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Resistance circuit

  1. Sep 26, 2011 #1
    1. The problem statement, all variables and given/known data

    http://i1103.photobucket.com/albums/g480/Alfredo24PR/physicssucks.jpg" [Broken]

    2. Relevant equations


    3. The attempt at a solution

    I tried adding the resistance:

    1/8 + 1/ (6+5+10) = 1/8 + 1/21 = 0.1726

    1/0.1726 = 5.793 ohm
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 26, 2011 #2
    You could start out by determining the resistance of the whole circuit. R2 and R3 are in parallel. You can find a resistance for them. Together, they are in parallel with R1. You can determine this resistance. This is in series with R4. The result is the total resistance of the circuit.

    Keep in mind the sum of the currents passing through R1, R2, R3 must equal the current passing through R4. Be careful how you compute effective resistance for resistors in parallel and series.
  4. Sep 26, 2011 #3
    Ok, I figured out the middle ones and you explained the last one.

    How do I start with the first question: "What potential difference must be applied to the external terminals?"
  5. Sep 27, 2011 #4
    One way to go at it would be to guess a voltage over the whole system. This allows you to compute the voltage drop through R4. Subtract that from the applied guessed voltage and you get the voltage drop across the inner part of the circuit. Knowing that you can compute the current through the 'inside' resistors. Keep reguessing until you get 4 amps through R2.
  6. Sep 27, 2011 #5


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    Staff: Mentor

    Your formula doesn't look right. Note that while R1, R2, and R3 are in parallel, R4 is in series with that parallel group.

    Here's a big hint :smile:. If you're given the current through resistor R2 then you automatically know the voltage across it (Ohm's Law). What do you know about the voltage across parallel components?
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