1.8.7 · D1 · Physics › Electromagnetism › Applications — sphere, cylinder, infinite plane
Electric charge apne aas-paas puri space mein invisible "push arrows" (ek field) bhar deta hai, aur agar tum ek khayal ki band bag kisi charge ke around draw karo, toh us bag se nikalne wale arrows ki total sankhya sirf us charge par depend karti hai jo us bag ke andar baitha hai. Jab bag ki shape charge ki symmetry se match karti hai, toh un arrows ko count karna simple multiplication ban jaata hai — aur yahi Gauss's Law ke peeche ka poora engine hai.
Yeh page parent note Applications — sphere, cylinder, infinite plane mein use hone wale har symbol ko zero se build karta hai. Agar parent ne koi symbol bina explain kiye likha hai, toh tum use pehle yahan explained paoge.
Definition Electric charge
Ek property jo kuch particles carry karte hain, jis wajah se woh ek doosre ko push ya pull karte hain. Coulombs mein measure hota hai, symbol C . Hum charge ke ek bade blob ko Q likhte hain aur ek tiny test blob ko q .
Picture: ek glowing dot. Positive charge = ek source jo arrows outward shoot karta hai; negative charge = ek sink jo arrows inward nigal jaata hai.
Yeh topic ko kyun chahiye: parent mein har formula "kitna charge hai?" se shuru hota hai — ek sphere Q hold karta hai, ek wire charge per length hold karti hai, ek sheet charge per area hold karti hai. Charge ke bina compute karne ke liye koi field hi nahi.
Picture mein arrows hi electric field hain, jise hum aage milenge.
Definition Electric field
E
Space mein har point par, E ek arrow hai jo batata hai ki ek positive test charge wahan rakhne par kis taraf (aur kitni strongly) push hoga. Upar laga chhota arrow, , matlab hai "is quantity ki ek direction bhi hai, sirf size nahi."
Size (magnitude) plain likhi jaati hai, E , newtons per coulomb mein, N/C .
Direction woh hai jis taraf arrow point karta hai.
Arrow kyun, sirf ek number kyun nahi?
Ek push tab tak bekar hai jab tak tum nahi jaante ki woh kis taraf push kar rahi hai. Ek number akela (5 N/C ) tumhe left aur right nahi bata sakta. Toh humein ek aisi cheez chahiye jo magnitude aur direction dono store kare — woh cheez ek vector hai, aur hat use mark karta hai.
Yeh topic ko kyun chahiye: E woh unknown hai jise hum har case mein solve karte hain (sphere, cylinder, plane).
Parent teen alag distance letters use karta hai. Yeh interchangeable nahi hain — inhe mix karna #1 galti hai.
Definition Distance symbols
R (capital) — object ka ek fixed size: sphere ka radius, cylinder ka radius. Yeh problem ke dauran kabhi nahi badalta.
r (small) — sphere ke centre se us point tak ki doori jahan tum field chahte ho. Yeh vary karta hai.
s (small) — cylinder/line ke axis se seedha sideways aapke point tak ki doori. Yeh vary karta hai.
Intuition Do alag small letters kyun?
Ek sphere ek point ke baare mein symmetric hai, toh doori us point se measure hoti hai → r . Ek cylinder ek line ke baare mein symmetric hai, toh doori us line se seedha bahar measure hoti hai → s . Cylinder ke liye r use karna secretly galat jagah se measure karna hai. Alag geometry, alag ruler.
Centre (sphere) se seedha door ya axis (cylinder) se seedha door point karna. Picture dekho: har arrow ruler line ke saath lie karta hai, kabhi sideways nahi.
Yeh topic ko kyun chahiye: poori trick yeh hai ki symmetry E ko radial hone par majboor karti hai aur sirf r par depend karne par (ya sirf s par), toh woh matching surface par constant hoti hai.
Real charge rarely ek single dot hota hai; yeh ek length, ek area, ya ek volume par spread hota hai. "Density" matlab hai "us cheez ke unit per kitna charge."
Definition Teen densities
λ (lambda) — linear charge density: length ke metre per coulombs, C/m . Picture: ek wire ke along charge paint kiya gaya.
σ (sigma) — surface charge density: square metre per coulombs, C/m 2 . Picture: ek flat sheet par charge paint kiya gaya.
ρ (rho) — volume charge density: cubic metre per coulombs, C/m 3 . Picture: ek solid ball mein fog ki tarah charge bhara hua.
Intuition Densities ki zaroorat hi kyun hai
Apne khayal ki bag ke andar charge find karne ke liye, tum density ko multiply karte ho jitna object ka bag ke andar hai usse:
andar ki length → Q = λ × L
andar ki area → Q = σ × A
andar ka volume → Q = ρ × 3 4 π r 3
Yahi woh Q e n c expressions hain jo parent use karta hai.
Common mistake Kaunsi density kaunsi shape ke saath jaati hai, yeh confuse karna
Fix: dimensions gino. Ek line 1-D hai → λ (per metre). Ek sheet 2-D hai → σ (per m²). Ek solid 3-D hai → ρ (per m³).
d A
Ek surface ko tiny flat patches mein kato. Har patch ka ek size hota hai (uski area, d A ) aur ek direction : arrow d A seedha patch se bahar nikalta hai, us par perpendicular, enclosed region se outward point karta hua.
d ka matlab hai "ek infinitesimally small piece of."
Picture: ek curved surface par ek tiny tile jisme se ek pin seedha uske face se bahar nikal rahi hai.
Yeh topic ko kyun chahiye: yeh measure karne ke liye ki kitne field arrows ek patch ko pierce karte hain, hum field arrow E ko patch ke apne arrow d A se compare karte hain. Woh comparison dot product hai, next.
Definition Do arrows ka dot product
E ⋅ d A = E d A cos θ , jahan θ do arrows ke beech ka angle hai.
Agar woh same taraf point karte hain (θ = 0 , cos 0 = 1 ): poori value E d A . Field patch ko sidha head-on pierce karta hai.
Agar woh perpendicular hain (θ = 9 0 ∘ , cos 9 0 ∘ = 0 ): value 0 . Field surface ke saath skim karta hai, kuch pierce nahi karta.
Intuition Dot product kyun aur plain multiplication kyun nahi?
Hum chahte hain "kitna field patch ke through jaata hai," yeh nahi ki kitna uske saath slide karta hai. Sirf E ka woh part jo d A ke along point karta hai woh count hota hai. Cosine exactly woh machine hai jo through-part ko rakhti hai aur sideways-part ko phek deti hai. Isliye ek cylinder ke end caps (field unke parallel hai) aur pillbox sides zero contribute karti hain — cos 9 0 ∘ = 0 .
Is concept ko apne topic ke roop mein dekhne ke liye Electric Flux dekho.
∮ S
∮ ek sum (∫ ) hai jisme ek circle hai, matlab "ek closed surface par add karo" — ek surface jisme koi hole nahi, jaise ek balloon skin jo ek region ko poori tarah wrap karti hai.
∮ S E ⋅ d A = (closed bag S ke har patch par E ⋅ d A add karo)
Picture: poore balloon se nikalne wale har field arrow ko count karna, jahan andar poking karne wale arrows negative count hote hain.
Gauss's law arrows-out ko charge-inside se relate karta hai. "Inside" tab hi samajh aata hai jab surface ek region ko poori tarah seal kar de — ek open sheet ka koi inside nahi hota. Isliye hum hamesha ek poori tarah sealed Gaussian surface use karte hain.
Q e n c
Tumhare chosen Gaussian surface ke andar baitha total charge — aur sirf wahi. Bag ke baahir wala charge net arrows out mein kuch contribute nahi karta.
ε 0 (epsilon-nought)
Permittivity of free space , universe ka ek fixed constant:
ε 0 = 8.85 × 1 0 − 12 C 2 / ( N ⋅ m 2 )
Yeh "charge ki matra" aur "field arrows ki matra" ke beech ka conversion factor hai. Coulomb's constant se related: k = 4 π ε 0 1 = 9 × 1 0 9 N ⋅ m 2 / C 2 (dekho Coulomb's Law ).
ε 0 se divide kyun karte hain?
Nature tumhe ek field arrow per coulomb nahi deti; woh tumhe 1/ ε 0 arrows per coulomb deti hai. ε 0 sirf exchange rate set karta hai. Bada ε 0 (zyada "permitting" medium) matlab charge per kam arrows.
Pieces ko ek saath rakhne par woh master tool milta hai jo parent shuru karta hai:
∮ S E ⋅ d A = ε 0 Q e n c
Definition Symmetry (yahan)
Ek shape mein symmetry hoti hai jab woh move/turn/flip karne ke baad identical dikhti hai. Ek sphere apne centre ke baare mein kisi bhi rotation ke baad same dikhta hai; ek infinite line same dikhti hai jab tum us par slide karo ya us ke around spin karo; ek infinite sheet har jagah same dikhti hai jahan tum khade ho.
Intuition Symmetry master key kyun hai
Gauss's law hamesha sach hai lekin aksar bekar hai, kyunki integral ke andar E wildly vary kar sakta hai aur bahar aane se mana kar sakta hai. Symmetry us variation ko forbid karti hai: agar source har direction mein same dikhta hai, toh E ko matching surface par same magnitude hona hi chahiye aur seedha d A ke saath point karna chahiye. Sirf tabhi ∮ collapse hokar E × A banta hai. Dekho Symmetry in Physics .
Source symmetry
Matching Gaussian surface
Result power law
Spherical (point-like)
Concentric sphere
E ∝ 1/ r 2
Axial (line/cylinder)
Coaxial cylinder
E ∝ 1/ s
Planar (sheet)
Pillbox
E = constant
Definition Conductor in electrostatics
Ek material jiske charges freely move kar sakte hain. Ek baar settle hone ke baad (electrostatic equilibrium): metal ke andar field zero hoti hai, aur saara excess charge outer surface par baitha hota hai. Full topic: Conductors in Electrostatics .
Yeh topic ko kyun chahiye: isliye "ek conducting shell ke andar, E = 0 " (Q e n c = 0 ) aur isliye ek conductor ki surface E = σ / ε 0 deti hai (sirf ek taraf field) instead of isolated-sheet wali σ /2 ε 0 . Yeh aage wale Parallel Plate Capacitor aur Electric Potential chapters ko bhi underlie karta hai.
Densities rho lambda sigma
Closed integral total flux
E constant so E times area
Gauss law E times area equals Q enc over eps0
Sphere Cylinder Plane results
Khud ko test karo — sirf jawab dene ke baad reveal karo.
E mein hat tumhe kya batata hai?Ki
E ek vector hai — yeh sirf size nahi, direction bhi carry karta hai.
r aur s mein difference?r = sphere ke centre se doori; s = cylinder/line ke axis se seedha bahar doori.
R (capital) kya hai?Object ka fixed radius (sphere ya cylinder); problem ke dauran vary nahi karta.
Kaunsi density wire, sheet, solid ball ke saath jaati hai? Wire → λ (C/m), sheet → σ (C/m²), solid ball → ρ (C/m³).
d A kis taraf point karta hai?Surface patch se seedha bahar, us par perpendicular, enclosed region se outward.
E ⋅ d A = 0 kab hota hai?Jab
E d A ke perpendicular ho (
θ = 9 0 ∘ ,
cos 9 0 ∘ = 0 ) — field surface ke saath skim karta hai.
∮ par circle kyun hai?Matlab hai ki integral ek closed surface par hai, toh "inside" well-defined hai.
Q e n c kya count karta hai?Sirf woh charge jo tumhare chosen Gaussian surface ke andar hai — bahar wala charge irrelevant hai.
ε 0 ki value aur meaning?8.85 × 1 0 − 12 C 2 / ( N⋅m 2 ) ; charge aur field arrows ke beech exchange rate.
Symmetry Gauss ko solvable kyun banati hai? Yeh
E ko matching surface par constant hone aur
d A ke parallel hone par majboor karti hai, toh
∮ collapse hokar
E × A ban jaata hai.
Equilibrium mein conductor ke andar field? Zero; saara excess charge outer surface par baitha hota hai.