Alternating Currents
Telangana Board · Class 12 · Physics
Quick revision notes for Alternating Currents — Telangana Board Class 12 Physics. Key concepts, formulas, and definitions for last-minute revision.
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Key Topics to Revise
1. Introduction to Alternating Current and Voltage
- AC current and voltage vary sinusoidally with time, unlike DC which remains constant
- In India, AC frequency = 50 Hz, meaning current changes direction 100 times per second
- Peak values (Vm, Im) represent maximum values during oscillation
2. AC Circuit with Pure Resistor
- In pure resistive AC circuit, voltage and current are in phase (φ = 0°)
- Ohm's law applies directly: V = IR at every instant
- Power is always positive, indicating continuous energy dissipation as heat
3. Phasor Representation
- Phasors are rotating vectors representing AC quantities in vector form
- Length of phasor = peak value, projection on y-axis = instantaneous value
- Phasor diagram simplifies addition of AC quantities with different phases
4. AC Circuit with Pure Capacitor
- Current leads voltage by 90° (π/2 radians) in pure capacitive circuit
- Capacitive reactance XC = 1/ωC decreases with increasing frequency
- Average power = 0, energy alternates between electric field and source
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