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Homework Help: Electric circuit, reversing current

  1. Feb 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Lets say that I have an electric circuit with an amp and volt meter attached to it and i reverse the direction of the current from the power supply.. what will happen to the reading of both the volt meter and the amp meter?

    2. Relevant equations

    3. The attempt at a solution

    My guess is that the amp meter have the opposite sign of the original one and the volt meter will read the same as the normal one... correct me if I am wrong
  2. jcsd
  3. Feb 21, 2009 #2
    First of all, the meters must be connected the right way (i.e. the red cable to + -terminal and black to the - -terminal) otherwise it won't show the correct values. But if there was a meter that worked no matter how you connect it, in your case what would the resistance be? Not the value, but the sign?
  4. Feb 21, 2009 #3
    it would be negative??
  5. Feb 21, 2009 #4
    Exactly. Are there resistors that can be negative? If you put negative resistance to Ohm's law V=R*I, you get -V. What does that mean in the circuit and is that effect possible?
    Last edited: Feb 21, 2009
  6. Feb 21, 2009 #5
    is there such thing as a negative resistance?
  7. Feb 21, 2009 #6
    That's the million dollar question! If there were, wouldn't it raise the voltage in the circuit? Is it possible to raise voltage without energy?
  8. Feb 21, 2009 #7
    I guess there isn't a way to raise voltage without energy
  9. Feb 21, 2009 #8
    That's right. So to return to your original question, what the amp and volt meter read? Would they be as you originally thought or would they be altered some way?
  10. Feb 21, 2009 #9
    I guess the amp meter would read negative and as well as the volt meter
  11. Feb 21, 2009 #10
    What does the sign in front of the amp value indicate? Is it just the magnitude of the value or could it be something else?
  12. Feb 21, 2009 #11
    the sign indicates the direction of the flow of the current.. so if it were reading positive the first time and negative when we reverse it, means that the current if flowing in the opposite direction now
  13. Feb 21, 2009 #12
    Amp meters always read the current so that, what goes in through their "input" and goes out through "output" is positive. After we reversed the current, the meter would have its "input" where the "output" was, because of the current's flow. So if it read negative values, what would that mean?
  14. Feb 21, 2009 #13
    as I said the current is flowing in the opposite direction
  15. Feb 21, 2009 #14
    No. Eventhough we don't physically alter the amp meter's position, it would reverse its output and input. And that way the negative reading would indicate the current is flowing in the same direction as we started. So: the readings would stay the same, both still positive.

    But after all this complicated and perhaps confusing discussion, you should always remember that the meters work only if they are correctly connected and that there aren't meters that would work as we assumed.
  16. Feb 21, 2009 #15
    yes that is correct... I am pretty sure that it's connected accordingly...
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