Class 8 Science · Chapter 4
Short answer:
Chapter 4 — Electricity: Magnetic and Heating Effects (Class 8 Science, Curiosity textbook, NCERT 2026-27) — sirf 2 mool concepts: current apne around magnetic field banata hai (Oersted, electromagnet), aur current resistance se guzarne par heat generate karta hai (heating effect, fuse, appliances). Power = V×I, Energy = P×t formula-based numericals bhi is chapter ka core part hain.
Chapter 4 poore Class 8 Science ke electricity concepts ki practical neev hai — yaha hum seekhte hain ki current sirf bulb jalane ke liye nahi, balki magnetism paida karne aur heat generate karne ke liye bhi use hota hai. Ye vahi chapter hai jo aage Class 10 ke "Magnetic Effects of Electric Current" chapter ka base banata hai. Is chapter me hum Oersted ke experiment se shuru karte hain (jisne pehli baar dikhaya ki current aur magnetism connected hain), phir electromagnet ka working aur uske real-life uses (crane, electric bell) padhte hain, aur end me current ke heating effect, electric fuse, aur ghar ki electrical safety cover karte hain — saath me kuch simple power/energy numericals bhi.
Chapter 4 Summary — 5 Minute Revision
Is chapter me humne dekha ki electric current sirf circuit me flow nahi karta — woh do important physical effects bhi paida karta hai. Pehla, magnetic effect: Oersted ke experiment ne prove kiya ki current-carrying conductor ke around magnetic field banti hai, aur isi principle par electromagnet kaam karta hai jiska strength current, turns aur core material se control hoti hai — cranes, electric bell jaise applications me use hota hai. Doosra, heating effect: jab current resistance wale conductor se guzarta hai to electrical energy heat me convert hoti hai, jo electric iron, heater, geyser jaise appliances ka basis hai — lekin yehi effect fault conditions me khatarnaak bhi ho sakta hai, isliye fuse lagaya jaata hai jo low melting point ki wajah se time par circuit todd deta hai. Saath hi Power (P=VI) aur Energy (E=Pt) ke numericals se hum electricity bill calculate karna bhi seekhte hain. Ye concepts Class 8 me foundation ban jaate hain aur aage Class 10 ke advanced magnetic-effects chapter ki taiyaari karte hain.
In-Text Questions — Solutions
Compass needle current-carrying wire ke paas kyu deflect ho jaati hai?
Kyunki current-carrying wire apne around ek magnetic field create karti hai (Oersted ka observation), aur ye field compass ki needle ko force karke apni direction se align kar deta hai.Electromagnet ki strength kis-kis factor par depend karti hai?
Electromagnet ki strength current ki magnitude, coil ke turns ki number, aur core material (soft iron sabse effective) par depend karti hai — inhe badhaane se strength badh jaati hai.Heating effect of current kab hota hai?
Jab bhi current kisi resistance wale conductor (jaise nichrome wire) se guzarta hai, kuch electrical energy heat me convert ho jaati hai — isse hi electric iron, heater, geyser kaam karte hain.Exercise Questions — Solutions (Q1–Q5)
1. Give reasons for the following: (a) An electromagnet is used in cranes to lift heavy iron scraps. (b) A coil of wire behaves like a magnet.
(a) Electromagnet ki strength current control karke change ki ja sakti hai — jab current on hota hai woh strong magnet ban jaata hai jo bhaari iron scrap ko easily uthaa leta hai, aur current off karte hi magnetism khatam ho jaati hai jisse load asaani se drop ho jaata hai. Permanent magnet me ye control possible nahi hota.
(b) Jab current ek coiled wire (solenoid) se guzarta hai, to har turn apna khud ka magnetic field banata hai aur saare turns ke fields aapas me add hoke ek strong resultant magnetic field bana dete hain jiski ek north pole aur ek south pole hoti hai — bilkul bar magnet jaisa behavior.
2. Ek electric iron 220 V par kaam karta hai aur 5 A current leta hai. Uska power rating aur 2 ghante me consume hui electrical energy (in kWh) nikalo.
P = V × I = 220 × 5 = 1100 W = 1.1 kW
Energy = P × t = 1.1 kW × 2 h = 2.2 kWh
Answer: Power rating = 1100 W, Energy consumed = 2.2 kWh (units)
3. Explain karo ki fuse wire electrical circuit ko kaise protect karta hai. Fuse wire ka melting point kaisa hona chahiye?
Fuse wire ek thin, low-melting-point metal (jaise tin-lead alloy) ka piece hota hai jo circuit me series me lagaya jaata hai. Jab short-circuit ya overload ki wajah se current normal safe value se zyada ho jaata hai, to heating effect of current ke karan fuse wire itni jaldi garam ho jaati hai ki woh pighal (melt) kar circuit ko todd deti hai — isse baaki appliances aur wiring bade current se bach jaate hain. Fuse ka melting point normal operating current par safe lekin fault current par jaldi melt hone jitna low hona chahiye, taaki damage hone se pehle hi circuit break ho jaaye.
4. Ek electric heater 750 W ka hai aur roz 4 hours chalta hai. 30 din me consume hui energy units (kWh) me nikalo, agar 1 unit ka rate ₹6 hai to mahine ka bill kya hoga?
Daily energy = (750/1000) × 4 = 3 kWh
Monthly energy = 3 × 30 = 90 kWh (units)
Bill = 90 × ₹6 = ₹540
Answer: 90 units, bill = ₹540
5. Electromagnet aur permanent magnet me koi teen differences likho.
| Electromagnet | Permanent Magnet |
|---|---|
| Magnetism sirf current flow ke time hoti hai | Magnetism hamesha rehti hai |
| Strength current/turns adjust karke control kar sakte hain | Strength fixed hoti hai |
| Poles ki polarity current direction reverse karke badal sakte hain | Polarity fixed hoti hai |
↔ Table ko side me swipe karein
Important Equations — Ek Nazar Me
| Concept | Formula / Rule |
|---|---|
| Electric Power | P = V × I (Watt) |
| Power (using resistance) | P = I² × R |
| Electrical Energy | Energy (E) = P × t |
| 1 Unit (commercial) | 1 kWh = 1000 W × 1 h |
| Oersted's Rule | Current-carrying conductor ke around magnetic field banti hai (compass deflection se detect) |
| Electromagnet strength badhane ke tareeke | Current badhao, turns badhao, soft iron core use karo |
| Heating effect condition | Jab current kisi resistance wale conductor se guzarta hai to electrical energy heat me convert hoti hai |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Electromagnet aur permanent magnet ko same samajh lena — jabki electromagnet ki magnetism sirf current flow hone tak hi rehti hai
- Power ka formula galat likhna, jaise P = V/I likh dena, jabki correct formula P = V × I hai
- Energy consumption calculate karte waqt watt ko kilowatt me convert karna bhool jaana (÷1000 karna zaroori hai)
- Fuse wire ko circuit breaker ya switch samajh lena — fuse ek baar melt hone ke baad replace karna padta hai, reset nahi hota
- Oersted ke experiment me ye bhool jaana ki compass needle sirf current ON hone par hi deflect hoti hai, current OFF hote hi wapas normal position me aa jaati hai
- Heating effect aur magnetic effect dono ko same cause samajh lena, jabki dono current ke alag-alag independent effects hain
Board-Style Important Questions
- Explain Oersted's experiment with a labelled diagram and state the conclusion drawn from it.
- An electric bulb rated 100 W, 220 V is used for 5 hours daily. Calculate the units of energy consumed in 30 days.
- Why does an electromagnet lose its magnetism as soon as the current is switched off? Give one practical use of this property.
- Describe the working of an electric fuse and explain why its melting point must be low.
- State three factors on which the strength of an electromagnet depends, and explain how each factor affects it.
- Distinguish between the magnetic effect and heating effect of electric current with one example each.
Aksar Poochhe Jaane Wale Sawaal
Class 8 Science Chapter 4 Electricity Magnetic and Heating Effects me kya-kya topics cover hote hain?
Is chapter me current ka magnetic effect (Oersted ka experiment), electromagnet aur uske uses, current ka heating effect, electric fuse, aur ghar me electrical safety cover hoti hai — plus simple numericals power aur energy consumption ke.
Oersted ka experiment kya prove karta hai?
Oersted ne dikhaya ki jab current-carrying wire ke paas compass needle rakhi jaati hai, needle deflect ho jaati hai — isse pata chalta hai ki electric current apne aas-paas magnetic field paida karta hai, yani electricity aur magnetism aapas me judi hui hain.
Electromagnet ke real-life uses kya hain?
Electromagnets ka use cranes (scrap lifting), electric bell, electric motor, MRI machines, aur maglev trains me hota hai — jaha temporary aur controllable magnetism chahiye hoti hai.
Fuse wire tin-lead alloy se hi kyu banaya jaata hai, copper se kyu nahi?
Fuse wire ka melting point low hona chahiye taaki fault current par jaldi pighal jaaye. Copper ka melting point bahut high hota hai (~1085°C) isliye woh time par nahi todega aur circuit protect nahi hoga — isliye low melting point wala tin-lead alloy use hota hai.
Electrical energy consumption ka SI unit kya hai aur ghar me kis unit me bill aata hai?
SI unit joule hai, lekin practical use ke liye 'kilowatt-hour (kWh)' use hota hai jise ghar me 'unit' bolte hain. 1 unit = 1000 watt ka appliance 1 ghante chale to jitni energy use hoti hai.
Class 8 Science Curiosity book pdf kaha se download kar sakte hain?
NCERT ki official website ncert.nic.in se latest session ki Curiosity textbook ka PDF free download kiya ja sakta hai — waha se chapter-wise PDF milte hain.
Class 8 Science — Saare Chapters
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