Lenz's law — opposing induced current: Question bank

1.8.27 · D5 · Physics › Electromagnetism › Lenz's law — opposing induced current

Definition Is page ke symbols — pehle yeh padho

Kisi bhi trap se pehle, har symbol ko theek se samajhte hain taaki kuch surprise na kare.

Magnetic flux $Φ_{B}$ — magnetic field $B$ ko ek loop ko ched karte arrows ke bundle ki tarah socho. Flux yeh hai ki kitne arrows loop ke area se guzar rahe hain — ek "kitna field net mein pakda gaya" number. Ek uniform field mein area $A$ wale flat loop ke liye, $Φ_{B} = B A cos θ$ , jahan $θ$ $B$ aur loop ke normal (loop ke face se seedha bahar nikla ek arrow) ke beech ka angle hai. Agar field seedha loop se guzre, toh $θ = 0$ aur $Φ_{B} = B A$ maximum hota hai; agar field loop ke plane ke saath saath skim kare, toh $θ = 9 0^{\circ}$ aur $Φ_{B} = 0$ . Dekho Magnetic flux.
Induced EMF $ε$ — woh "electrical push" (electromotive force, volts mein measure hoti hai) jo changing flux ek loop ke around create karta hai. Yahi induced current ko drive karta hai. Faraday's law kehta hai $ε = - \frac{d Φ _{B}}{d t}$ , matlab push flux kitni tezi se change ho raha hai ke barabar hai. Dekho Faraday's law of induction.
Drag force $F$ — woh mechanical force jo induced current tab push back karta hai jab kuch move karta hai. Resistance $R$ wali rails par field $B$ mein speed $v$ se move karte length $L$ ke rod ke liye, yeh hai $F = \frac{B ^{2} L ^{2} v}{R}$ , hamesha motion ke against point karta hai. Kyunki baaki sab fixed hai, $F$ $v$ ke saath badhta hai — iska matlab hai $F \propto v$ (" $F$ $v$ ke proportional hai"). Dekho Motional EMF and sliding rod.
Kisi word ke around ==double equals== sirf is vault ka highlighter hai — ek cue ki woh word ek key term hai jo yaad karne layak hai. Iska koi physics meaning nahi hai.

Woh ek sentence jis se neeche sab kuch nikalta hai: induced current change in flux ko oppose karta hai, field ko nahi, directly motion ko nahi, aur current ko khud ko bhi nahi. Yeh pinned rakhho.

Neeche diya figure convention aur dono core scenarios pictures mein fix karta hai — shuru karne se pehle ek nazar daalo.

Figure — Lenz's law — opposing induced current

True ya false — justify karo

Trap: lagbhag har ek "almost true" hai. Jo half galat hai wahi tumhe pakadna hai.

Induced current hamesha external field

B

ke opposite flow karta hai.

False. Yeh flux ki change ko oppose karta hai. Agar flux decrease ho raha hai, toh induced field

B

ke saath hi point karta hai taaki use prop up kar sake.

Ek coil ke andar motionless ruka hua steady magnet ek induced current drive karta hai.

False. Flux mein koi change nahi matlab

\frac{d Φ _{B}}{d t} = 0

, isliye induced EMF

ε = 0

. Lenz change ko oppose karta hai; ek constant flux mein oppose karne ko kuch nahi.

Lenz's law ko sirf conservation of energy se derive kiya ja sakta hai.

True. Agar current change mein help karta toh self-amplifying motion aur free energy milti; sirf opposing direction hi mechanical work ko heat ke against balance karta hai.

Magnet ki speed double karne se induced EMF ki magnitude double ho jaati hai.

True (roughly).

ε = - \frac{d Φ _{B}}{d t}

flux ke rate of change ke saath scale karta hai, aur do guna tezi se move karna flux ko lagbhag do guna tezi se change karta hai.

Lenz's law ki drag force ek chalte magnet ko dead stop par la sakti hai aur phir use peeche dhakka de sakti hai.

False. Drag force

F

\propto v

hai; jaise

v \to 0

force

\to 0

. Yeh kinetic energy ko drain karta hai lekin motion ko kabhi reverse nahi karta.

Agar tum magnet ko flip karo (S-pole pehle) coil ki taraf aate hue, toh induced current direction reverse ho jaata hai.

True. Pole flip karne se coil ke through

B

reverse ho jaata hai, isliye flux change reverse hota hai, aur opposing current bhi iske saath reverse ho jaata hai.

Induced current tab bhi flow karta agar coil ek open (tooti hui) ring hoti.

False. Ek EMF

ε

phir bhi induce hota hai, lekin koi closed path nahi hone se koi current flow nahi hoti — aur isliye practically koi opposing force nahi. Lenz direction deta hai, lekin current ke liye circuit chahiye.

Ek coil jo ek uniform field mein move kar rahi hai, poori tarah field ke andar, Lenz drag feel karti hai.

False. Agar poora loop uniform field ke andar fully rehe, toh enclosed flux

Φ_{B} = B A

change nahi hota, isliye koi induced current nahi aur koi drag nahi. Dekho Motional EMF and sliding rod.

Error dhundho

Har statement mein ek broken step hai. Use name karo.

"N-pole approach karta hai → coil ka near-face S-pole ban jaata hai flux rakhne ke liye, isliye magnet attract hota hai."

Error: ek approaching magnet flux badhata hai, isliye coil ko increase oppose karna chahiye → near-face N ban jaata hai aur repel karta hai. S-pole/attract case withdrawing magnet ke liye hota hai.

"Page mein jaata flux badh raha hai, isliye induced field ko bhi page mein point karna chahiye match karne ke liye."

Error: ek increase ko oppose karne ke liye induced field page ke bahar point karta hai, uske saath nahi. "Match" bilkul ulta hai.

"Maine wire par right-hand rule se current direction find ki, phir field figure out ki."

Error: order reversed hai. Pehle "change ko oppose karo" se loop ke andar required field direction decide karo, phir current read off karne ke liye Right-hand rule use karo.

"Magnet ek copper tube mein brake karne mein koi energy lose nahi karta — magnetic forces koi kaam nahi karti."

Error: ek single charge par magnetic force koi kaam nahi karti, lekin yahan source (gravity / tumhara haath) drag force

F

ke against kaam karta hai, aur woh kaam metal mein heat ban jaata hai. Energy conserved hai, free nahi. Dekho Eddy currents and magnetic braking.

"Faraday's law

ε = - \frac{d Φ _{B}}{d t}

pehle se hi current direction deta hai, isliye Lenz's law redundant hai."

Error: Faraday induced EMF

ε

ki magnitude deta hai; minus sign meaningless hai jab tak tum sign convention (normal + circulation ka choice, jaise upar bataya) fix nahi karte, aur Lenz's law precisely woh rule hai jo us sign ko physically interpret karta hai. Dekho Faraday's law of induction.

"Kyunki drag force hamesha motion ko oppose karti hai, isliye yeh eventually magnet ki velocity reverse kar degi."

Error: motion oppose karne ka matlab sirf decelerate karna hai. Kyunki

F \propto v

, force exactly tab zero ho jaati hai jab speed zero ho jaati hai — yeh slow kar sakti hai lekin direction kabhi flip nahi kar sakti.

Why questions

Slogan se nahi, mechanism se jawab do.

Kyun induced current change ko oppose karta hai help karne ki bajaye?

Agar help karta, toh motion self-amplify hoti aur kuch se energy generate hoti, Conservation of energy ka violation hota. Opposition hi woh direction hai jo tumse electrical energy ke liye payment karata hai.

ε = - \frac{d Φ _{B}}{d t}

mein minus sign kyun hai?

Jab tum normal + circulation convention fix karo, toh minus hi Lenz's law hai: yeh current ko iss tarah force karta hai ki uska apna field flux change se ladte, isliye mechanical work in equals heat out. Isse drop karo aur energy bookkeeping fail ho jaati hai.

Ek copper pipe se guzarta magnet plastic pipe se slower kyun girta hai?

Chalta magnet copper ke har ring se guzarte

Φ_{B}

ko change karta hai, looping eddy currents induce karta hai jinke fields magnet ke approach/departure ko oppose karte hain — ek velocity-dependent drag force. Plastic un currents ko carry nahi kar sakta.

Sliding-rod drag force motion ke against kyun point karti hai chahe rod kisi bhi direction mein move kare?

Jis bhi direction se enclosed flux badhe usse oppose kiya jaata hai, aur ek area increase ko oppose karne ka matlab hamesha ek force

F

hai jo expansion resist karta hai. Motion reverse karo aur dono flux trend aur force flip ho jaate hain,

v

ke anti-parallel rehte hain.

Lenz's law ko "loop chahta hai flux same rahe" kyun describe kiya jaata hai?

Induced field hamesha ek increase cancel karne ya ek decrease replenish karne ke liye point karta hai — kisi bhi tarah yeh total flux

Φ_{B}

ko uski previous value ki taraf nudge karta hai, jaise ek thermostat change resist karta hai.

Ek faster magnet "infinite braking" runaway kyun cause nahi karta?

Opposing force speed ke saath badhti hai (

F \propto v

), lekin yeh sirf already present kinetic energy dissipate karti hai; yeh tumhari supply ki energy se zyada nahi ho sakti, isliye smoothly decelerate karta hai

v = 0

ki taraf jahan force bhi vanish ho jaati hai.

Ek self-inductor mein induced current ka direction apni khud ki changing current ko kyun oppose karta hai?

Changing current apna khud ka flux change karta hai, aur Lenz us change ko oppose karta hai — ek back-EMF produce karta hai jo current ke increase ya decrease ko resist karta hai. Dekho Self-inductance and back-EMF.

Edge cases

Woh scenarios jahan naive rule ko dhyan se apply karna padta hai.

Exactly us instant par induced current kya hai jab flux maximum ho (turning point of

Φ_{B}

Maximum par

\frac{d Φ _{B}}{d t} = 0

, isliye

ε = 0

aur current momentarily zero hai — chahe flux khud large ho. Rate of change, value nahi, induction drive karta hai.

Ek loop rotate hota hai taaki uska plane

B

ke parallel rahe (normal

B

ke perpendicular). Flux? Induced current?

Normal

⊥ B

ke saath,

Φ_{B} = B A cos 9 0^{\circ} = 0

us instant par, lekin jaise yeh us orientation se rotate hota hai, flux sabse tezi se change ho raha hota hai, isliye induced current wahan maximum par hota hai. Dekho Magnetic flux.

Ek magnet ek loop ke paas aata hai, plane se guzarta hai, aur door jaata hai. Current kya karta hai?

Yeh approach par ek taraf flow karta hai (repel), zero ho jaata hai jab magnet plane cross karta hai jahan flux peak karta hai aur

\frac{d Φ _{B}}{d t} = 0

hota hai, phir recession par reverse ho jaata hai (attract). Current sign change karta hai, flux rate ke sign ko track karta hai.

Do identical magnets ek loop ki opposite faces se equal speed se approach karte hain. Net induced current?

Agar dono loop mein same sense mein flux add karein, effects add ho jaati hain; agar woh opposite senses mein flux add karein, toh flux changes cancel ho ke zero net induced current de sakte hain chahe do moving magnets ho.

Ek perfectly circular loop uniform field mein uniformly expand karta hai jo uske perpendicular hai. Induced current ki direction?

Growing area matlab increasing flux

Φ_{B} = B A

, isliye induced current aise flow karta hai ki uska field andar

B

ko oppose kare — sign sirf flux ke trend par depend karta hai, kisi "cut" wire par nahi.

Agar resistance

R \to \infty

(open circuit) ho toh induced current ka kya hota hai?

EMF

ε

abhi bhi present hai (Lenz abhi bhi ek direction assign karta hai) lekin

I = ε / R \to 0

: koi current nahi, koi opposing force nahi, koi heat nahi. Tum magnet ko "free mein" move kar sakte ho kyunki kuch dissipate nahi hota.

Ek superconducting loop ka resistance

R \to 0

hai. Jab tum iska flux change karne ki koshish karo toh kya hota hai?

R \to 0

ke saath koi bhi tiny EMF ek huge induced current drive karta hai, jiska field attempted change ko almost perfectly cancel kar deta hai — isliye

Φ_{B}

essentially apni initial value par frozen rehta hai. Zero resistance loop ko sabse zyada strong flux-defender banata hai, "flux freely flows" ka opposite.

Connections

Faraday's law of induction — EMF magnitude supply karta hai; Lenz iske sign interpret karta hai.
Magnetic flux — woh quantity $Φ_{B}$ jiske change ko yahan har question track karta hai.
Right-hand rule — required field direction ko current direction mein convert karta hai.
Motional EMF and sliding rod — "loop fully inside field" edge case aur drag force $F$ .
Eddy currents and magnetic braking — copper-pipe traps.
Conservation of energy — woh reason ki "opposes" direction forced kyun hai.
Self-inductance and back-EMF — Lenz circuit ke apne current par apply hota hai.