Power factor of filament lamp is

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Examoogle · Accepted Answer

unity

Explanation: A filament lamp is essentially a purely resistive load because it consists of a tungsten filament that converts electrical energy into heat and light through Joule heating. In a purely resistive circuit, the voltage and current are in phase, resulting in a power factor of unity (cos 0° = 1).

Working Principle: The filament lamp behaves as a linear resistor following Ohm's Law $V = IR$. Since there are no inductive or capacitive components (like windings or plates) to cause a phase shift between the sinusoidal voltage and current waveforms, the power factor remains 1.0.

Key Formulas: $P = VI \cos \phi$ — Power in AC circuit; $\cos \phi = \frac{R}{Z}$ — Definition of power factor

Key Points: Filament lamps have no magnetic or electric field storage elements.; The impedance $Z$ of a filament lamp is purely resistive ($Z = R$).; In resistive loads, the phase angle $\phi$ between voltage and current is $0^\circ$.

Advantages: High efficiency in power utilization; No requirement for reactive power compensation

Disadvantages: Low overall energy efficiency (conversion of most energy to heat); Short operational life compared to LED technology

Applications: Incandescent lighting; Basic resistive heating elements

Additional Info: Option B and C (Zero leading/lagging) refer to pure capacitors and pure inductors, respectively.. Option D is characteristic of inductive loads like motors or transformers which are standard in industrial environments.

Answer: A is correct — The filament lamp is a purely resistive load where voltage and current are in phase, resulting in a power factor of unity.

Exam Tip: Always remember: resistive loads (lamps, heaters) have unity power factor, inductive loads (motors, coils) have lagging power factor, and capacitive loads (capacitors, synchronous condensers) have leading power factor.

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