Class 12 Physics Chapter 4: Moving Charges and Magnetism

the genius April 08, 2026

 

Moving Charges and Magnetism: Class 12 Physics Chapter 4 Notes

Pichle chapter mein humne dekha ki electric current kya hota hai. Is chapter mein hum ek bahut hi interesting cheez padhenge: Jab bhi koi charge move karta hai, toh wo apne charon taraf ek Magnetic Field paida karta hai. Is discovery ne science ki duniya badal di thi. Chaliye iske core concepts ko samajhte hain.

1. Oersted’s Discovery

Hans Christian Oersted ne pehli baar bataya tha ki agar kisi wire se current flow ho raha hai, toh uske paas rakhi magnetic needle deflect hoti hai. Iska matlab Electric Current magnetic field generate karta hai.

2. Magnetic Force (Lorentz Force)

Agar koi charge $q$, magnetic field $B$ mein $v$ velocity ke saath enter karta hai, toh uspar ek force lagta hai:

$$\vec{F} = q(\vec{v} \times \vec{B}) = qvB \sin \theta$$

  • Agar $\theta = 0^\circ$ ya $180^\circ$, toh force zero hoga.

  • Agar $\theta = 90^\circ$, toh force maximum ($qvB$) hoga.

3. Biot-Savart Law

Ye law batata hai ki ek chote se current element ($dl$) ki wajah se kisi point par kitna magnetic field ($dB$) hoga.

$$dB = \frac{\mu_0}{4\pi} \frac{I dl \sin \theta}{r^2}$$

Yahan $\mu_0$ Permeability of Free Space hai.

4. Ampere’s Circuital Law

Jaise Electrostatics mein Gauss Law tha, waise hi Magnetism mein Ampere’s Law hai.

$$\oint B \cdot dl = \mu_0 I_{enclosed}$$

Iska use karke hum Solenoid aur Toroid ka magnetic field nikalte hain.

5. Cyclotron

Cyclotron ek aisi device hai jo charged particles (jaise protons) ko bahut high energy tak accelerate karne ke kaam aati hai. Isme electric field particles ko speed up karti hai aur magnetic field unhe circular path mein ghumati hai.

6. Moving Coil Galvanometer

Ye current detect karne waali device hai. Iska principle ye hai ki jab kisi current-carrying coil ko magnetic field mein rakhte hain, toh uspar Torque lagta hai.

$$\tau = NIAB \sin \theta$$

7. Top Important Questions (Chapter 4)

Blog post ke end mein ye questions add karne se students ko revision mein help milti hai:

Short Questions:

  1. Define 1 Tesla: Magnetic field ki intensity jab $1C$ charge $1m/s$ ki speed se chalte hue $1N$ force experience kare.

  2. Fleming’s Left-Hand Rule: Force, magnetic field aur current ki direction nikalne ke liye use hota hai.

  3. Galvanometer ko Ammeter mein kaise badle? Ek chota resistance (Shunt) parallel mein lagakar.

Long Derivations:

  1. Magnetic field at the axis of a circular current loop: Biot-Savart Law ka use karke derivation.

  2. Force between two parallel current-carrying wires: Isse 1 Ampere ki definition bhi nikalti hai.

  3. Working principle and construction of Moving Coil Galvanometer.

  4. Magnetic field inside a Long Solenoid: $B = \mu_0 nI$.

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