Alternating Current Revision Notes for NEET Physics PDF Download
🔌 ALTERNATING CURRENT · AC Circuits
RMS • Phasors • LCR circuits • Resonance • Power • Transformer • NEET problems
⚡ AC BASICS & RMS VALUE
Alternating current: I = I₀ sin(ωt), V = V₀ sin(ωt) (for resistive circuit).
RMS (root mean square) value: Irms = I₀/√2, Vrms = V₀/√2 (for sinusoidal AC).
Peak value I₀ = √2 Irms.
Average value over half cycle: Iavg = (2I₀)/π.
📈 Sinusoidal AC waveform
🔧 AC CIRCUIT ELEMENTS
Resistor (R)
I = I₀ sin(ωt), V = V₀ sin(ωt)
Voltage & current in phase. φ = 0°.
Impedance = R.
Inductor (L)
I = I₀ sin(ωt), V = V₀ sin(ωt + 90°)
Voltage leads current by 90°.
Inductive reactance: XL = ωL.
Capacitor (C)
I = I₀ sin(ωt), V = V₀ sin(ωt – 90°)
Current leads voltage by 90°.
Capacitive reactance: XC = 1/(ωC).
📐 Phasor diagrams for R, L, C
🔗 LCR SERIES CIRCUIT
Impedance: Z = √[R² + (XL – XC)²]
Phase angle φ: tan φ = (XL – XC)/R
Current: Irms = Vrms/Z
Resonance: when XL = XC → ωL = 1/(ωC) → ω0 = 1/√(LC), f0 = 1/(2π√(LC))
At resonance: Zmin = R, Imax = V/R, current and voltage in phase.
📊 Resonance curve (I vs ω)
💡 POWER IN AC CIRCUITS
Instantaneous power: P = V I = V₀ I₀ sin(ωt) sin(ωt + φ)
Average power: Pavg = Vrms Irms cos φ = Irms² R (only resistor dissipates power).
Power factor: cos φ = R/Z = Pavg/(Vrms Irms).
In purely reactive circuit: cos φ = 0 (no power).
🌀 LC OSCILLATIONS
In LC circuit, charge oscillates: q = q₀ cos(ωt), ω = 1/√(LC).
Energy transfers between capacitor (UE = q²/2C) and inductor (UB = ½ L I²).
⚙️ TRANSFORMER
Works on mutual induction. For ideal transformer (no losses):
Voltage ratio: Vs/Vp = Ns/Np
Current ratio: Is/Ip = Np/Ns (power in = power out for ideal).
Step‑up transformer: Ns > Np → Vs > Vp, Is < Ip.
Step‑down transformer: Ns < Np → Vs < Vp, Is > Ip.
🔧 Transformer core & windings
💡 NEET TIPS & SHORTCUTS
- At resonance, impedance is minimum, current is maximum, and power factor = 1.
- For LCR circuit, the peak current occurs at resonance frequency.
- Power factor cos φ = R/Z. For pure L or C, cos φ = 0.
- In AC circuits, average power is dissipated only in resistor.
⚠️ COMMON MISTAKES
- Using DC formulas for AC (e.g., V = IR for L or C).
- Forgetting that reactances depend on frequency (XL ∝ f, XC ∝ 1/f).
- Confusing phase relationships (voltage leads current in inductor, lags in capacitor).
- Using peak values instead of RMS in power formulas.
📌 QUICK REVISION CARD
RMS: Irms = I₀/√2, Vrms = V₀/√2
Impedance: Z = √[R² + (XL – XC)²]
Resonance: ω₀ = 1/√(LC), f₀ = 1/(2π√(LC))
Power factor: cos φ = R/Z
Avg power: P = Vrms Irms cos φ
Transformer: Vs/Vp = Ns/Np = Ip/Is