Alternating Current for NEET UG
The electricity in our homes reverses direction 50 times a second. Learn rms values, reactance, impedance, resonance and transformers.
PhysicsClass 1265 MCQs3 lessons13 flashcards3 memory tricks
Practise Alternating Current free
What you'll learn
- AC voltage and rms values
Relate peak and rms values, and explain why rms is used. - Reactance, impedance and phase
Find the reactance of L and C, the impedance of an LCR circuit, and the phase difference. - Resonance and transformers
Find the resonant frequency of an LCR circuit and use the transformer equation.
Key formulas
| AC voltage | v = V₀ sin ωt |
|---|---|
| rms voltage | V_rms = V₀/√2 ≈ 0.707V₀ |
| rms current | I_rms = I₀/√2 |
| Average power in a resistor | P = V_rms I_rms |
| Impedance of series LCR | Z = √(R² + (X_L − X_C)²) |
| Phase angle | tan φ = (X_L − X_C)/R |
| Power factor | cos φ = R/Z |
| Average power | P = V_rms I_rms cos φ |
Memory tricks
In an inductor (L), voltage E leads current I by 90°. In a capacitor (C), current I leads voltage E by 90°.
At resonance X_L = X_C, impedance is at its minimum (just R) and current is at its maximum. f₀ = 1/(2π√LC).
Transmission power P = VI. Higher V means lower I, and the line loss I²R falls with the square of the current.
Try 3 NEET-style questions
Q1. A series circuit contains R = 30 Ω and a capacitor of reactance 40 Ω. Its impedance and power factor are
- 70 Ω and 0.43
- 10 Ω and 3
- 50 Ω and 0.8
- 50 Ω and 0.6
Show answer
Answer: (4) 50 Ω and 0.6
Z = √(R² + X_C²) = √(900 + 1600) = 50 Ω. Power factor cos φ = R/Z = 30/50 = 0.6.
Q2. The capacitive reactance of a 10 μF capacitor on a 50 Hz supply is about
- 3.18 Ω
- 318 Ω
- 3180 Ω
- 31.8 Ω
Show answer
Answer: (2) 318 Ω
X_C = 1/(2πfC) = 1/(2π × 50 × 10 × 10⁻⁶) = 1/(3.14 × 10⁻³) ≈ 318 Ω.
Q3. A choke coil is preferred over a resistor for controlling AC current because it
- stores charge
- dissipates negligible average power
- has zero reactance
- is cheaper
Show answer
Answer: (2) dissipates negligible average power
An ideal choke coil has reactance but no resistance, so it limits the current without dissipating average power as heat.
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