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Homework Help: Current phase angle

  1. Nov 21, 2007 #1
    1. The problem statement, all variables and given/known data

    We are working with phasors and R-L-C circuits. Here is some given data:

    Vsource = 120 Volts, 60 Hz
    Resistor = 100[tex]\Omega[/tex]
    Capacitor = 50[tex]\mu[/tex]F
    Inductor = 0.2 H
    Inductor Resistance = 10[tex]\Omega[/tex]

    My question is this, when solving for current using V = IZ, can you set your current phase angle to 0 degrees?

    As there was no angle associated with the source, I did I = (120V [tex]\angle[/tex]0 degrees)/(112.2[tex]\Omega[/tex][tex]\angle[/tex]11.49 degrees)

    You get 1.07 A[tex]\angle[/tex]-11.49 degrees

    My instructor is saying we can set the angle of the current to zero because it is the only common thing in the circuit. Therefore the angle on the source is 11.49 Degrees, Not -11.49.

    Can you do this? He doesn't really elaborate on the reasoning behind this. He just says they are all the same, I just need alittle more explenation is all.

    2. Relevant equations

    Using V = IZ

    Solving for I, I = V / Z

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 29, 2007 #2
    Think about it in real life. The only thing that matters when you're measuing a circuit is the phase difference between the current and voltage; really they have no phase, by themselves.
  4. Nov 29, 2007 #3

    The Electrician

    User Avatar
    Gold Member

    I think it's helpful to realize that what are called two-terminal
    circuit elements (R, L, and C are the fundamental components)
    *enforce* a relationship between voltage and current. The voltage
    *across* a component and the current *through* it are not independent.
    It is what two-terminal components do; they establish a relationship
    between voltage and current, *for that component only*.

    So, if some two-terminal circuit elements are in series, the
    current in each of them *must* be identical. If they are in parallel,
    the voltage seen (applied across) by each of them *must* be identical.

    Thus, if they are in series, the currents must be the same and
    *only* the voltages across each can be different. If they are in
    parallel, only the *currents* in each can be different; the voltage
    seen by each is the same.

    For components in series, since the current in all of them is the
    same, it makes sense to use current as a reference, and speak of the
    phase of the voltages *across* (not to ground) each component with
    respect to the current through all of them.

    For components in parallel, it is appropriate to use the voltage
    *across* them as the reference since this is the same for all of them,
    and speak of the phase of the current
    in each with respect to the voltage across all of them.
  5. Dec 2, 2007 #4
    You can do it either way. What you have assumed is the more common approach that I have seen. When you take advanced classes ofPower transmission, you will often start our with V angle 0 as the current is generally unknown in power transmission and thats what you have to find out.

    Since you cannot 100% say that the angle of either voltage or current is exaclt x degrees, you only have the option to find out which is lagging/leading the other. If V lags I then your answer for current will be x degrees greater than the angle for voltage and vice versa (with this idea clear, you can start off with even V angle -23.1254987 degrees. I hope you get my point.)
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