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Internal resistance of inductance (or other devices) are said to be in series. But parasitic capacitance is said to be in parallel (in case of an inductor). Why is that so? What determines whether an internal property is in series or parallel?

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From a circuit theory perspective, an inductor is effectively a short-circuit (wire) at DC while a capacitor is an open-circuit.

Thus, any parasitic capacitance must be in parallel since, if it were in series, an inductor would be an open-circuit at DC.

From an AC perspective, if the parasitic capacitance were in series, the inductor would appear capacitive at low frequencies and inductive at frequencies above the self-resonance frequency:

$$Z = j \omega L + \frac{1}{j \omega C} = \frac{1}{j\omega C} \left( 1 - \omega^2LC\right)$$

But, in fact, we see the opposite, the physical inductor is inductive at low frequencies and appears capacitive at frequencies above the self-resonance frequency.

$$Z = j\omega L || \frac{1}{j\omega C} = j \omega L\left(\frac{1}{1 - \omega^2LC}\right) $$

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  • $\begingroup$ Will it hold good in AC too? My friend is working in RF generator and told me about the parasitic capacitance. And AC is used in RF generation. $\endgroup$ Jun 27, 2014 at 12:10
  • $\begingroup$ @karthikeyan, see update. $\endgroup$ Jun 27, 2014 at 12:21
  • $\begingroup$ Beautiful answer, Thank you ! $\endgroup$
    – A Onsi
    May 15, 2020 at 9:42

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