I have a RLC circuit, and I know if we wish to calculate the phase difference between Voltage and Input current then the formula is

$$ \phi = \tan^{-1} \frac{X_L -X_C}{R} $$

But as we already know, phase difference between inductor voltage and input current is 90°. I know how to calculate it through waveform. $$\phi = 360 f \,\mathrm dT $$

But I have no idea how do I calculate it through first formula. In short, what will be the question mark in the following expression and why? $$\phi = \tan^{-1} \frac{X_L}{?}$$.

Also for the capacitor voltage and input current.

I know am weak in electronics and my basics too that's why am seeking help from you guys, kindly correct me if I was wrong in any of my presumptions.


1 Answer 1


But I have no idea how do I calculate it through first formula.

For a pure inductor the phase relationship between voltage and current is that current always lags voltage by 90°. This comes from the basic but fundamental inductor, voltage and current equation: -

$$V = L\dfrac{\mathrm di}{\mathrm dt}$$

Or, if you insist on using your original formula, ARCTAN(infinity)=90 degrees. It's infinity because \$X_L/0\$ is infinity and this is because resistance is 0 Ω.

Similarly for the pure capacitor, the phase relationship is that current leads voltage by 90°.

  • \$\begingroup\$ Thanks sir and one more follow up query, if there is practical inductor suppose I have inductor with internal resistance 10 ohm them the formula will be $$X_L / 10 $$ right? \$\endgroup\$
    – somanshu
    Jul 31, 2021 at 8:50
  • \$\begingroup\$ @Elecguy oops, sorry I missed your follow-up. Yes the ARCTAN operates on that. \$\endgroup\$
    – Andy aka
    Aug 1, 2021 at 15:30

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