Lenz's law — opposing induced current — Hinglish notes

1.8.27 · Physics › Electromagnetism

Lenz's law EXIST kyun karta hai?

KYA problem solve karta hai ye? Faraday's law humein induced EMF ki size batata hai: $ε = - \frac{d Φ _{B}}{d t}$ lekin bare magnitude $\frac{d Φ _{B}}{d t}$ ye nahi batata ki current kis direction mein jayegi. Lenz's law minus sign ka physical meaning hai.

KYUN must karta hai current change ko oppose? Suppose karo ki usne change ko help kiya hota. Tab:

Ek magnet ko coil ki taraf push karo → induced current magnet ko aur tezi se khichti hai → wo apne aap speed up hota hai → current badhti hai → aur zyada khinchti hai → kuch nahi se infinite energy.

Ye conservation of energy ko violate karta hai. Isliye physically allowed direction sirf wahi hai jo motion/change ko resist kare. Tumhe magnet ko push karne ke liye work karna padta hai, aur wahi work electrical energy ban jaata hai.

HOW to apply karo (4-step recipe)

Figure — Lenz's law — opposing induced current

Minus sign ko energy se derive karna

KYUN sign negative hai aur positive nahi? Chalo build karte hain.

Ek bar magnet (N-pole pehle) ko speed $v$ se loop ki taraf move karo. Loop mein into flux badh raha hai: $\frac{d Φ _{B}}{d t} > 0$ .

Agar induced current kuch bhi oppose nahi karta, koi force nahi, free energy — forbidden.
Isliye magnet ke sabse paas loop ka face N-pole ban jaata hai taaki magnet ko wapas push kare (repel like poles). Iske liye current ek specific sense mein chahiye.

Is repulsion ke against tumhara jo mechanical work hota hai wo electrical energy mein convert hota hai: $W_{you} = \int F d x = \int I^{2} R d t = heat in resistor .$ Bookkeeping balance karne ke liye ( $P_{mech} = P_{elec} > 0$ ), induced EMF aisi direction mein hona chahiye ki current change ko oppose kare — isliye $ε = - d Φ_{B} / d t$ mein minus sign hai. Minus sign hi energy conservation hai.

Worked examples

Worked example Example 1 — Magnet approaching a coil

Ek bar magnet ka N-pole circular coil ki taraf move karta hai. Current kis direction mein flow hogi (magnet ki side se dekha jaaye)?

Step 1: N-pole se field magnet se door, yaani coil ke andar point karti hai. Kyun? Field lines N se bahar nikalti hain.
Step 2: Magnet paas aa raha hai → coil mein flux increase ho raha hai. Kyun? Jaise distance ghatata hai, coil par $B$ badhta hai.
Step 3: Coil increase ko oppose kare → induced field coil ke bahar magnet ki taraf point kare → coil face N-pole ban jaata hai (repels). Kyun? Increase oppose karne ka matlab hai incoming field ko cancel karna.
Step 4: Right-hand rule: field coil se bahar tumhari taraf ⇒ current counterclockwise flow karti hai jaise magnet se dekha jaaye. Kyun? Fingers current ke along curl karo, thumb field direction deta hai.

✔ Result: counterclockwise, aur magnet ko push-back force feel hota hai.

Common mistakes (Steel-manned)

Recall Feynman: 12-saal ke bachche ko samjhao

Socho tumhare paas ek soyal guard dog (coil) hai. Jab ek magnet uski taraf sneaks karta hai, dog growl karta hai aur use door dhakelta hai. Jab magnet jaane ki koshish karta hai, dog uski tail pakad ke wapas kheenchta hai. Dog hamesha chahta hai ki cheezein bilkul waise rahen jaise thi. Yahi "waise rehna chahna" Lenz's law hai — aur dog ko growl karne ke liye energy lagti hai (tumhara pushing), isliye tumhe kabhi free magic energy nahi milti.

Active-recall flashcards

Lenz's law kya determine karta hai jo Faraday's magnitude akele nahi karta?

Induced current ki direction (minus sign ka meaning).

Lenz's law ko ek sentence mein state karo.

Induced current aisi direction mein flow karta hai ki uska magnetic field us flux change ko oppose kare jisne use produce kiya.

Lenz's law kin fundamental principle ke equivalent hai?

Conservation of energy.

Agar loop ke through flux decrease ho raha ho, toh induced field loop ke andar external field ke saath point karta hai ya against?

Uske saath (same direction), flux maintain karne ke liye.

Ek N-pole ek coil ki taraf aata hai; coil ka paas wala face kaunsa pole banta hai?

North pole (ye repel karta hai, approach oppose karta hai).

Ek N-pole coil se withdraw hota hai; paas wala face kaunsa pole?

South pole (ye attract karta hai, magnet ko rakhne ki koshish karta hai).

Length

L

ke rod ke liye jo

v

speed se field

B

mein move kare,

∣ ε ∣

kya hai?

B Lv

Sliding rod par force hamesha uski motion ke opposite kyun hoti hai?

Kyunki induced current increasing flux ko oppose karna chahiye (Lenz), drag create karta hai

F = B^{2} L^{2} v / R

Lenz sign ke saath Faraday's law likho.

ε = - \frac{d Φ _{B}}{d t}

Physically kya hota agar induced current change ko aid karta?

Self-amplifying motion → infinite free energy → energy conservation violate.

Connections

Faraday's law of induction — Lenz sign/direction supply karta hai.
Magnetic flux — wo quantity jiska change induction drive karta hai.
Right-hand rule — induced-field direction ko current direction mein convert karta hai.
Motional EMF and sliding rod — quantitative Lenz example.
Eddy currents and magnetic braking — bulk conductors mein Lenz drag.
Conservation of energy — minus sign ke peeche ki deep wajah.
Self-inductance and back-EMF — Lenz apni khud ki changing current par apply hota hai.