Class 12 Physics Chapter 7: Alternating Current (AC)

the genius April 08, 2026

 

Alternating Current: Class 12 Physics Chapter 7 Notes in Hindi

Ab tak humne DC (Direct Current) ke baare mein padha jo batteries se milta hai. Lekin hamare gharon mein jo bijli aati hai, wo Alternating Current (AC) hai. Is post mein hum AC ke behavior ko resistors, inductors aur capacitors ke saath samjhenge.

1. Alternating Current (AC) Kya Hai?

Aisa current jiski value aur direction samay ke saath periodic tareeke se badalti rehti hai, use Alternating Current kehte hain.

  • Equation:

    $$I = I_0 \sin(\omega t)$$

    aur

    $$V = V_0 \sin(\omega t)$$

  • Yahan $I_0$ aur $V_0$ Peak Value (maximum value) hain.

2. RMS Value (Root Mean Square Value)

AC ki direction badalti rehti hai, isliye uska average full cycle mein zero hota hai. Isliye hum RMS Value ka use karte hain, jise "Effective Value" bhi kehte hain.

  • $V_{rms} = \frac{V_0}{\sqrt{2}} \approx 0.707 V_0$

  • Hamare gharon mein aane waali 220V bijli asal mein RMS value hoti hai.

3. AC Circuit with L, C, and R

Jab AC alag-alag components se guzarta hai, toh current aur voltage ke beech "Phase" badal jata hai:

  1. Pure Resistor (R): Voltage aur Current saath-saath (in phase) chalte hain.

  2. Pure Inductor (L): Voltage current se $90^\circ$ aage (leads) rehta hai.

  3. Pure Capacitor (C): Current voltage se $90^\circ$ aage (leads) rehta hai.

4. LCR Series Circuit (Most Important)

Jab Inductor, Capacitor aur Resistor teeno series mein jude hon, toh total rukawat ko Impedance ($Z$) kehte hain.

  • Impedance Formula:

    $$Z = \sqrt{R^2 + (X_L - X_C)^2}$$

  • Yahan $X_L = \omega L$ (Inductive Reactance) aur $X_C = \frac{1}{\omega C}$ (Capacitive Reactance) hai.

5. Resonance (Anunaad)

LCR circuit mein ek aisi frequency aati hai jahan $X_L = X_C$ ho jata hai. Is condition mein:

  • Impedance ($Z$) minimum ho jata hai ($Z = R$).

  • Current ($I$) maximum ho jata hai.

  • Resonant Frequency:

    $$f_r = \frac{1}{2\pi \sqrt{LC}}$$

6. Power in AC Circuit

AC circuit mein average power sirf Resistor mein consume hoti hai.

  • Formula:

    $$P_{avg} = V_{rms} I_{rms} \cos \phi$$

  • Yahan $\cos \phi$ ko Power Factor kehte hain.

7. Transformer

Transformer ek aisi device hai jo low AC voltage ko high mein (Step-up) ya high ko low mein (Step-down) badalti hai.

  • Principle: Ye Mutual Induction par kaam karta hai.

  • Efficiency: Transformer mein Energy loss hota hai (Copper loss, Iron loss, Eddy currents).

8. Top Important Questions (Chapter 7)

Blog post ko SEO rank karwane ke liye ye questions add karein:

Short Questions:

  1. Wattless Current kya hai? Aisa current jo koi power consume nahi karta ($\phi = 90^\circ$).

  2. AC, DC se zyada khatarnak kyun hai? Kyunki 220V AC ki peak value $\approx 311V$ tak jati hai.

  3. Choke Coil ka kya kaam hai? Ye bina energy waste kiye AC current ko control karti hai.

Long Derivations:

  1. Derive the expression for Impedance in LCR Series circuit.

  2. What is Sharpness of Resonance and Q-factor?

  3. Explain the construction and working of a Transformer.

  4. Prove that average power in a pure inductor or capacitor is zero.

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  • Labels: Class 12 Physics, Alternating Current, NCERT Hindi Notes, LCR Circuit.

  • Description: Is post mein AC current, RMS value, LCR circuit aur Transformer ke important notes aur derivations hain. Board exams ke liye perfect guide.

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