π ALTERNATING CURRENT Β· AC Circuits
RMS β’ Phasors β’ LCR circuits β’ Resonance β’ Power β’ Transformer β’ NEET problems
β‘ AC BASICS & RMS VALUE
Alternating current: I = Iβ sin(Οt), V = Vβ sin(Οt) (for resistive circuit).
RMS (root mean square) value: Irms = Iβ/β2, Vrms = Vβ/β2 (for sinusoidal AC).
Peak value Iβ = β2 Irms.
Average value over half cycle: Iavg = (2Iβ)/Ο.
π Sinusoidal AC waveform
π§ AC CIRCUIT ELEMENTS
Resistor (R)
I = Iβ sin(Οt), V = Vβ sin(Οt)
Voltage & current in phase. Ο = 0Β°.
Impedance = R.
Inductor (L)
I = Iβ sin(Οt), V = Vβ sin(Οt + 90Β°)
Voltage leads current by 90Β°.
Inductive reactance: XL = ΟL.
Capacitor (C)
I = Iβ sin(Οt), V = Vβ sin(Οt β 90Β°)
Current leads voltage by 90Β°.
Capacitive reactance: XC = 1/(ΟC).
π Phasor diagrams for R, L, C
π LCR SERIES CIRCUIT
Impedance: Z = β[RΒ² + (XL β XC)Β²]
Phase angle Ο: tan Ο = (XL β XC)/R
Current: Irms = Vrms/Z
Resonance: when XL = XC β ΟL = 1/(ΟC) β Ο0 = 1/β(LC), f0 = 1/(2Οβ(LC))
At resonance: Zmin = R, Imax = V/R, current and voltage in phase.
π Resonance curve (I vs Ο)
π‘ POWER IN AC CIRCUITS
Instantaneous power: P = V I = Vβ Iβ sin(Οt) sin(Οt + Ο)
Average power: Pavg = Vrms Irms cos Ο = IrmsΒ² R (only resistor dissipates power).
Power factor: cos Ο = R/Z = Pavg/(Vrms Irms).
In purely reactive circuit: cos Ο = 0 (no power).
π LC OSCILLATIONS
In LC circuit, charge oscillates: q = qβ cos(Οt), Ο = 1/β(LC).
Energy transfers between capacitor (UE = qΒ²/2C) and inductor (UB = Β½ L IΒ²).
βοΈ 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).
Stepβup transformer: Ns > Np β Vs > Vp, Is < Ip.
Stepβdown transformer: Ns < Np β Vs < Vp, Is > Ip.
π§ Transformer core & windings
π‘ 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 Ο = R/Z. For pure L or C, cos Ο = 0.
- In AC circuits, average power is dissipated only in resistor.
β οΈ COMMON MISTAKES
- Using DC formulas for AC (e.g., V = IR for L or C).
- Forgetting that reactances depend on frequency (XL β f, XC β 1/f).
- Confusing phase relationships (voltage leads current in inductor, lags in capacitor).
- Using peak values instead of RMS in power formulas.
π QUICK REVISION CARD
RMS: Irms = Iβ/β2, Vrms = Vβ/β2
Impedance: Z = β[RΒ² + (XL β XC)Β²]
Resonance: Οβ = 1/β(LC), fβ = 1/(2Οβ(LC))
Power factor: cos Ο = R/Z
Avg power: P = Vrms Irms cos Ο
Transformer: Vs/Vp = Ns/Np = Ip/Is
π ALTERNATING CURRENT β’ NEET REVISION NOTES
πΈ NOTES PREVIEW
Preview of Alternating Current Notes
π₯ DOWNLOAD ALTERNATING CURRENT NOTES PDF
Download Alternating Current Revision Notes for NEET Physics PDF for quick revision and strong conceptual clarity. This chapter is one of the most important and scoring topics in Electrodynamics for NEET.
These Alternating Current handwritten notes PDF free download include RMS value, impedance, LCR circuit, resonance, power factor, and all important formulas, tricks, and PYQ-based concepts.
- High probability of questions in NEET
- LCR circuit and resonance are frequently asked
- Concept-based + numerical questions
- RMS and power factor are important topics
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π WEIGHTAGE ANALYSIS
2 Questions
LCR + RMS + impedance
High Weightage
Theory + numericals
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