Alternating_Current_Class12fullnotesforrevision.pptx

Revision ac current

1. Alternating Current (AC) Class 12 Physics | As per CBSE 2026 Syllabus

2. Introduction • Alternating current (AC) is an electric current that changes its magnitude and direction periodically. • The most common form of electrical energy used in homes and industries. • AC is generated using an alternator.

3. Difference Between AC and DC • AC: Changes direction periodically. • DC: Flows only in one direction. • AC can be transmitted over long distances with less power loss. • DC is used in electronic devices like batteries.

4. Mathematical Expression of AC Instantaneous value of AC: i = i₀ sin(ωt) Where: • i = instantaneous current • i₀ = peak current • ω = angular frequency = 2πf • t = time

5. RMS and Average Value • RMS (Root Mean Square) value: i_rms = i₀ / √2 ≈ 0.707 i₀ • Average value (over half cycle): i_avg = 2i₀ / π ≈ 0.637 i₀ • These values help represent the effective current in AC circuits.

6. Phase and Phase Difference • Phase: The fraction of a complete cycle elapsed relative to a reference point. • Phase difference: The angular displacement between two alternating quantities. • Represented in degrees or radians.

7. AC Through Resistor, Inductor, Capacitor 1. Pure Resistor: Voltage and current are in phase. V = V₀ sin(ωt) 2. Pure Inductor: Current lags voltage by 90°. V = L(di/dt) 3. Pure Capacitor: Current leads voltage by 90°. Q = CV

8. Series LCR Circuit • Contains resistor (R), inductor (L), and capacitor (C) in series. • Impedance: Z = √(R² + (ωL – 1/ωC)²) • Current: I = V / Z • Phase angle: tanφ = (ωL – 1/ωC) / R

9. Resonance in LCR Circuit • Occurs when inductive reactance = capacitive reactance. ωL = 1/ωC • Impedance is minimum and current is maximum. • Resonant frequency: f₀ = 1 / (2π√(LC))

10. Power in AC Circuit • Instantaneous power: P = VI = V₀I₀ sin²(ωt) • Average power: P_avg = VI cosφ • cosφ = power factor • Power factor = 1 for pure resistors, 0 for pure L or C.

11. Applications of AC • Used in power transmission and distribution. • Electric motors and transformers operate on AC. • Domestic and industrial power supply. • Easier to step up or down using transformers.

12. Summary • AC changes direction and magnitude periodically. • Represented by i = i₀ sin(ωt). • RMS and average values indicate effective current. • LCR circuit shows resonance at a specific frequency. • AC is the backbone of modern electrical systems.