Alternating Current – Class 12 Physics Notes | Full NCERT Summary with Important Diagrams

 ⚡️ Chapter 7 – Alternating Current (AC) – Class 12 Physics

---

🔹 What is Alternating Current?

Alternating Current (AC): An electric current that changes its magnitude and direction with time.

Represented as:

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

 = Peak current

 = Angular frequency 

---

🔹 AC vs DC

Property Alternating Current (AC) Direct Current (DC)

Direction Changes with time Constant

Source Generator Battery

Transmission Less loss More loss

---

🔹 Peak, RMS, and Average Value

1. Peak Value (I₀, V₀): Maximum value of current or voltage.

2. Root Mean Square (RMS) Value:

I_{\text{rms}} = \frac{I_0}{\sqrt{2}}, \quad V_{\text{rms}} = \frac{V_0}{\sqrt{2}}

📌 Important Diagram: Graph of AC waveform showing I vs t

---

🔹 AC through Resistor (R)

Voltage and current are in the same phase.

📌 Diagram: Phasor diagram for AC through R

---

🔹 AC through Inductor (L)

Current lags voltage by .

Inductive reactance:

X_L = \omega L

📌 Diagram: Phasor diagram for AC through L

---

🔹 AC through Capacitor (C)

Current leads voltage by .

Capacitive reactance:

X_C = \frac{1}{\omega C}

📌 Diagram: Phasor diagram for AC through C

---

🔹 LCR Series Circuit

Contains Resistor, Inductor, and Capacitor in series.

Net impedance:

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

I = \frac{V}{Z}

\tan\phi = \frac{X_L - X_C}{R}

📌 Diagram: LCR series circuit with phasor diagram

---

🔹 Resonance in LCR Circuit

Condition: 

Resonant frequency:

f_0 = \frac{1}{2\pi\sqrt{LC}}

📌 Diagram: Resonance curve in LCR circuit

---

🔹 Power in AC Circuit

Instantaneous Power:

P = VI = V_0 I_0 \cos(\phi) \sin(\omega t) \sin(\omega t + \phi)

P_{\text{avg}} = V_{\text{rms}} I_{\text{rms}} \cos(\phi)

\cos(\phi)

📌 Diagram: Power vs phase angle graph

---

🔹 Transformer

Device to step up or step down AC voltage.

Based on mutual induction.

Formula:

\frac{V_s}{V_p} = \frac{N_s}{N_p}, \quad \text{Efficiency} = \frac{P_s}{P_p}

📌 Diagram: Working of transformer (step-up / step-down)

---

🔹 Uses of AC over DC

AC can be easily transformed to higher or lower voltages.

Easier to transmit over long distances with fewer losses.

Generators and transformers work on AC.

---

✅ Summary Table:

Component Voltage-Current Phase Relation Reactance

Resistor In Phase 

Inductor Voltage leads current by 90° 

Capacitor Current leads voltage by 90° 

---

📊 Important Diagrams to Include:

1. AC waveform (I vs t)

2. Phasor diagrams: R, L, C, and LCR circuit

3. LCR circuit diagram

4. Resonance curve

5. Transformer working

6. Power vs Phase angle graph

---

✍️ Written by: Shahid – Study with Shahid

🔗 Visit our blog for more Physics notes