Physics XII - Chapter 7: Alternating Current
Practice Alternating Current (AC) MCQs. Learn AC voltage, LCR circuits, power factor, resonance, transformers, and phasors for Class 12 physics exams.
Quick Revision Box
- AC Current: I = I₀ sin(ωt + φ); sinusoidal variation with time.
- AC Voltage: V = V₀ sin(ωt); sinusoidal variation in circuits.
- Peak Value: Maximum value of AC quantity (I₀, V₀).
- RMS Value: I_rms = I₀/√2, V_rms = V₀/√2; equivalent DC value for same power.
- Angular Frequency: ω = 2πf = 2π/T; rate of change of phase.
- Phase Difference: φ; angle by which current leads or lags voltage.
- Purely Resistive Circuit: V and I in phase (φ = 0); P = V_rms I_rms.
- Purely Inductive Circuit: Current lags voltage by 90° (φ = π/2).
- Inductive Reactance: X_L = ωL = 2πfL; opposition by inductor.
- Purely Capacitive Circuit: Current leads voltage by 90° (φ = -π/2).
- Capacitive Reactance: X_C = 1/ωC = 1/(2πfC); opposition by capacitor.
- Series LCR Circuit: Impedance Z = √[R² + (X_L - X_C)²].
- Phase Angle: tanφ = (X_L - X_C)/R; determines current phase.
- Resonance: X_L = X_C; Z_min = R, I_max at ω₀ = 1/√(LC).
- Quality Factor (Q): Q = ω₀L/R = 1/(ω₀CR); sharpness of resonance.
- Power in AC: P = V_rms I_rms cosφ; real power depends on power factor.
- Power Factor: cosφ = R/Z; ratio of real power to apparent power.
- Wattless Current: I_rms sinφ; component that doesn't contribute to power.
- LC Oscillations: Energy transfer between L and C; ω = 1/√(LC).
- Transformer: V₂/V₁ = N₂/N₁ = I₁/I₂; ideal transformer equations.
- Transformer Efficiency: η = (Output power)/(Input power) × 100%.
- Choke Coil: Reduces AC with minimal power loss; high L, low R.
Basic Level Questions
Chapter Summary
Alternating Current takes us into the rhythmic, pulsating world of electricity that powers our homes and industries - a world where current and voltage dance together in sinusoidal harmony. Unlike the steady flow of DC, AC constantly changes direction, creating a dynamic interplay between voltage and current that leads to fascinating phenomena like phase differences and resonance.
We discover how resistors, inductors, and capacitors each respond differently to alternating current - resistors maintaining perfect sync between voltage and current, inductors making current lag behind like a hesitant dancer, and capacitors pushing current ahead like an eager partner. The concept of reactance gives us mathematical insight into why these components behave so differently in AC circuits compared to their DC counterparts.
The series LCR circuit emerges as the star performer, where the competition between inductive and capacitive reactance can lead to the spectacular phenomenon of electrical resonance. At resonance, circuits can selectively amplify specific frequencies, making possible everything from radio tuning to musical instrument design. The quality factor tells us how sharply a circuit can discriminate between frequencies, much like a skilled musician distinguishing subtle pitch differences.
Practical applications bring these concepts to life - transformers that efficiently change voltage levels for power distribution, choke coils that control current with minimal energy loss, and the crucial understanding of power factor that helps industries optimize their electrical efficiency. This chapter reveals why AC won the historic "current war" and continues to be the lifeblood of our electrical infrastructure.