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Homework Help: Load voltage with shorted load / open load

  1. Aug 23, 2011 #1
    1. The problem statement, all variables and given/known data

    1. R = 470 ohms. Connect a jumper wire between points A and B. Measure the voltage across the load resistor. Explain why the load voltage with a shorted load is zero using Ohm's law.

    2. Remove the jumper wire and open the load resistor. Measure the voltage between the AB terminals. Why is the load voltage with an open load approximately equal to the source voltage. Use ohm's law and kirchhoff's voltage law to explain.

    2. Relevant equations

    ***Voltage source = 10V
    3. The attempt at a solution

    1. I am confused how to explain this. Would it simply be because the wire has no resistance it is taking all the current up? V=IR , V=0*r V= 0

    2. I'm lost on this one. Opening the load resistor would just make it a 10V battery in series with a 470 resistor. That doesn't exactly make it near the source voltage?

    edit: still lost :( not sure what else to add, noticed a lot of views
    Last edited: Aug 23, 2011
  2. jcsd
  3. Aug 23, 2011 #2


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    I'd say your answer to 1) is correct. There's no resistance between A and B, so they must be at the same potential, by Ohm's law. For 2), now there is no current flow. So the potential at A and B must be the same as the potential at the corresponding end of the battery by the same argument, right?
    Last edited: Aug 23, 2011
  4. Aug 23, 2011 #3


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    I think the question would make more sense if it read "Measure the voltage across the points A and B", since the load resistor is detached from the circuit.

    Use Ohm's law to calculate the voltage across R when no current is flowing.
    Then use Kirchhoff's voltage law to determine the voltage across A and B.
  5. Aug 23, 2011 #4
    I edited it to the words the book used D: my bad

    Ohms law: V=IR , V=0*470 = 0V across R...?

    The voltage law applies to a closed circuit :/ Not sure how I would get the sums of the voltages to come out to 0.
  6. Aug 23, 2011 #5


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    I would use Ohm's law on the second part as well. See post 2.
  7. Aug 23, 2011 #6
    Not seeing how you can make the same argument

    On the 1st I understand it as:
    The wire has no resistance so all the current went through it. The load resistor got no current so, V = 0 * R , the V = 0

    But on the 2nd question:
    No current at all anywhere, V = 0*R = 0
    Yet I need to show that it equals the source voltage, 10V.
  8. Aug 23, 2011 #7


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    V is a potential difference between two points. Can you show the potential difference between point A and the positive terminal of your battery is zero? And that the potential difference between B and the negative terminal is also zero? Wouldn't that do it? Kirchoff's law is what tells you there is no current. If there is no loop, there is no current.
    Last edited: Aug 23, 2011
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