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Current change

  1. Mar 25, 2012 #1
    Question b. in paint doc.

    My answer is: Becuase they said the current in question a. through the 6.0 ohm resistor is 1.0 A im going to say the only two that will change is resistor 7.0 Ω and the newly replaced 3.0 Ω resistor... Im saying the current through the 6.0 ohm resistor will not change, but the solutions is saying it will... why? How does the different resistor (in place of the 3.0 ohm resistor) in parallel with the 6.0 ohm resistor have any effect on changing the current that banches off at the banch point???
     

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  3. Mar 25, 2012 #2

    gneill

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    Staff: Mentor

    The branch consists of the 3 and 6 Ohm resistors in parallel which are in turn in series with the 7 Ohm resistor. That 7 Ohm resistor is critical. What happens if the current through the 7 Ohm resistor changes?
     
  4. Mar 25, 2012 #3
    Well if the current through the 7 ohm resistor changes from say 7 to 5 amps then the current through the 6 and 3 ohm resistors would not longer be 1.0 A + 6.0 A from each resistor respectfully, it would then be 1.0 A + 4.0A through the 6 and 3 ohm resistor. That is how I am thnking of it.
     
  5. Mar 25, 2012 #4

    gneill

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    Staff: Mentor

    No. If the 3 Ohm resistor is changed, then the current through the 6 Ohm resistor will change.

    If you are told that for the original configuration the current through the 6 Ohm resistor is 1.0 A, then you should be able to work out the current that flows through the 7 Ohm resistor for that configuration (as well as the current through the 3 Ohm resistor, the potential across the 3 and 6 ohm resistors, the potential across the 7 Ohm resistor, and the battery potential).

    If the 3 Ohm resistor is changed to another value, then the only thing you can count on not changing is the values of the other components and the battery potential. You'll have to find the new current values and voltage drops accordingly and see what has changed.
     
  6. Mar 26, 2012 #5

    NascentOxygen

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    Ah! But if the current through the 7Ω changes, then (by Ohms Law) the voltage across that 7Ω will likewise change. As you can see from the circuit, the voltage across the 6Ω (and also the 3Ω) is determined as the battery voltage minus the voltage drop across the 7Ω.

    So if you are correct in saying the current through the 7Ω resistor will change, then you must at the same time acknowledge that the voltage across the 6Ω will also change. And it follows that, by Ohms Law, if voltage across the 6Ω changes so must current through it.

    You can't mess with logic. :smile:
     
  7. Mar 26, 2012 #6
    If 3Ω replaced means new voltage across it.
    Across 6Ω voltage changes accordingly. Which means different value of current.
     
    Last edited: Mar 26, 2012
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