Overview

Alternating Current (AC) is a vital chapter in NEET Physics that deals with electric current and voltage varying sinusoidally with time. AC is extensively used in household and industrial applications, making it an important topic for both conceptual understanding and numerical problem-solving in NEET. This chapter connects AC voltage and current, reactance, impedance, power, and resonance. Mastering the formulas for AC circuits, including RMS and peak values, reactance, impedance, and power factor, is essential for solving NEET questions efficiently. This guide provides a structured and comprehensive overview of all key formulas and concepts in Alternating Current.

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AC Voltage and Current

In an AC circuit, the instantaneous voltage and current are given by:

• v(t) = V₀ sin(ωt)

• i(t) = I₀ sin(ωt)

Where:

• V₀ and I₀ are peak voltage and current

• ω = 2πf is the angular frequency

• t = time

Key points for NEET:

• Current and voltage vary sinusoidally

• Peak values and RMS values are different and often asked in problems

RMS and Average Values

• RMS voltage: V_rms = V₀ / √2

• RMS current: I_rms = I₀ / √2

• Average power over a cycle: P_avg = V_rms I_rms cosφ

Where φ = phase difference between voltage and current.

Key points:

• RMS values are used for calculating real power in AC circuits

• Average current or voltage over a full cycle is zero

Reactance and Impedance

• Inductive reactance: X_L = ω L

• Capacitive reactance: X_C = 1 / (ω C)

• Impedance in series LCR circuit: Z = √(R² + (X_L - X_C)²)

Key points for NEET:

• Impedance determines current in AC circuits

• Phase difference φ = arctan((X_L - X_C) / R)

AC Power

• Instantaneous power: p(t) = v(t) i(t) = V₀ I₀ sin²(ωt + φ)

• Average power: P = V_rms I_rms cosφ

• Maximum power occurs when φ = 0, i.e., in resonance

Key points:

• Power factor cosφ is crucial for AC power calculations

• Useful for solving problems in R, L, and C circuits

Resonance in AC Circuits

• Resonant frequency: f₀ = 1 / (2π √(LC))

• At resonance: X_L = X_C → Z = R → I = I_max

• Voltage across L and C can exceed applied voltage at resonance

Key points for NEET:

• Resonance condition is often tested in numerical problems

• Maximum current flows at resonance

Current and Voltage Relations in L, C, and R Circuits

1. Purely Resistive Circuit:

   • Current in phase with voltage (φ = 0)

2. Purely Inductive Circuit:

   • Current lags voltage by 90° (φ = 90°)

3. Purely Capacitive Circuit:

   • Current leads voltage by 90° (φ = -90°)

Key points:

• Phase relationships are important in AC circuit questions

• Helps in solving LCR series and parallel circuit problems

Power in AC Circuits

• Instantaneous power: p(t) = V I sin(ωt + φ)

• Average power: P_avg = V_rms I_rms cosφ

• Reactive power: Q = V_rms I_rms sinφ

• Apparent power: S = V_rms I_rms

Key points for NEET:

• P = S cosφ relates real, reactive, and apparent power

• Phase angle φ is essential for understanding power factor

Importance of Alternating Current Formulas in NEET

These formulas help NEET aspirants:

• Calculate RMS, peak, and instantaneous values of voltage and current

• Solve AC circuit problems with resistors, inductors, and capacitors

• Analyze resonance, impedance, and phase relationships

• Calculate average, reactive, and apparent power in circuits

Key formulas to remember:

• v(t) = V₀ sin(ωt), i(t) = I₀ sin(ωt)

• V_rms = V₀ / √2, I_rms = I₀ / √2

• X_L = ω L, X_C = 1 / (ω C), Z = √(R² + (X_L - X_C)²)

• P_avg = V_rms I_rms cosφ, φ = arctan((X_L - X_C)/R)

• Resonance: f₀ = 1 / (2π √(LC)), I_max at resonance

Memorizing these formulas ensures efficient problem-solving in NEET exams.

Practical Applications

Alternating Current concepts are widely applied in:

• Household electricity supply (AC)

• Transformers for voltage conversion

• AC motors and generators

• Resonance in radio and communication circuits

• Power factor correction in industrial circuits

Connecting theory with practical applications enhances understanding and retention for NEET aspirants.

Preparation Tips for NEET Alternating Current

1. Understand Conceptually – Focus on RMS, peak values, reactance, impedance, and phase difference.

2. Create a Formula Sheet – Include LCR series formulas, resonance, and power factor.

3. Use Diagrams – Draw phasor diagrams, LCR circuits, and resonance curves.

4. Regular Revision – Frequent practice ensures quick recall during exams.

5. Connect with Real Life – Relate AC circuits to household electricity and electrical devices.

Conclusion

Alternating Current is a high-yield chapter for NEET Physics that connects voltage, current, impedance, resonance, and power. Mastering AC voltage and current, RMS values, reactance, impedance, phase relationships, and resonance allows students to solve both numerical and conceptual problems efficiently. Understanding the physical significance of each formula, visualizing phasor relationships, and revising regularly enhances confidence, accuracy, and speed. This guide provides NEET aspirants with a structured approach to learn, revise, and master Alternating Current effectively, making it an essential resource for exam success.