Power Factor — Prelims Strategy

Prelims Strategy

For NEET prelims, a strong grasp of the fundamental definitions and formulas related to power factor is essential.

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Formulas: — Memorize the key formulas: $ext{Power Factor} = cos phi = R/Z = P/S$. Also, know the impedance formulas for series RL, RC, and RLC circuits: $Z_{RL} = sqrt{R^2 + X_L^2}$, $Z_{RC} = sqrt{R^2 + X_C^2}$, $Z_{RLC} = sqrt{R^2 + (X_L - X_C)^2}$. Remember $X_L = omega L$ and $X_C = 1/(omega C)$.
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Phase Angle: — Understand how to determine the phase angle $phi$ from voltage and current equations ($V = V_m sin(omega t)$, $I = I_m sin(omega t pm phi)$). If current leads voltage, it's a leading power factor (capacitive). If current lags voltage, it's a lagging power factor (inductive).
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Special Cases: — Pay close attention to special cases: purely resistive (PF=1, $phi=0$), purely inductive (PF=0 lagging, $phi=90^circ$), purely capacitive (PF=0 leading, $phi=-90^circ$), and resonance (PF=1, $phi=0$). These are common conceptual traps.
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Numerical Problems: — For numerical problems, systematically calculate reactances ($X_L, X_C$), then impedance (Z), and finally the power factor. Be careful with units and trigonometric values.
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Trap Options: — Be wary of trap options that confuse leading/lagging, or values greater than 1. Always check if the calculated power factor makes physical sense (between 0 and 1). Practice identifying the nature of the circuit (inductive/capacitive) from the phase angle.