Class 10 Science · Chapter 12
Short answer:
Is chapter me 21 in-text questions (saat page-wise sets) aur 18 exercise questions — total 39 solved questions — NCERT ke exact order me cover kiye gaye hain. Coverage: magnetic field aur field lines ke properties, seedhe conductor ka field (right-hand thumb rule), circular loop aur solenoid ka field, current-carrying conductor par force (Fleming ka left-hand rule), electric motor aur split ring ka kaam, electromagnetic induction, electric generator (AC vs DC, slip ring vs commutator, Fleming ka right-hand rule), aur domestic electric circuits (live/neutral/earth wire, short circuit, overloading, fuse aur earthing). Saath me ek rules reference table, 6 sabse common exam mistakes, board-style practice questions marks-wise, aur 6 FAQs.
Ye chapter formula ka nahi, direction ka hai — har rule me kaun si ungli kya represent karti hai, bas wahi decide karta hai ki number sahi aayega ya ulta. Class 10 Science ka Chapter 12 batata hai ki current sirf heat aur light hi nahi deta, uske around ek magnetic field bhi banti hai. Aur ye do-tarfa rasta hai: current se magnetism (Oersted ka experiment, electromagnet, electric motor) aur magnetism se current (Faraday ka electromagnetic induction, electric generator). Chapter ke aakhir me yahi physics aapke ghar ki wiring tak pahunch jati hai — live-neutral-earth wire, fuse, short circuit aur overloading. Padhte waqt ek cheez pakde rehna: har baar poochho ki current kis direction me hai, field kis direction me hai, aur force ya motion kis direction me nikal rahi hai. Ye teen cheezein set ho gayi to poora chapter set ho gaya.
Chapter 12 Summary — 5 Minute Revision
1. Magnetic Field aur Field Lines
- Magnetic field = magnet ke aas-paas ka wo region jahan uska magnetic force feel hota hai. Ye ek vector quantity hai — magnitude bhi hai, direction bhi. SI unit: tesla (T).
- Field line ki direction = us point par rakhi compass needle ke north pole ke point karne ki direction.
- Magnet ke bahar field lines N se S ki taraf jati hain; magnet ke andar S se N — isliye har field line ek closed loop hoti hai.
- Jahan lines paas-paas (crowded) hoti hain wahan field strong; jahan door-door hoti hain wahan weak. Bar magnet me lines sabse zyada crowded poles par hoti hain — yani field poles par sabse strong hai, centre par nahi.
- Do field lines kabhi cross nahi karti — agar karti to us point par compass needle ko do alag directions milti, jo impossible hai.
- Uniform field = barabar spacing wali parallel straight lines (jaise horse-shoe magnet ke poles ke beech ka region).
2. Oersted ka Discovery
Current-carrying wire ke paas rakhi compass needle deflect ho jati hai — yani current ke around magnetic field banti hai. Current ki direction ulti karo to needle bhi ulti taraf deflect hoti hai. Yahi poore chapter ki neenv hai.
3. Straight Conductor ka Magnetic Field — Right-Hand Thumb Rule
- Field lines wire ke around concentric circles (concentric vritt) hoti hain, jinka centre wire par hota hai. Circles ka plane wire ke perpendicular hota hai.
- Right-hand thumb rule (Maxwell's corkscrew rule): daaye haath me wire ko is tarah pakdo ki angootha (thumb) current ki direction me point kare — to mudi hui ungliyan (curled fingers) field ki direction batati hain.
- Field strength: current badhne par badhti hai (B ∝ I), aur wire se door jaane par ghatti hai (B ∝ 1/r). Isliye wire ke paas circles paas-paas, door jaakar door-door.
4. Circular Loop ka Magnetic Field
- Loop ke har chhote tukde ke around concentric circles banti hain, par loop ke centre par sabhi lines lagbhag straight aur ek hi direction me ho jati hain — is liye centre par field sabse strong.
- Field ki strength badhti hai: current badhane se, loop ki radius ghatane se, aur turns badhane se. n turns wale coil me field n guna ho jata hai (kyunki har turn ka field ek hi direction me judta hai).
- Direction nikalne ka tarika: loop ke kisi ek tar par right-hand thumb rule lagao — ya seedha: agar aapki taraf se dekhne par current anticlockwise hai to us taraf loop ka North pole hai aur field aapki taraf (loop se bahar) nikal rahi hai; clockwise hai to wo South pole hai aur field andar ja rahi hai.
5. Solenoid aur Electromagnet
- Solenoid = cylinder par close-close lipti hui insulated wire ke bahut saare circular turns.
- Current bahne par solenoid ka field pattern bilkul bar magnet jaisa hota hai — ek sira North pole, doosra South pole.
- Solenoid ke andar field uniform hoti hai — har point par same strength aur same direction, aur lines parallel straight lines hoti hain. (Ye chapter ka bahut poocha jane wala point hai.)
- Solenoid ke andar soft iron ka core daal do to bahut strong magnet ban jata hai — usi ko electromagnet kehte hain. Current band, magnetism band.
6. Current-carrying Conductor par Force — Fleming ka Left-Hand Rule
- Magnetic field me rakhe current-carrying conductor par force lagta hai (kyunki field field se interact karti hai).
- Force maximum tab hota hai jab current ki direction field ke perpendicular (90°) ho; current aur field parallel ho to force zero.
- Fleming's left-hand rule: baaye haath ka pehli ungli (forefinger) = magnetic Field, beech wali ungli (middle finger) = Current, angootha (thumb) = Force / motion — teeno ko ek doosre ke mutually perpendicular rakho.
- Yaad rakhne ka tarika: Forefinger→Field, Centre→Current, thumb→thrust (force).
7. Electric Motor
- Principle: magnetic field me rakhe current-carrying coil par force lagta hai; coil ke do opposite sides par current opposite direction me hone se force bhi opposite hoti hai, jisse couple/torque banta hai aur coil ghoomti hai.
- Energy conversion: electrical → mechanical.
- Split ring (commutator) ka kaam: har aadhe chakkar (half rotation) ke baad coil me current ki direction ulti kar dena, jisse force ki direction bani rahe aur coil ek hi direction me lagatar ghoomti rahe.
- Commercial motors me: permanent magnet ki jagah electromagnet, zyada turns wali coil, aur soft iron core — teeno milkar power badhate hain. Coil + core = armature.
8. Electromagnetic Induction (Faraday)
- Coil aur magnet ke beech relative motion ho ya coil se guzarne wala magnetic field badle, to coil me induced current paida hota hai.
- Current tabhi banta hai jab change ho rahi ho — magnet coil ke andar sthir rakhne par galvanometer zero dikhata hai.
- Induced current tab zyada jab: relative motion tez ho, magnet strong ho, coil me turns zyada ho.
- Fleming's right-hand rule: daaye haath ka pehli ungli (forefinger) = magnetic Field, angootha (thumb) = Motion (conductor ki movement), beech wali ungli (middle finger) = induced Current — teeno mutually perpendicular.
9. Electric Generator
- Principle: electromagnetic induction — magnetic field me coil ko ghumane par usme induced current banta hai. Energy conversion: mechanical → electrical.
- Brushes ka kaam: ghoomti hui coil (rings ke through) se sthir bahri circuit tak current pahunchana — sliding electrical contact.
- AC generator me do slip rings — har aadhe chakkar par current ki direction apne aap badalti hai, isliye output alternating current.
- DC generator me split ring (commutator) — ye har aadhe chakkar par external circuit se connection swap kar deta hai, jisse bahri circuit me current ek hi direction me bahta hai.
- Bharat me supply AC, 220 V, 50 Hz — yani current apni direction 1 second me 100 baar badalta hai (har 1/100 s par).
10. Domestic Electric Circuits
| Wire | Insulation ka rang (NCERT) | Kaam |
|---|---|---|
| Live | Red | Supply ka high-potential wire (220 V) |
| Neutral | Black | Return path, potential lagbhag zero |
| Earth | Green | Metal body ko zameen se jodta hai — safety |
↔ Table ko side me swipe karein
- Short circuit: live aur neutral wire seedhe touch ho jaye (insulation kharab ya fault), resistance lagbhag zero, current bahut zyada badh jata hai → heat aur aag.
- Overloading: ek hi socket/circuit par bahut saare high-power appliances chalane se ya supply voltage badhne se current safe limit paar kar jata hai.
- Fuse: live wire ke saath series me juda patla tar — nirdharit current se zyada bahne par pighal kar circuit tod deta hai.
- Earthing: agar live wire galti se metal body ko chhoo jaye to current earth wire ke through zameen me chala jata hai, body ka potential nahi badhta → user ko shock nahi lagta.
In-Text Questions — Solutions
Q1. Why does a compass needle get deflected when brought near a bar magnet?
Compass needle khud ek chhota bar magnet hi hai — uska apna north aur south pole hota hai. Jab use bar magnet ke paas laate hain, to bar magnet ke magnetic field me aakar needle ke poles par force lagta hai: needle ka north pole magnet ke south pole ki taraf kheencha jata hai aur needle ka south pole magnet ke north pole ki taraf. In do opposite forces se needle par ek torque banta hai aur needle ghoom kar us point ki field ki direction me set ho jati hai — isi ghoomne ko hum deflection kehte hain.
Q1. Draw magnetic field lines around a bar magnet.
(Diagram based question — likhne wala description yaad rakho, kyunki board me diagram banana hota hai.)
- Bar magnet ke bahar lines North pole se nikalti hain aur South pole me ghusti hain; magnet ke andar wahi lines South se North jaati hain — is tarah har line closed loop banati hai.
- Lines poles ke paas sabse crowded (paas-paas) hoti hain — yahi batata hai ki field poles par sabse strong hai.
- Magnet ke beech wale hisse (middle) ke around lines kaafi door-door aur baahar ki taraf phaili hui hoti hain — field wahan weak hai.
- Har line par teer ka nishaan (arrow) lagana zaroori hai — arrow North se South (bahar ki taraf) dikhna chahiye.
- Koi bhi do lines aapas me cross nahi karengi.
Q2. List the properties of magnetic lines of force.
- Ye closed continuous curves hoti hain: magnet ke bahar N→S, magnet ke andar S→N.
- Kisi bhi point par line ki tangent us point par magnetic field ki direction batati hai — yani jahan compass needle ka north pole point karega.
- Do field lines kabhi ek doosre ko nahi kaatti.
- Jahan lines paas-paas hain wahan field strong, jahan door-door hain wahan weak.
- Barabar spacing wali parallel straight lines ka matlab hai uniform field.
- Ye lines ek dusre ko side se dhakelti hain (repel) aur apni lambai ke saath sikudne ki koshish karti hain — isi se attraction/repulsion samjha ja sakta hai.
Q4. Why don't two magnetic field lines intersect each other?
Kyunki agar do field lines kisi point par kaat deti, to us ek hi point par field ki do alag-alag directions hoti (dono lines ki apni-apni tangent). Iska matlab wahan rakhi compass needle ko ek hi samay par do directions me point karna padta — jo asambhav hai. Kisi bhi point par magnetic field ki sirf ek hi direction ho sakti hai, isliye field lines kabhi intersect nahi karti.
Q1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop anticlockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Loop table ke plane me hai aur upar se dekhne par current anticlockwise hai.
Right-hand thumb rule ka use: loop ke kisi ek chhote hisse ko daaye haath se is tarah pakdo ki angootha us hisse me current ki direction me ho — mudi ungliyan batati hain ki loop ke andar wale region me field upar ki taraf nikal rahi hai.
- Loop ke andar (inside): magnetic field table ke plane ke perpendicular, upar ki taraf — yani observer ki taraf bahar aati hui. Loop ka upar wala face North pole ki tarah kaam karta hai.
- Loop ke bahar (outside): field neeche ki taraf — table ke plane me andar jaati hui.
Yaad rakhne ka shortcut: Anticlockwise = North pole aapki taraf (field bahar aati hui), Clockwise = South pole aapki taraf (field andar jaati hui).
Q2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Uniform field ka matlab — us poore region me field ki magnitude aur direction dono same hain.
Diagram: ek hi direction me jaati hui parallel straight lines banao jinke beech ka gap har jagah barabar ho, aur har line par ek hi taraf ka arrow lagao. Barabar spacing = barabar strength; parallel lines = same direction.
Practical example: horse-shoe (U-shaped) magnet ke do poles ke beech ka region lagbhag uniform field deta hai; aise hi lambe current-carrying solenoid ke andar ka region.
Q3. Choose the correct option. The magnetic field inside a long straight solenoid-carrying current: (a) is zero (b) decreases as we move towards its end (c) increases as we move towards its end (d) is the same at all points.
Sahi uttar: (d) is the same at all points.
Lambe solenoid ke andar field lines parallel straight lines hoti hain aur barabar spacing par hoti hain — yani field uniform hai, har point par same magnitude aur same direction. Ye ek permanent bar magnet ke andar jaisi hi situation hai.
Note: Field kam hone ki baat solenoid ke sire ke bahar par lagoo hoti hai, andar ke uniform region par nahi.
Q1. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum
Sahi uttar: (c) velocity aur (d) momentum.
Magnetic field charged particle par jo force lagata hai wo hamesha uski velocity ke perpendicular hota hai. Perpendicular force sirf direction badalta hai, speed nahi (kyunki wo koi kaam nahi karta, kinetic energy same rehti hai).
- Mass — nahi badlegi.
- Speed — nahi badlegi (magnitude same).
- Velocity — badlegi, kyunki direction badal rahi hai (velocity vector hai).
- Momentum — badlega, kyunki momentum = mass × velocity, aur velocity ki direction badal rahi hai.
Q2. In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
Rod par lagne wala force current (I), magnetic field (B) aur field me maujood rod ki lambai (L) — teeno ke seedhe anupat me badhta hai. Force badhega to displacement bhi badhega.
- (i) Current badhane par — force badhega, isliye rod AB ka displacement badh jayega.
- (ii) Zyada strong horse-shoe magnet — field B badhega, force badhega, displacement badhega.
- (iii) Rod AB ki lambai badhane par — field me rakhi effective lambai badhegi, force badhega, displacement badhega.
Q3. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (b) towards east (c) downward (d) upward.
Sahi uttar: (d) upward (upar ki taraf).
Positive charge hai, isliye conventional current ki direction wahi hai jidhar particle ja raha hai — west.
Fleming's left-hand rule lagao:
- Thumb (angootha) = Force → North ki taraf rakho (deflection idhar hai).
- Middle finger (beech ki ungli) = Current → West ki taraf rakho.
- Ab jo forefinger (pehli ungli) in dono ke perpendicular banti hai, wo upar (upward) point karti hai — aur forefinger hi magnetic field ki direction batati hai.
Isliye magnetic field vertically upward hai.
Q11. State Fleming's left-hand rule.
Fleming's left-hand rule: apne baaye haath ki pehli ungli (forefinger), beech wali ungli (middle finger) aur angoothe (thumb) ko is tarah failao ki teeno ek doosre ke mutually perpendicular ho. Ab:
- Forefinger → magnetic Field ki direction
- Middle finger → Current ki direction
- Thumb → conductor par lagne wale Force (motion) ki direction
Ye rule wahan lagta hai jahan current se motion nikalni ho — yani electric motor me. Yaad rakhne ka tarika: Forefinger → Field, Central finger → Current, thumb → thrust.
Q2. What is the principle of an electric motor?
Electric motor is siddhant par kaam karta hai ki magnetic field me rakhe current-carrying conductor (coil) par force lagta hai, jiski direction Fleming's left-hand rule se milti hai.
Rectangular coil ki do opposite bhujaon (arms) me current ulti-ulti direction me bahta hai, isliye un par force bhi opposite directions me lagta hai — ek upar, doosra neeche. Ye do barabar aur opposite forces milkar ek torque (couple) banati hain jo coil ko ghumati hai.
Energy conversion: electrical energy → mechanical energy.
Q3. What is the role of the split ring in an electric motor?
Split ring (commutator) har aadhe chakkar (half rotation) ke baad coil me bahne wale current ki direction ulti kar deta hai, kyunki ye coil ke sire aur brushes ke beech connection swap kar deta hai.
Kyu zaroori hai: aadha chakkar poora hone par coil ki jo bhuja pehle upar thi wo neeche aa jati hai. Agar current ki direction wahi rehti, to force ulti taraf lagta aur coil aage ghoomne ki jagah wapas peeche jhoolne lagti. Current ulat jaane se force ki direction bhi ulat jati hai — isliye torque hamesha ek hi direction me lagta rehta hai aur coil lagatar ek hi direction me ghoomti hai.
Q1. Explain different ways to induce current in a coil.
Coil me current tabhi induce hota hai jab coil se guzarne wali magnetic field badalti hai. Iske teen tarike:
- Magnet ko hilana: coil sthir rakhkar bar magnet ko coil ke andar ghusao ya bahar nikalo — galvanometer deflection dikhayega.
- Coil ko hilana: magnet sthir rakhkar coil ko uske paas laao ya door le jao — phir bhi current induce hoga. (Zaroori sirf relative motion hai.)
- Doosre coil ka current badalna: paas rakhi ek aur coil (primary) me current on/off ya kam-zyada karo — uske badalte field se hamari coil (secondary) me current induce hota hai. Isi ko mutual induction kehte hain. Coil ko field me ghumakar bhi current induce kiya ja sakta hai — yahi generator ka tarika hai.
Q2. State the principle of an electric generator.
Electric generator electromagnetic induction ke siddhant par kaam karta hai: jab koi conductor (rectangular coil) magnetic field me ghumaya jata hai, to coil se guzarne wala magnetic field lagatar badalta hai aur usme induced current paida ho jata hai.
Induced current ki direction Fleming's right-hand rule se nikalti hai. Energy conversion: mechanical energy → electrical energy.
Q3. Name some sources of direct current.
Direct current (DC) ke sources — jahan current hamesha ek hi direction me bahta hai:
- Cell / dry cell (torch wali battery)
- Battery (car ki lead-acid battery, mobile battery)
- DC generator (dynamo) — split ring/commutator wala
- Solar cell (photovoltaic cell)
Q4. Which sources produce alternating current?
Alternating current (AC) ke sources — jahan current apni direction samay-samay par badalta rehta hai:
- AC generator (alternator) — do slip rings wala
- Power house se aane wali ghar ki mains supply — Bharat me 220 V, 50 Hz, yani current 1 second me 100 baar apni direction badalta hai (har 1/100 second par).
- Hydro-electric, thermal aur nuclear power plants — sabhi me AC generator hi lage hote hain.
Q5. Choose the correct option. A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each: (a) two revolutions (b) one revolution (c) half revolution (d) one-fourth revolution
Sahi uttar: (c) half revolution (aadha chakkar).
Har aadhe chakkar ke baad coil ki jo bhuja pehle magnetic field me upar ki taraf badh rahi thi, wo ab neeche ki taraf badhne lagti hai — yani uski motion ki direction field ke sapeksh ulat jati hai. Fleming's right-hand rule ke anusaar motion ulti hone par induced current bhi ulta ho jata hai. Isliye har half rotation par current ki direction ek baar badalti hai — yahi AC banne ka karan hai.
Q1. Name two safety measures commonly used in electric circuits and appliances.
- Fuse (ya MCB): har circuit ke live wire ke saath series me juda hota hai. Nirdharit se zyada current bahne par fuse ka tar pighal jata hai aur circuit toot jata hai — short circuit aur overloading se aag rukti hai.
- Earthing: metallic body wale appliances (iron, fridge, toaster) ki body green earth wire se zameen se jodi jaati hai. Agar live wire galti se body ko chhoo jaye to leakage current zameen me chala jata hai, body ka potential nahi badhta aur user ko shock nahi lagta.
Q2. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Diya hai: P = 2 kW = 2000 W, V = 220 V, circuit rating = 5 A
I = P / V = 2000 / 220 ≈ 9.09 A
Oven ko lagbhag 9.09 A current chahiye, jabki circuit sirf 5 A ka rated hai.
Natija: circuit me safe limit se lagbhag dugna current bahega — yani overloading ho jayegi. Wires zyada garam ho jayenge; fuse pighal kar circuit tod dega (ya MCB trip ho jayega). Agar fuse na hota to wiring garam hokar insulation jal sakta hai aur aag lag sakti hai.
Q3. What precaution should be taken to avoid the overloading of domestic electric circuits?
- Ek hi socket par bahut saare appliances ek saath mat jodo — khaaskar high-power wale (heater, geyser, AC, iron).
- Har circuit me sahi rating ka fuse/MCB zaroor lagao (na bahut zyada rating ka, na jugaad wala tar).
- Wire ki current-carrying capacity load ke hisaab se honi chahiye — patle tar par bhaari load mat chalao.
- Insulation kharab ya damaged wire turant badlo, taaki short circuit na ho.
- High-power appliances ke liye alag circuit banwao.
Poore Class 10 Science ke handwritten colour notes
IITian & district toppers ke banaye short notes — revision-ready, diagram ke saath. Board se pehle poora syllabus 3 din me revise.
Exercise Questions — Solutions (Q1–Q18)
Q1. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire (b) The field consists of straight lines parallel to the wire (c) The field consists of radial lines originating from the wire (d) The field consists of concentric circles centred on the wire
Sahi uttar: (d) The field consists of concentric circles centred on the wire.
Lambe seedhe current-carrying wire ke around magnetic field lines concentric circles banati hain jinka centre wire par hota hai aur jinka plane wire ke perpendicular hota hai. Wire ke paas circles chhote aur paas-paas (strong field), door jaakar bade aur door-door (weak field). Direction right-hand thumb rule se milti hai.
Q2. The phenomenon of electromagnetic induction is (a) the process of charging a body (b) the process of generating magnetic field due to a current passing through a coil (c) producing induced current in a coil due to relative motion between a magnet and the coil (d) the process of rotating a coil of an electric motor
Sahi uttar: (c) producing induced current in a coil due to relative motion between a magnet and the coil.
Electromagnetic induction ka matlab hi hai — badalti hui magnetic field (ya magnet aur coil ke beech relative motion) se coil me current paida hona. Option (b) uska ulta hai (current se field), aur (d) motor hai, generator nahi.
Q3. The device used for producing electric current is called a (a) generator (b) galvanometer (c) ammeter (d) motor
Sahi uttar: (a) generator.
Generator mechanical energy ko electrical energy me badal kar current produce karta hai. Galvanometer sirf chhote current ki presence aur direction detect karta hai, ammeter current maapta hai, aur motor electrical energy ko mechanical energy me badalta hai (current banata nahi, use karta hai).
Q4. The essential difference between an AC generator and a DC generator is that (a) AC generator has an electromagnet while a DC generator has permanent magnet. (b) DC generator will generate a higher voltage. (c) AC generator will generate a higher voltage. (d) AC generator has slip rings while the DC generator has a commutator.
Sahi uttar: (d) AC generator has slip rings while the DC generator has a commutator.
AC generator me do alag-alag slip rings hote hain — har ring hamesha coil ke ek hi sire se juda rehta hai, isliye external circuit me current har aadhe chakkar par apni direction badalta rehta hai (AC milta hai).
DC generator me split ring / commutator hota hai, jo har aadhe chakkar par brushes se connection swap kar deta hai — isliye bahri circuit me current ek hi direction me bahta hai (DC milta hai).
Baaki options galat hain: dono me electromagnet ya permanent magnet ho sakta hai, aur voltage design par depend karta hai, AC/DC hone par nahi.
Q5. At the time of short circuit, the current in the circuit (a) reduces substantially (b) does not change (c) increases heavily (d) vary continuously.
Sahi uttar: (c) increases heavily.
Short circuit me live aur neutral wire seedhe aapas me touch ho jaate hain, jisse circuit ka resistance lagbhag zero ho jata hai. Ohm's law se
I = V / R
R bahut chhota hone par I bahut bada ho jata hai. Isi bhaari current se wires garam hokar aag lag sakti hai — isliye fuse/MCB lagaya jata hai.Q6. State whether the following statements are true or false. (a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with a green insulation is usually the live wire of an electric supply.
- (a) False. Motor ulta kaam karta hai — electrical energy → mechanical energy. Mechanical se electrical wala kaam generator karta hai.
- (b) True. Generator electromagnetic induction par hi chalta hai — field me coil ghumane se induced current banta hai.
- (c) True. Lambe circular coil (solenoid) ke andar/centre par field lines parallel straight lines hoti hain, yani field uniform hai.
- (d) False. Green insulation earth wire ka hota hai. Live wire ka insulation red hota hai (neutral ka black).
Q7. List three sources of magnetic fields.
- Permanent magnet — jaise bar magnet ya horse-shoe magnet.
- Current-carrying conductor — seedha tar, circular loop ya solenoid; current bahne par uske around field banti hai.
- Electromagnet — soft iron core wala current-carrying solenoid.
(Prithvi khud bhi ek magnetic field ka source hai — isi wajah se compass needle hamesha north-south me set hoti hai.)
Q8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Solenoid magnet jaisa kaise: jab solenoid me current bahta hai to uske around ka field pattern bilkul bar magnet ke pattern jaisa hota hai — ek sira North pole ban jata hai aur doosra South pole. Field lines ek sire se nikal kar doosre me ghusti hain, aur solenoid ke andar uniform hoti hain. Aise solenoid ko dhage se latkao to wo ghoom kar north-south me set ho jayega, bilkul bar magnet ki tarah.
Poles pata karna (bar magnet se): ek known bar magnet ka North pole solenoid ke ek sire ke paas laao —
- Agar repulsion (dhakka) ho, to solenoid ka wo sira bhi North pole hai (kyunki repulsion sirf same poles me hota hai — yahi magnetism ka pakka test hai).
- Agar attraction ho, to wo sira South pole hai.
Bina magnet ke bhi kar sakte ho: solenoid ke sire ki taraf se dekhne par agar current anticlockwise dikhe to wo North pole, clockwise dikhe to South pole.
Q9. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Force maximum tab hota hai jab conductor me current ki direction magnetic field ki direction ke perpendicular (90°) ho.
Jaise-jaise current aur field ke beech ka kona 90° se kam hota jata hai, force ghatta jata hai; aur jab current field ke parallel (0° ya 180°) ho jata hai to force zero ho jata hai.
Q10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Uttar: magnetic field vertically downward (neeche ki taraf) hai.
Reasoning step by step:
- Electron beam back wall se front wall ki taraf ja raha hai — yani aapke saamne ki taraf.
- Electron par negative charge hai, isliye conventional current ki direction ulti hogi — yani front se back ki taraf, aapki taraf.
- Deflection (force) aapki daayi taraf hai.
Fleming's left-hand rule: thumb ko daayi taraf (force), middle finger ko apni taraf/peeche ki taraf (current) rakho — to forefinger apne aap neeche point karegi. Forefinger hi field batati hai, isliye magnetic field vertically downward hai.
Sabse badi galti yahan: electron ki direction ko hi current maan lena. Electron ke liye current hamesha ulti direction me hoti hai — ye step chhoote to answer bilkul ulta (upward) aa jayega.
Q11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Diagram ke labels (ye sab likhna zaroori hai): rectangular coil ABCD, horse-shoe/permanent magnet ke N aur S pole, split ring (commutator) ke do halves, do carbon brushes, battery aur switch/key, aur coil ke ghoomne ki direction ka arrow.
Principle: magnetic field me rakhe current-carrying coil par force lagta hai, jiski direction Fleming's left-hand rule se milti hai.
Working:
- Battery se current brushes aur split ring ke through coil ABCD me bahta hai.
- Coil ki do opposite bhujaon (AB aur CD) me current opposite direction me bahta hai, isliye Fleming's left-hand rule se un par force bhi opposite lagti hai — ek bhuja par upar ki taraf, doosri par neeche ki taraf.
- Ye do barabar aur ulti forces milkar ek couple/torque banati hain, jisse coil apni axis par ghoomne lagti hai.
- Aadha chakkar poora hone par split ring ke halves brushes badal lete hain, current ki direction ulat jati hai, torque phir se usi direction me lagta hai — coil lagatar ghoomti rehti hai.
Split ring ka function: har aadhe chakkar ke baad coil me current ki direction reverse karna, taaki torque hamesha ek hi direction me lage aur coil ek hi direction me lagatar ghoome. Isi wajah se ise commutator kehte hain.
Energy conversion: electrical → mechanical.
Q12. Name some devices in which electric motors are used.
- Electric fan aur exhaust fan
- Washing machine
- Refrigerator aur air conditioner (compressor motor)
- Mixer-grinder / juicer
- Water pump aur borewell motor
- Electric drill machine, MP3/CD player, computer ka cooling fan
Q13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?
- (i) Magnet ko coil me ghusane par: coil se guzarne wala magnetic field badhta hai, isliye coil me induced current banta hai aur galvanometer ki needle ek taraf deflect hoti hai.
- (ii) Magnet ko bahar nikalne par: field ab ghat raha hai, isliye phir se induced current banta hai par uski direction ulti ho jati hai — galvanometer ki needle doosri taraf deflect hoti hai.
- (iii) Magnet ko coil ke andar sthir rakhne par: koi relative motion nahi, isliye field me koi change nahi — koi current induce nahi hoga aur galvanometer zero par hi rahega.
Nichod: induced current paida karne ke liye field ka badalna zaroori hai, sirf field ka hona kaafi nahi.
Q14. Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Haan, coil B me current induce hoga.
Karan: Coil A me current badalne par uske around ki magnetic field bhi badalti hai. Kyunki coil B paas rakhi hai, uske through guzarne wala magnetic field bhi badalta hai — aur badalte field se coil B me induced current paida ho jata hai. Ise electromagnetic induction (yahan par mutual induction) kehte hain.
Dhyan raho: coil B me current sirf usi kshan bahega jab A ka current badal raha ho (on karte waqt, off karte waqt, ya rheostat ghumate waqt). Agar A me steady current bahta rahe to B me kuch induce nahi hoga.
Q15. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.
(i) Right-hand thumb rule (Maxwell's corkscrew rule): daaye haath me conductor ko is tarah pakdo ki angootha current ki direction me ho — to mudi hui ungliyan magnetic field ki direction batayengi (field concentric circles me hoti hai).
(ii) Fleming's left-hand rule: baaye haath ki pehli ungli = magnetic Field, beech wali ungli = Current, angootha = Force — teeno mutually perpendicular.
(iii) Fleming's right-hand rule: daaye haath ki pehli ungli = magnetic Field, angootha = Motion (conductor ki gati), beech wali ungli = induced Current — teeno mutually perpendicular.
Pakka yaad rakho: left hand = motor (current do, motion lo), right hand = generator (motion do, current lo). Dono me forefinger hamesha Field hi hoti hai.
Q16. Explain the underlying principle and working of an electric generator. What is the function of brushes?
Principle: Electromagnetic induction — jab conductor/coil magnetic field me ghumaya jata hai to coil se guzarne wali field lagatar badalti hai aur usme induced current paida ho jata hai. Energy conversion: mechanical → electrical.
Working:
- Rectangular coil ABCD ko strong magnetic field (N aur S pole ke beech) me axle par rakha jata hai aur bahar se ghumaya jata hai.
- Ghoomte waqt coil ki ek bhuja upar ki taraf move karti hai aur doosri neeche — is wajah se dono me induced current opposite directions me banta hai, jo circuit me judkar ek hi direction me bahta hai. Direction Fleming's right-hand rule se milti hai.
- Aadha chakkar poora hone par bhujaon ki motion ki direction badal jati hai, isliye induced current ki direction bhi badal jati hai — yahi AC hai.
- AC generator: do slip rings — external circuit me current har half rotation par direction badalta hai.
DC generator: split ring (commutator) — connection swap ho jane se external circuit me current ek hi direction me bahta hai.
Brushes ka function: carbon brushes rings par sate rehte hain aur ghoomti hui coil se sthir bahri circuit tak current pahunchate hain — yani rotating part aur fixed circuit ke beech sliding electrical contact ka kaam karte hain, bina taar ulajhe.
Q17. When does an electric short circuit occur?
Short circuit tab hota hai jab live wire aur neutral wire seedhe aapas me touch ho jaate hain. Iske common karan:
- Wire ka insulation ghis jana ya damage ho jana
- Appliance me fault aa jana
- Wiring me dhile ya khule connection, ya paani/nami ka aa jana
Kya hota hai: circuit ka resistance lagbhag zero ho jata hai, isliye
I = V / R
ke anusaar current bahut bhaari maatra me bahne lagta hai. Wires turant garam ho jaati hain aur aag lag sakti hai — isi liye live wire ke saath series me fuse ya MCB lagaya jata hai jo circuit ko turant tod deta hai.Q18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Earth wire ka function: ye green insulation wala wire appliance ki metallic body ko ghar ke bahar zameen me gadi metal plate se jodta hai, jisse leakage current ke liye bahut kam resistance wala rasta ban jata hai.
Metallic appliances ko earth karna kyu zaroori hai:
- Agar kisi fault me live wire appliance ki metal body ko chhoo jaye, to body high potential par aa jayegi.
- Aise me koi us body ko chhoo le to current uske sharir se hokar zameen me jayega — ghatak electric shock lag sakta hai.
- Earthing hone par ye current sharir ki jagah earth wire ke aasan raste se zameen me chala jata hai. Body ka potential zameen ke barabar (lagbhag zero) bana rehta hai.
- Saath hi, achanak bahut zyada current bahne se fuse/MCB trip ho jata hai aur supply kat jati hai.
Isi liye iron, refrigerator, geyser, washing machine jaise metal-body appliances me hamesha 3-pin plug (jiska mota pin earth ka hota hai) istemal hota hai.
Important Equations — Ek Nazar Me
Rules Reference Table — kaun sa rule, kahan, aur kaun si ungli kya
| Rule | Kis kaam ke liye | First finger (forefinger) | Second finger (middle finger) | Thumb (angootha) |
|---|---|---|---|---|
| Right-hand thumb rule (Maxwell's corkscrew rule) daaya haath |
Seedhe current-carrying conductor ke around magnetic field ki direction nikalne ke liye | Ungliyan alag-alag nahi — saari ungliyan wire ke around mod kar (curl) pakdi jaati hain; ye magnetic field ki (concentric circles ki) direction batati hain | Current ki direction (conductor ke saath) | |
| Fleming's left-hand rule baaya haath — MOTOR |
Magnetic field me rakhe current-carrying conductor par force / motion ki direction nikalne ke liye | Magnetic Field (N → S) | Current (conventional, + se −) | Force / motion (thrust) |
| Fleming's right-hand rule daaya haath — GENERATOR |
Magnetic field me hilte/ghoomte conductor me induced current ki direction nikalne ke liye | Magnetic Field (N → S) | Induced current | Motion (conductor kis taraf ja raha hai) |
| Clock-face rule (circular loop / solenoid ka sira) |
Loop ya solenoid ke sire ka pole pehchanne ke liye | Us sire ki taraf se dekho: current Anticlockwise → wo sira North pole (field aapki taraf bahar aati hui) | current Clockwise → wo sira South pole (field andar jaati hui) | ||
| Compass needle convention | Kisi bhi point par field ki direction define karne ke liye | Field ki direction wahi hai jidhar us point par rakhi compass needle ka North pole point karta hai | ||
↔ Table ko side me swipe karein
Yaad rakhne ke pakke short-cuts
| Trick | Matlab |
|---|---|
| Left = Motor, Right = Generator | Motor me current DETE hain, motion LETE hain (left hand). Generator me motion DETE hain, current LETE hain (right hand). |
| Forefinger hamesha Field | Dono Fleming rules me pehli ungli ka matlab kabhi nahi badalta — sirf thumb aur middle finger ka role swap hota hai. |
| F-C-T (left hand) | Forefinger–Field, Central finger–Current, Thumb–Thrust (force). |
| ACW = North | Anti-ClockWise dikhne wala sira North pole; clockwise wala South pole. |
↔ Table ko side me swipe karein
Kuch zaroori quantitative points (relation, formula ratta nahi)
| Situation | Field / force kis par depend karta hai |
|---|---|
| Seedha conductor | Current badhne par field badhti hai; conductor se doori badhne par field ghatti hai |
| Circular loop | Current badhne par badhti, radius badhne par ghatti, turns (n) badhne par n guna |
| Solenoid | Andar field uniform; current, turns per unit length aur soft iron core se badhti hai |
| Conductor par force | Current, magnetic field aur field me maujood lambai badhne par force badhta hai; current ⊥ field par maximum, parallel par zero |
| Induced current | Relative motion tez hone par, magnet strong hone par, aur coil ke turns zyada hone par badhta hai |
| Bharat ki AC supply | 220 V, 50 Hz → current 1 second me 100 baar direction badalta hai |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Fleming ke left aur right hand rule ka aapas me swap ho jana. Ye is chapter ki sabse zyada dohrayi jane wali galti hai. Yaad rakho: left hand = motor (current do, motion milegi), right hand = generator (motion do, current milega). Aur dono me forefinger hamesha Field hi rehti hai — sirf thumb aur middle finger ka kaam badalta hai (motor me thumb = force, generator me thumb = motion). Exam me pehle likho ki device motor hai ya generator, phir hi haath uthao.
- Solenoid ke andar field ko 'sire ki taraf badhti/ghatti hai' likh dena. Lambe solenoid ke andar field poori tarah uniform hoti hai — har point par same magnitude aur same direction, lines parallel straight. Ghatne wali baat solenoid ke sire ke bahar ke liye hai. MCQ me 'is the same at all points' hi sahi option hota hai, 'decreases towards its end' trap hai.
- Bar magnet ke centre par field sabse strong bata dena. Bilkul ulta hai — field lines poles par sabse crowded hoti hain, isliye field poles par sabse strong aur magnet ke beech (centre) me sabse weak. Field line diagram banate waqt lines poles par paas-paas aur beech me door-door dikhani chahiye.
- AC aur DC generator ka antar galat likhna — 'AC me electromagnet, DC me permanent magnet'. Ye difference magnet ka nahi, rings ka hai: AC generator me do slip rings (current har half rotation par direction badalta hai), DC generator me split ring / commutator (external circuit me current ek hi direction me). Magnet dono me kuch bhi ho sakta hai, aur voltage ka AC/DC hone se koi lena-dena nahi.
- Electron beam wale questions me electron ki direction ko hi current maan lena. Electron par negative charge hai, isliye conventional current electron ki gati ke bilkul ulti direction me hoti hai. Ye ek step chhootne se Fleming ke rule ka poora answer ulta aa jata hai (upward ki jagah downward). Positive charge (alpha particle, proton) ke liye current usi direction me hoti hai jidhar particle ja raha hai — dono cases alag hain.
- Motor aur generator ki energy conversion aur split ring ka kaam gadd-madd kar dena. Motor: electrical → mechanical; yahan split ring har half rotation par current ki direction ulti karta hai taaki coil ek hi taraf ghoomti rahe. Generator: mechanical → electrical; DC generator me split ring connection swap karke external circuit me current ek hi direction me rakhta hai. Aur brushes ka kaam dono me ek hi hai — rotating coil aur fixed circuit ke beech contact — brushes current ki direction nahi badalte, ye kaam ring ka hai.
Board-Style Important Questions
- 1 mark: Lambe seedhe current-carrying conductor ke around banne wali magnetic field lines ka aakaar kaisa hota hai? Unki direction kis rule se nikalti hai?
- 1 mark: Domestic circuit me earth wire ke insulation ka rang kya hota hai, aur wo kis se juda hota hai?
- 2 marks: Do magnetic field lines aapas me kyun nahi kaat sakti? Karan sahit samjhaiye.
- 3 marks: Fleming's right-hand rule likhiye. Kisi coil me induced current paida karne ke teen alag-alag tarike bataiye.
- 3 marks: AC generator aur DC generator me mool antar kya hai? Dono me lage rings ka naam aur kaam likhiye.
- 5 marks: Electric motor ka labelled diagram banaiye. Uska principle aur working samjhaiye. Split ring aur brushes ka kaam alag-alag likhiye.
Quick Quiz — Score Check Karein
Q1. Bar magnet ke andar magnetic field lines ki direction kya hoti hai?
Q2. Do magnetic field lines kabhi ek doosre ko cross kyun nahi karti?
Q3. Ek seedhe current-carrying conductor ke around magnetic field ki direction nikalne ke liye kaunsa rule use hota hai?
Q4. Ek straight conductor me current double kar diya jaaye aur distance same rahe, to us point par magnetic field B ka kya hoga (B ∝ I aur B ∝ 1/r ke hisaab se)?
Q5. Ek circular loop me agar turns ki sankhya 3 guni (triple) kar di jaaye, current same rakhte hue, to centre par field ki strength ka kya hoga?
Q6. Solenoid ke andar magnetic field ke baare me kaunsa statement sahi hai?
Q7. Fleming ke left-hand rule me beech wali ungli (middle finger) kya represent karti hai?
Q8. Current-carrying conductor par magnetic field me force kab maximum hota hai?
Q9. Electric motor me split ring (commutator) ka kya kaam hota hai?
Q10. Electromagnetic induction me coil me induced current kab paida hota hai?
Aksar Poochhe Jaane Wale Sawaal
Left-hand rule aur right-hand rule me confusion hamesha ho jata hai — koi pakka tarika hai yaad rakhne ka?
Haan. Sirf ek line yaad rakho: LEFT = motor, RIGHT = generator. Motor me hum current dete hain aur motion lete hain, isliye baaya haath. Generator me hum motion dete hain aur current lete hain, isliye daaya haath. Ek aur cheez pakki hai — dono rules me pehli ungli (forefinger) hamesha magnetic Field hi batati hai, wo kabhi nahi badalti. Sirf angootha aur beech ki ungli apna kaam swap karte hain: motor me angootha force hai, generator me angootha motion hai. Exam me pehle likho ki question motor ka hai ya generator ka, uske baad hi haath uthao.
Right-hand thumb rule aur Fleming's right-hand rule — ye do alag cheezein hain kya?
Bilkul alag hain, naam mil-jul jaata hai. Right-hand THUMB rule sirf ek kaam ke liye hai: current-carrying wire ke around magnetic field ki direction nikalna — usme wire ko mutthi me pakadte hain, angootha current ki taraf, mudi ungliyan field batati hain. Fleming's RIGHT-HAND rule bilkul alag hai — usme teen ungliyan alag-alag failai jaati hain aur wo generator me induced current ki direction batata hai (forefinger field, thumb motion, middle finger induced current). Ek field banane ke liye, doosra current dhoondhne ke liye.
Magnet ko coil ke andar rakhe rehne par current kyun nahi banta?
Kyunki induced current banane ke liye magnetic field ka BADALNA zaroori hai, sirf field ka hona kaafi nahi. Magnet sthir hone par coil se guzarne wali field constant rehti hai, koi change nahi, isliye galvanometer zero dikhata hai. Jaise hi magnet ko andar ghusao ya bahar nikalo, field badalti hai aur current induce ho jata hai. Isi liye generator me coil ko lagatar ghumana padta hai — rukte hi bijli band.
Solenoid aur electromagnet me kya farak hai?
Solenoid matlab close-close lipte hue insulated wire ke bahut saare circular turns. Usme current bahane par wo bar magnet ki tarah behave karta hai. Ab agar us solenoid ke andar soft iron ka core daal do, to field bahut zyada strong ho jati hai aur use electromagnet kehte hain. Sabse badi baat — electromagnet ka magnetism current par nirbhar hai: current band, magnetism khatam. Isliye crane, electric bell aur relay me permanent magnet ki jagah electromagnet lagaya jaata hai. Core hamesha soft iron ka hi hota hai, steel ka nahi, kyunki soft iron current hatte hi apna magnetism chhod deta hai.
Ghar ki supply 220 V, 50 Hz hai — iska matlab current ek second me kitni baar direction badalta hai?
50 Hz ka matlab hai 50 complete cycles per second. Har ek cycle me current do baar apni direction badalta hai (ek baar aage, ek baar peeche), isliye ek second me kul 100 baar direction badalti hai — yani har 1/100 second par. Ye NCERT me seedha likha hua point hai aur board me chhote question ke roop me aata rehta hai.
Earthing aur fuse dono safety ke liye hain — dono ka kaam alag kaise hai?
Fuse circuit ko bachata hai, earthing insaan ko bachati hai. Fuse live wire ke saath series me juda patla tar hai — jab current safe limit se zyada bahta hai (short circuit ya overloading me), to wo pighal kar circuit tod deta hai aur aag lagne se rokta hai. Earthing bilkul alag kaam karti hai — agar live wire galti se appliance ki metal body ko chhoo jaye, to leakage current green earth wire ke aasan raste se zameen me chala jata hai, body ka potential nahi badhta, aur chhoone wale ko shock nahi lagta. Isi liye metal-body appliances me 3-pin plug hota hai jiska mota pin earth ka hota hai.
Class 10 Science — Saare Chapters
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Class 10 Science ke saare chapters ke colour handwritten short notes — diagrams, formulas aur important points ek jagah.
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