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Homework Help: Introductory circuit problems

  1. Sep 7, 2010 #1
    1. The problem statement, all variables and given/known data

    i have a simple circuit with 115 V and a resistance of 10 ohms. i have to find :

    a) load current for the circuit
    b)current through circuit if the resistance becomes open circuited
    c)voltage across circuit if the resistance becomes open circuited
    d)current through circuit if the resistance becomes short circuited
    e)voltage across circuit if the resistance becomes short circuited
    f)voltage across resistor

    2. Relevant equations

    ohms law: I=V/R

    3. The attempt at a solution

    a) I am not sure
    b)since its open circuit, there would be no current
    c)no voltage because there is no current
    d)not sure
    e)not sure
    f)11.5 volts
  2. jcsd
  3. Sep 7, 2010 #2


    User Avatar

    Staff: Mentor

    Is the circuit in the problem the voltage source and its source resistance (the 11.5 Ohms)?

    Your answer for b) is correct, but c) is not. You do not need a current to have a voltage. An open circuit has infinite resistance, so no current flows even if you have a voltage across the open circuit.
  4. Sep 7, 2010 #3
    a: have you tried using the equation you posted?
    b: correct
    c: no. As an easily accepted counterexample, consider a battery of 1.5V not hooked up to anything. Clearly, as it is not hooked up to anything, zero current flows through it. However, 1.5V still exists across its terminals. In a similar fashion, a 115V voltage source is connected to the "terminals" of the "resistor of infinite resistance" aka the open circuit. How much voltage is across that resistor's terminals?
    d: have you tried using the equation you posted (taking the limit as R -> 0)?
    e: by the definition of an ideal wire, it has the same voltage at every point on the wire that is on the same node. So V_a - V_a = ?
    f: 11.5 or 115?
  5. Sep 7, 2010 #4
    Heres a crude paint drawing of the circuit in question:

    Attached Files:

  6. Sep 7, 2010 #5
    Thank you for all your help. i figured the problem out
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