Alternating Current β Class 12 CBSE Physics revision notes
A 4-page visual revision sheet for Alternating Current, written for Class 12 CBSE Physics. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.
Page 1 as PDF
Alternating Current in one look
the source keeps flipping, and every element answers it differently
- AC source: voltage reverses every half cycle
- Instantaneous value: current, generally out of step with voltage by phase Ο
- RMS value: the steady DC that delivers the same average power
Mean and RMS, said precisely
- average of sinΟ t over a full cycle is zero, positive and negative halves cancel exactly
- RMS squares first (always positive), averages that, then square-roots: v_rms = v_0/β2, i_rms = i_0/β2
- power and heating always use RMS values: the whole reason RMS exists
which reactance is bigger, X_L or X_C, at this frequency?
- XL > XC β circuit is inductive: current lags voltage, net reactance is positive
- XL = XC β resonance: reactances cancel, circuit behaves as pure R, current peaks
- XL < XC β circuit is capacitive: current leads voltage, net reactance is negative
AC through R, L, C alone
- current in phase with voltage, Ο = 0
- opposition is R, the same value at any frequency
- current lags voltage by 90Β°
- reactance X_L = Ο L, grows with frequency
- current leads voltage by 90Β°
- reactance X_C = 1/Ο C, shrinks with frequency
Page 1 of 4. The full sheet, all 4 pages, opens free in the app.
What this Alternating Current sheet covers
- AC source
- Instantaneous value
- RMS value
- Mean and RMS, said precisely
- AC through R, L, C alone
- Resistor
- Inductor
- Capacitor
- How the two reactances move with frequency
- Reactance and impedance
- inductive reactance
- capacitive reactance
- series LCR impedance
- phase angle
- Limits of reactance and impedance
- Resonance in a series LCR circuit
- resonant angular frequency
- resonant frequency
- impedance at resonance
- sharpness of resonance
- Power in an AC circuit
- The transformer
- turns ratio equals voltage ratio
- ideal transformer, power in = power out
- so current ratio is inverse
- Where a real transformer loses energy
- Symbols
- Traps that cost marks
- Tricks that save time
How CapCompass uses this sheet
These notes are not a standalone PDF. CapCompass plans Physics chapter by chapter against your real exam dates and free hours, and when the plan reaches Alternating Current the sheet opens inside the study block itself β at the page covering the sub-topic that block is on, with the timer still running. Revision arrives at the moment the plan says revise, instead of having to be gone looking for.