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Homework Help: Electricity-Resistance Question

  1. Mar 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A Student needs to construct a circuit in which there is a voltage drop of 5.0v across a resistance combination, and a total current of 2.0mA flowing through the combination. She has a 4000 Ohm ( 4.0kOhm) resistor which she wants to use and proposes to add another resistor in parallel with it.

    2. Relevant equations

    a) What value should she use for the second resistor

    b) Determine the effective resistance of the combination

    3. The attempt at a solution

    I have no idea how to work this out. The answers are

    a) 6.7 kOhms

    b) 2.5 kOhms

    I just need to know how to get to that answer
  2. jcsd
  3. Mar 17, 2010 #2
    Voltage drop across the "equivalent" resistance, meaning the resistance shown by the 2 resistances in parallel, equals 5V.

    V = I*R => R = V/I

    R = 5V/2mA = 2,500 Ohms

    Now this R is the equivalent resistance R_eq, which can be found by:

    1/R_eq = 1/R1 + 1/R2

    or after rearranging: R_eq = R1*R2/(R1+R2) , where you know one of the resistors (R1 = 4 kOhm).
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