Power factor of incandescent bulb is ______

Question

Examoogle · Accepted Answer

Unity

Explanation: An incandescent bulb is a purely resistive load, meaning the voltage and current waveforms are in phase with each other. Consequently, the power factor, defined as the cosine of the phase angle between voltage and current, is equal to unity ($\cos(0^{\circ}) = 1$).

Working Principle: In an incandescent bulb, electrical energy is converted into thermal energy through a tungsten filament. Since the filament exhibits negligible inductive or capacitive reactance at power frequencies, the load behaves as a pure resistor. In a pure resistor, the phase angle $\phi$ between the supply voltage $V$ and the current $I$ is $0^{\circ}$, resulting in a power factor of 1.

Key Formulas: $PF = \cos\phi$; $P = VI \cos\phi$

Key Points: Incandescent bulbs act as pure resistive loads.; For resistive loads, current is in phase with voltage.; The phase difference $\phi$ is zero, making $\cos(0) = 1$.; Unity power factor implies maximum efficiency in terms of current utilization for the given power.

Advantages: No reactive power consumption ($VAR = 0$).; Does not contribute to transmission line losses caused by reactive currents.

Disadvantages: High current draw compared to efficient light sources like LEDs.; Significant thermal loss occurs as the heating element is the source of light.

Applications: Residential lighting; Standardized test loads in electrical laboratories

Additional Info: Inductive loads (like motors) have lagging power factors.. Capacitive loads have leading power factors.. Option A is incorrect as power factor is never zero for a consuming load (that would imply a pure reactive element).

Answer: D is correct — The incandescent bulb is a purely resistive load, where the current and voltage are perfectly in phase, resulting in a power factor of unity.

Exam Tip: Always remember: Resistive loads = Unity PF; Inductive loads (motors, transformers) = Lagging PF; Capacitive loads (capacitor banks) = Leading PF.

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