Ham Radio: Impedance-3a: Reactance Physics

[last updated: 2021-04-11]
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• So what's going on when an inductor or a capacitor creates Reactance?

  
       The essence of Reactance: Voltage and Current are out-of-phase with each other.

• The shift can happen in either direction: Capacitors shift the current to lead the voltage waveform. Inductors shift the current to lag the voltage.
• By convention, phase angle is measured from voltage to current. So if voltage leads (inductors), the phase angle is positive. If voltage lags (capacitors), the phase angle is negative.
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• For capacitors:
  
  
  

  The current leads the voltage - it rushes in first, and the voltage builds later.
  The voltage lags the current.
  They are out of phase by -90deg.

  The physics behind it may help your understanding (or not, LOL):
  • Recall that the simple structure of a capacitor is two parallel conducting plates, separated by a very thin insulating layer.
  • When voltage is first applied to a capacitor, an inrush of electrons (current) collect on the negative plate of the capacitor,
    balanced by an equal number of positive charges attracted to ('opposite' charges attract each other)
    and collected on the positive plate of the capacitor.
  • As the electrons continue to collect however, the additional electrons are slightly repelled (delayed) by the electrons already there, ('like' charges repel each other),
    with the result that there is a delay in building a full charge of voltage on the capacitor.
  • This creates a phase shift between the voltage and current waveforms.
    The current leads the voltage - it rushes in first, and the voltage builds later.

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• For inductors:
  The voltage leads the current - the voltage first builds the magnetic field, which delays the current
  The current lags the voltage
  They are out of phase by +90deg.
  The physics behind it
  • Recall that the simple structure of an inductor is a coil of wire.
  • When the first current comes into the coil, it starts building a magnetic field around it.
    The magnetic field starts from zero, and the instant current starts to flow, the field builds.
  • This constitutes a "changing magnetic field".
    And magnetic field theory tells us that when a conductor is inside a changing magnetic field,
    current is induced in the conductor.
  • The direction of this induced current is opposite to the current that created it. This is Back EMF.
  • The net result is that the current that wants to come into the coil is impeded, pushed back, delayed, by the back EMF,
    and this creates the out-of-phase situation: the current lags the voltage in the inductor.

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