JEE · Physics
Select a frequency-dependent AC model, relate phasors to impedance and power, and separate circuit response from the origin of induced emf.
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In short
In a sinusoidal AC circuit, voltage and current vary with time and may differ in phase. A resistor dissipates average power, an ideal inductor and capacitor exchange energy with the source, and a series LCR circuit combines their frequency-dependent reactances into an impedance. At series resonance, inductive and capacitive reactances cancel.
This page owns sinusoidal response, phasors, impedance, resonance, average power and transformer behaviour. Electromagnetic Induction owns the flux-change laws and inductance origins that feed into this circuit response.
The official JEE documents define content scope. They do not publish chapter weightage, so none is asserted here.
Verified against the current JEE Main 2026 syllabus and JEE Advanced 2026 syllabus on 7 September 2026. This is a wording and scope mapping, not a claim about question difficulty or frequency.
Sources: JEE Main 2026 syllabus and JEE (Advanced) 2026 syllabus, both linked in the sources section below.
Prerequisite
Current Electricity
Stabilise potential difference, resistance and Kirchhoff's laws before Alternating Current.
Prerequisite
Electromagnetic Induction
Confirm induced emf and self-inductance before treating inductors in AC circuits.
Prerequisite
Capacitance
Revisit capacitance before treating the capacitor's frequency-dependent response.
Prerequisite
Simple Harmonic Motion
Use SHM phase and angular frequency reasoning to read AC phasors.
Parent
JEE Physics
Subject hub for the Physics chapter set.
Parent
JEE syllabus
Exam-level official scope for every Physics, Chemistry and Mathematics unit.
This is a readiness check, not a weightage or scoring-priority list.
State whether a value is instantaneous, peak, or RMS, and whether the waveform is sinusoidal.
State whether the supplied value is instantaneous, peak, or RMS, and whether the waveform is sinusoidal. Standard AC formulas assume a sinusoidal steady state unless stated otherwise.
Resistance, inductance and capacitance produce different magnitude and phase relations.
Resistance, inductance, and capacitance produce different magnitude and phase relations between voltage and current. Identify which element or combination is present before writing any equation.
In a series LCR circuit, the same instantaneous current passes every element, while voltage phasors combine vectorially rather than as ordinary scalars.
Inductive reactance grows with frequency and capacitive reactance falls with frequency.
Inductive reactance grows with frequency and capacitive reactance falls with frequency. Frequency therefore changes impedance and phase, and this comparison drives resonance behaviour.
Average power depends on the in-phase component of voltage and current through the power factor. A purely reactive element carries current with zero average power.
An ideal transformer exchanges voltage and current ratios according to turns ratio while conserving input and output power. It requires a sinusoidal varying flux and does not work on steady DC.
Six decisions cover most Alternating Current questions. Select the model before any algebra.
Voltage equals peak voltage times sine of omega t, and current equals peak current times sine of omega t minus phi.
Sinusoidal voltage and current, with current lag phi in this convention.
Use when — Steady sinusoidal state.
Common trap — Mixing peak, RMS and instantaneous values.
RMS voltage equals peak voltage divided by the square root of two, and similarly for RMS current.
Effective values for heating equivalence.
Use when — Pure sinusoid.
Common trap — Applying the factor to a nonsinusoidal waveform.
Inductive reactance equals angular frequency times inductance.
Inductive reactance.
Use when — Ideal inductor in sinusoidal steady state.
Common trap — Treating it as DC resistance.
Capacitive reactance equals one divided by angular frequency times capacitance.
Capacitive reactance.
Use when — Ideal capacitor in sinusoidal steady state, omega greater than zero.
Common trap — Substituting ordinary frequency for omega equals 2 pi f.
Impedance equals the square root of resistance squared plus the squared difference of inductive and capacitive reactance.
Series LCR impedance magnitude.
Use when — Series circuit in sinusoidal steady state.
Common trap — Adding reactances arithmetically without sign.
Tangent of phi equals the difference of inductive and capacitive reactance divided by resistance.
Phase of supply voltage relative to current.
Use when — Same series LCR convention.
Common trap — Reporting lead or lag without the sign of X_L minus X_C.
Average power equals RMS voltage times RMS current times cosine of phi.
Average real power.
Use when — Sinusoidal steady state.
Common trap — Using peak values without the factor one half.
Resonant angular frequency equals one divided by the square root of inductance times capacitance.
Ideal series-resonance angular frequency.
Use when — Ideal inductor and capacitor in the series LCR model.
Common trap — Claiming impedance becomes zero when resistance is present.
Secondary to primary voltage ratio equals the turns ratio, and the current ratio is its inverse.
Ideal transformer ratios.
Use when — Ideal transformer, sinusoidal varying flux, negligible loss.
Common trap — Using a transformer with steady DC.
Answer: Below resonance the circuit is net capacitive and current leads. At resonance impedance equals resistance and current is maximum with zero phase difference. Above resonance the circuit is net inductive and current lags.
Using RMS formulas without confirming a sinusoidal waveform
Knowledge gapWhy it happens
The one over root two factor holds only for a pure sinusoid, not for arbitrary periodic waveforms.
How it is corrected
Confirm the waveform is sinusoidal before applying the standard RMS relation.
Adding resistor, inductor and capacitor voltages as ordinary scalars in a series circuit
Decision / selection errorWhy it happens
Series voltages are phasors with different phase angles, so they combine vectorially rather than arithmetically.
How it is corrected
Combine voltage phasors using the impedance triangle before reading a total.
Assuming current always lags in an AC circuit
Knowledge gapWhy it happens
Whether current leads or lags depends on whether capacitive or inductive reactance dominates.
How it is corrected
Compare inductive and capacitive reactance before deciding the direction of phase difference.
Claiming current is infinite at series resonance while ignoring resistance
Execution errorWhy it happens
At resonance impedance equals resistance, not zero, so current is maximum but finite when resistance is present.
How it is corrected
Keep resistance in the impedance expression even at resonance.
Applying a transformer turns ratio to steady DC
Decision / selection errorWhy it happens
A transformer depends on a changing flux, which a steady DC supply does not provide.
How it is corrected
Restrict transformer ratio relations to sinusoidal varying flux conditions.
Repeating Faraday-law depth on this page instead of linking to Electromagnetic Induction
Needs reviewWhy it happens
Flux-change laws and inductance origins belong to Electromagnetic Induction, not to circuit-response reasoning.
How it is corrected
Treat inductance and induced emf as given inputs here and route deeper flux questions to Electromagnetic Induction.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
It is the direct-current value that would produce the same average heating in a resistor. For a sinusoid, RMS current equals peak current divided by the square root of two.
Inductive reactance equals capacitive reactance, the impedance becomes equal to resistance, supply voltage and current are in phase, and current is maximum for a fixed supply amplitude.
It describes an ideal AC condition with zero average power, such as a purely reactive element where voltage and current are in quadrature.
Evidence boundary: the syllabus mapping is tied to the official 2026 JEE Main and JEE Advanced documents. No chapter weightage, question frequency, or forecast is asserted. Official papers are linked for evidence-safe practice, and any question classified by chapter requires human academic review first. Flux-change and Lenz-law depth is deliberately routed to Electromagnetic Induction rather than duplicated here.
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