Answer

  1. In LCR circuit: $\tan\varphi= \frac{\text{X}_{L} - \text{X}_{c}}{\text{R}} = \frac{\text{wL} - \frac{1}{\text{wC}}}{\text{R}}$

Now $\text{X}_{L} = \text{wL} = ( 1000\times100\times10^{-3})\Omega$

$= 100\Omega$

and $\text{X}_{c} = \frac{1}{\text{wc}} = \bigg(\frac{1}{100\times2\times10^{-6}}\bigg)\Omega$

$\therefore\text{X}_{c} = 500 \Omega$

$\therefore\tan\varphi = \frac{500 - 100}{400} = 1 $

$\tan\varphi = 1 $

$\varphi = 45^{o}$

  1. Power Factor:

When power factor = 1, we have XL=XC

$\therefore\text{X'}_{c} = \frac{1}{\omega\text{C}'} = 100 \Omega$

This gives $\text{C'} = \frac{1}{100\omega} = 10 \mu\text{F}$

We, therefore, need to add a capacitor of capacitance (10-2) μF = 8μF in parallel with the given capacitor.

Alternate Answer

Let addition capacitance C1 be connected:

$\text{X}'_{c} = \frac{1}{1000(2 + \text{C}_{1})\times10^{-6}}$

$\therefore100 = \frac{1}{1000(2 + \text{C}_{1})\times10^{-6}}$

$\therefore2 + \text{C}_{1} = 10 $

$\text{C}_{1} = 8 \mu\text{F}$.

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