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Homework Problem Stumping Me

  1. Dec 16, 2006 #1
    RLC Circuit Homework Problem

    Thia last HW problem is stumping me.

    This is my first year of physics class, and I am working on some homework problems tonight. It's the last m/c question out of 30 that I am doing tonight, and I'm having a bit of trouble figuring this one out:

    Question:
    When the instantaneous voltage and current in a RLC circuit are in phase...

    Answer Choices are as follows:
    A. Inductive Reactance is ZERO.
    B. Capacitive Reactance is ZERO.
    C. Total Reactance is ZERO.
    D. Resistance is ZERO.

    If someone can assist me with this last HW problem, I'd really appredciate it. Thanks! I tried o post this in the Introductory Physics Homework section, but could only start a new topic here in the advanced section.

    HookedOnPhysics
     
    Last edited: Dec 16, 2006
  2. jcsd
  3. Dec 16, 2006 #2
    Still can't figure this one out. Does the term being "in phase" mean the voltage and current are equal?

    I'll check back later.

    HookedOnPhysics
     
  4. Dec 16, 2006 #3

    Gokul43201

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    No, it means that the phase difference between the current and voltage is zero. Note that it's meaningless to say something like "voltage and current are equal". That's comparing apples and oranges.

    If you do not know what the terms 'phase' and 'phase difference' mean in the context of AC circuits, you need to take a step back and hit the textbooks! Have you been taught what a phasor diagram is?
     
    Last edited: Dec 16, 2006
  5. Dec 17, 2006 #4
    Hi Goku,
    I'm in HS and our teacher is starting to tell us about that. I'm trying to work ahead in the chapters and do some extra problems, and was just having trouble on this particular one.

    My bad, I believe our teacher said that being "in phase" was referring to, in essence, the voltage and current being mirrors of each other, so that would mean the difference between the two is zero. I guess I read my notes wrong and assumed it meant they were equal.

    Anyways, I guess this problem is stumping a lot of people on here. If anyone knows the correct answer, please let me know. I will keep trying to find the correct answer too.

    Thanks.

    HookedOnPhysics
     
  6. Dec 17, 2006 #5
    Try to explain in your own words what phase means.
    Check out this,
    http://en.wikipedia.org/wiki/Phase_(waves)

    read specifically the "Phase Shift" part.

    Also, as a hint, what causes a phase shift? Does a resistor? Does a capacitor? Does an inductor? What about both?
     
  7. Dec 17, 2006 #6
    Thanks for the help.

    I'm thinking the answer to this HW problem is "Total reactance is ZERO"

    Is that correct?

    The reason I'm thinking this is that if Total Reactance is ZERO in this RLC Circuit, then the circuit would behave as if it was a pure resistor, which would allow the voltage and current to be in phase.

    That right?

    HookedOnPhysics
     
  8. Dec 17, 2006 #7

    OlderDan

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    Homework Helper

    That statement is still comparing apples and oranges. You cannot subtract volts from amperes or vice versa and get zero.

    A better way to think of it is to compare dimensionless ratios. At some moment in time the current has a maximum value I_max and at some moment in time the voltage has a maximum value V_max. If the two are in phase, then

    I(t)/I_max = V(t)/V_max

    at all times t, and in that case I_max and V_max occur at the same time. You can also express this relationship as

    V(t)/I(t) = V_max/I_max

    which says that the ratio of voltage to current is the same at all times.

    Good thoughts.:smile:
     
    Last edited: Dec 17, 2006
  9. Dec 17, 2006 #8

    Gokul43201

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    This is correct. But it's VERY important to correctly learn the meaning of 'phase' of an AC signal, so don't skip past it because you've found the answer to this question.
     
  10. Dec 17, 2006 #9
    HookedOnPhysics: Please, please, please listen to this advice. The sooner you can understand this the better!
     
  11. Dec 17, 2006 #10
    Will do. Thanks a lot for the help and advice!! Now I know what I'll be studying over Christnmas vacation :)

    HookedOnPhysics
     
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