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Direct-current circuit/Kirchhoff's law problem

  1. Mar 11, 2013 #1
    1. The problem statement, all variables and given/known data
    "In the circuit shown in the figure below, ε1 = 27.0 V, R1 = 6.00 Ω, and R2 = 3.00 Ω."
    https://www.webassign.net/yf/27-29alt.gif

    (a) Find the current in resistor R.
    (b) Find the resistance R.
    (c) Find the unknown emf ε.
    (d) If the circuit is broken at point x, what is the current in resistor R?

    I need help with understanding part d, please!

    2. Relevant equations
    I = V/R


    3. The attempt at a solution
    I1 + I2 - I3 = 0

    ε - I2R2 - I1R2 = 0
    ε1 - ε + I1R1 - I3R3 = 0

    (a) Find the current in resistor R.
    2 A
    (b) Find the resistance R.
    4.5 Ω
    (c) Find the unknown emf ε.
    42 V
    (d) If the circuit is broken at point x, what is the current in resistor R?
    2 A (wrong)

    What am I doing wrong? Wouldn't the current in resistor R then be 9 V/4.5 Ω = 2 A?
     
  2. jcsd
  3. Mar 11, 2013 #2

    gneill

    User Avatar

    Staff: Mentor

    When you break the circuit at x it removes that branch from the circuit. It will no longer conduct current. This will change the distribution of currents and voltages in the remainder of the circuit.

    Assume that component values don't change, so the value of R that you found previously remains the same. Draw the "new" circuit accordingly and determine the current.
     
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