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Thread: Misguided Phase Relation

  1. #1 Misguided Phase Relation 
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    Another of my little imponderables troubling me forever, I expected to add it on in A.C. Voltage Sources, but that thing has gone hog-wild.

    At DeVry Technical Institute, my instructor, Dick Revor, an outstandingly proficient source of knowledge, often declared, "The voltage across, and the current through, a pure resistance is ALWAYS IN PHASE with each other." I took his word for it.

    My problem: Given an inductor in series with that resistor, (and, to gum up the works even more, the resistance inherent in the inductor), provide fodder gleamingly enticing: IF the voltage and current regarding the resistance are in phase, THEN the current flowing through the resistance MUST be OUT OF PHASE with the current flowing through the inductor. Right? If so, the current level in various parts of that series circuit must NOT be the same, instantaneously, as was always claimed as Gospel for series circuits: "Current in a series circuit is everywhere equal." Anyone else have this understanding handicap, or is it just me? jocular


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    Current in the series circuit is equal everywhere. The voltage across the resistors will be proportional to the current, according to Ohm's law. The voltage across the inductor will be out of phase with the current. The voltage across the inductor will also be out of phase with the voltage across the resistor.


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    Out of phase by 90 degrees lag vector, 43 lead for c and is freq dependent.
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    Quote Originally Posted by Harold14370 View Post
    Current in the series circuit is equal everywhere. The voltage across the resistors will be proportional to the current, according to Ohm's law. The voltage across the inductor will be out of phase with the current. The voltage across the inductor will also be out of phase with the voltage across the resistor.
    Thanks! Still having a problem with this. Equal everywhere, yes, on average, maybe? Given the flow of current as it progresses back and forth through the circuit, at any given instant as it passes through the inductor, it there lags the build-up of voltage, but at that same instant it is present in full amount through the resistance, as the full voltage dropped across the resistance is present. I understand that the collapsing field is preventing instantaneous current flow to follow presence of voltage. Still cannot completely comprehend.

    At least, the lifelong voltage phase relationship between resistor and inductor is clearer! Thank you! joc
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    Let's try a mechanical analogy. Say you are pushing a mass back and forth on a frictionless surface like an air hockey table. In this analogy, the mass is analogous to inductance, force is analogous to voltage, and velocity is analogous to current. When you are pushing the weight back and forth, the highest force is at the ends of travel where you are accelerating the mass to change its direction. The lowest force is in the middle, where you are changing the force from forward to reverse acceleration. This is where the velocity is highest. So the force is 180 degrees out of phase with the velocity. This is like the current in an inductor. When you put a voltage on an inductor, nothing much happens right away. But when the current builds up, it's hard to stop.

    Now let's add a "resistor" to the system. This would be like putting the mass in a tray of molasses instead of gliding on the air hockey table. The force on the tray of molasses is just proportional to the velocity of the mass you are dragging through it. No delay involved.
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    The analogy I use is two wires with the created magnetic field interfering with the flow. Back and forth and back and forth on the parallel wires.
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    Magic Pixel. You have earned 3 days off for posting nonsense.
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    Quote Originally Posted by Harold14370 View Post
    Let's try a mechanical analogy. Say you are pushing a mass back and forth on a frictionless surface like an air hockey table. In this analogy, the mass is analogous to inductance, force is analogous to voltage, and velocity is analogous to current. When you are pushing the weight back and forth, the highest force is at the ends of travel where you are accelerating the mass to change its direction. The lowest force is in the middle, where you are changing the force from forward to reverse acceleration. This is where the velocity is highest. So the force is 180 degrees out of phase with the velocity. This is like the current in an inductor. When you put a voltage on an inductor, nothing much happens right away. But when the current builds up, it's hard to stop.

    Now let's add a "resistor" to the system. This would be like putting the mass in a tray of molasses instead of gliding on the air hockey table. The force on the tray of molasses is just proportional to the velocity of the mass you are dragging through it. No delay involved.
    I appreciate your efforts, but am trodding through muddy waters. Current in a pure inductor lags voltage applied by 90` though, not 180`. I have studied and worked with all the mathematical approaches, though long ago, even a foray into the calculus usage determining area under repetitive waveforms to predict power in a course entitled "Pulse Circuits".

    For the life of me, the physical understanding is elusive. Mental block, perhaps. Consider this question, if you will: Given a series R, L, C circuit being fed sinusoidal A.C. power by a voltage source having ONLY internal resistance, no C or L of it's own, if L & C are pure reactances, no R involved, I understand the current lagging voltage 90` as the voltage appears across the L, current leading voltage appearance across the C, both in phase across the R, now, is E & I in phase with respect to the voltage source? The textbooks gave little real-world physical interpretation, and we "swallowed" the reactive effects as they were presented. joc
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    Quote Originally Posted by jocular View Post
    For the life of me, the physical understanding is elusive. Mental block, perhaps. Consider this question, if you will: Given a series R, L, C circuit being fed sinusoidal A.C. power by a voltage source having ONLY internal resistance, no C or L of it's own, if L & C are pure reactances, no R involved, I understand the current lagging voltage 90` as the voltage appears across the L, current leading voltage appearance across the C, both in phase across the R, now, is E & I in phase with respect to the voltage source? The textbooks gave little real-world physical interpretation, and we "swallowed" the reactive effects as they were presented. joc
    If the E you are considering is the E of the voltage source, and if the inductive and capacitive reactance are equal and opposite then the phase angle is zero as shown in the impedance triangle here,
    Series RLC Circuit Analysis and RLC Series Circuits
    The voltage across the individual reactive components will be out of phase, though.
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  11. #10  
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by jocular View Post
    For the life of me, the physical understanding is elusive. Mental block, perhaps. Consider this question, if you will: Given a series R, L, C circuit being fed sinusoidal A.C. power by a voltage source having ONLY internal resistance, no C or L of it's own, if L & C are pure reactances, no R involved, I understand the current lagging voltage 90` as the voltage appears across the L, current leading voltage appearance across the C, both in phase across the R, now, is E & I in phase with respect to the voltage source? The textbooks gave little real-world physical interpretation, and we "swallowed" the reactive effects as they were presented. joc
    If the E you are considering is the E of the voltage source, and if the inductive and capacitive reactance are equal and opposite then the phase angle is zero as shown in the impedance triangle here,
    Series RLC Circuit Analysis and RLC Series Circuits
    The voltage across the individual reactive components will be out of phase, though.
    Thanks for the link! Yes, I meant are E source and I source in phase at the source, irrespective of reactive components elsewhere in the circuit, assuming the source contains only pure resistance? joc
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  12. #11  
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    Quote Originally Posted by jocular View Post
    Thanks for the link! Yes, I meant are E source and I source in phase at the source, irrespective of reactive components elsewhere in the circuit, assuming the source contains only pure resistance? joc
    Only in a purely resistive network is the voltage across any element is in phase with the current through that same element. The source is also an element. If the network contains reactances, voltages and currents need not be in phase any longer. A trivial example is a voltage source driving a capacitor. The capacitor's voltage will be the same as that of the source, and it lags the current through the capacitor. Since the current through the voltage source is the same as that through the capacitor, you should be able to figure out the rest. The upshot is that the current through the source need not be in phase with the voltage of the source, as that relationship is determined by the elements connected to the source, and not by the source itself.
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