V=IR

    3. The attempt at a solution

    R1 and R2 are different.

    In a series the equation is just: R1+R2+R3 and for a parallel it is 1/R1+ 1/R2

    R1 and R2 must add up to get 910 (total).

    I'm just not sure how.
     
  2. jcsd
  3. Feb 8, 2009 #2
    I am a little confused by the wording. What do you mean by "equivalent resistance"? Do you mean the total resistance or that they are equivalent (the same)? If it is the former, then why must they add up to 910 ohm? Don't they add up to 710 when connected in serial?
     
  4. Feb 8, 2009 #3
    Equivalent resistance here means total... I think. The question seems easy and yet I can't do it.
     
  5. Feb 8, 2009 #4
    I think the wording may be confusing you. This may help clear it up:

    The total resistance when the 2 resistors are connected in series is 710 ohms.
    The total resistance when the 2 resistors are connected in parallel is 200 ohms.


    With this information, how many independent equations can you write? How many unknowns do you have?

    Once you have solved for the resistances, you will be able to calculate the current(s) using Ohm's Law.
     
  6. Feb 8, 2009 #5
    Parallel is 1 / [ (1/R1) + (1/R2) ]

    "The reciprocal of the sum of the reciprocals."

    Another form that you can use when there are exactly two resistors is R1 R2 / (R1 + R2)

    "The product over the sum."
     
  7. Feb 8, 2009 #6
    I have never done a problem like this. In class we are given the total resistance or a drawing. I'm not sure what to do when I am given the total resistance and have to find the two resistors.

    For parallel: 150=1/r1+1/r2

    Would I get the same answer for the two resistors.
    To solve for 1/r1 I could plug 1 in for r2 and I end up getting::

    R1=1/149= .00667
     
  8. Feb 8, 2009 #7
    You have two unknown, R1 and R2, and you have two equations, one for the connection in series and one for the connection in parallel. With two unknowns and two equations, you can simply set up your system of equations and solve for R1 and R2.

    Can you write the two equations?
     
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