This MCQ module is based on: Heating Effect of Electric Current
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Heating Effect of Electric Current
🎓 Class 8
Science
CBSE
Theory
Ch 4 — Materials: Metals and Non-Metals
⏱ ~28 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="chemistry" difficulty="basic"]
Probe and Ponder
Turn on a room heater — the coils glow orange. Turn on a bulb — the tiny wire inside glows blindingly white. Turn on a mobile charger — it gets slightly warm. Why does current heat some wires a lot, some a little, and others barely?
- Why don't the copper wires in your home's wiring glow like a heater?
- What special metal is hidden inside the little glass bulb of an incandescent lamp?
- When a fuse blows, has your house's safety actually worked — or failed?
- Why can a single overloaded power strip start a fire?
4.5 Heating Effect of Electric Current
Every wire has some resistance. When current is forced through this resistance, the wire heats up — exactly like your palms heat up when you rub them together against friction. This is the heating effect of electric current.
Key Idea: Electrical energy → Heat energy. The higher the resistance of a wire and the larger the current, the more heat is produced.
🔬 Activity 4.3 — Two Wires, One CurrentL3 Apply
🤔 Predict first: If you pass the same current through a thin copper wire and an equally thin nichrome wire, which one will heat up more?
- Take a short piece of nichrome wire and a similar piece of copper wire.
- Connect each, one at a time, across a battery of 4–6 cells through a switch.
- Observe each wire carefully (do not touch — they get HOT).
Observation: The nichrome wire glows bright red-hot. The copper wire becomes only slightly warm. Why? Nichrome has much higher resistance than copper — so the same current does much more heating work in nichrome.
Resistance of Different Metals
Metals differ widely in how much they resist current. Arranged from highest to lowest resistance (for a wire of the same size):
Nichrome > Iron > Steel > Copper > Silver
That is why heating appliances use nichrome, while house wiring uses copper — we want heat in one, but NOT in the other!
4.6 The Incandescent Lamp (Filament Bulb)
An ordinary glass bulb contains one of engineering's cleverest tricks — a tiny filament made of tungsten.
Why Tungsten?
Sky-high melting point
Tungsten melts at about 3400°C. The filament glows at ~2500°C without melting.
Coiled for more length
Coiling packs a long wire into a tiny space → much higher resistance → more heat & light.
Inert gas inside
Argon (or nitrogen) surrounds the filament so it cannot burn up in oxygen.
4.7 Heating Appliances — The Nichrome Family
Take a close look inside any electric heater — you will see coiled silvery wire glowing red. This is nichrome, an alloy of nickel, chromium, and a little iron. Nichrome is ideal for heaters because it has:
- High resistance — produces lots of heat for a given current.
- High melting point (~1400°C) — does not melt when glowing red-hot.
- Does not rust or oxidise easily — lasts for years.
Common principle: Current flows through a nichrome coil inside → coil glows / gets hot → heat transfers to water, bread, clothes, or air.
🔬 Activity 4.4 — Spot the NichromeL3 Apply
🤔 Predict first: Open (under adult supervision, device UNPLUGGED) an old electric iron or kettle. Where will you find the nichrome?
- Choose a broken, unplugged electric iron or kettle.
- With an adult, open the outer casing carefully.
- Look for the heating element — usually a coil of silvery wire.
- Compare its thickness and colour with normal copper wire.
Finding: The heating element is nichrome — silvery-grey, usually wound as a coil over a ceramic (insulating) former. The wire that brings current to the element is copper.
4.8 Electrical Safety — Fuses and MCBs
The heating effect is useful in appliances, but dangerous if it happens where we don't want it. Too much current in ordinary house wiring can melt the insulation and start a fire. Hence, every home needs a safety device that shuts off current when it becomes too much.
The Humble Fuse
A fuse is a short, thin wire made of a tin–lead alloy with a low melting point. When too much current flows, the fuse heats up, melts, and breaks the circuit — cutting off the current before bigger damage can happen.
⚠️ Very important: Never replace a blown fuse with ordinary copper wire! Copper has a very high melting point — it won't melt even when dangerously high current flows. Always use a fuse of the correct rating.
Modern Replacement — The MCB
Modern homes use Miniature Circuit Breakers (MCBs) instead of fuses. An MCB uses a small switch that automatically trips (turns OFF) when current becomes too high. Once the fault is fixed, you simply flip the MCB back to ON — no wires to replace.
Safety Rules Everyone Must Know
Don't overload sockets
Plugging many high-wattage devices into one strip can overheat wiring.
Use proper earthing
The green wire safely carries any leaked current into the earth — saves you from shocks.
Keep water away
Never touch a switch or appliance with wet hands — water conducts.
🎯 Try It: Spot the Safety Hazard L4 Analyse
Click each setup: is it Safe or Unsafe?
1. Wet floor near heater
Electric heater running in a bathroom with water on the floor.
2. Correct-rated fuse used
Electrician replaces a blown 5 A fuse with a fresh 5 A fuse.
3. Ten plugs in one strip
Heater + iron + kettle + TV + charger all in one extension.
4. Copper wire instead of fuse
Someone replaces a blown fuse with copper wire.
5. Switch off before repair
Main MCB turned OFF before fixing a wall plug.
6. Bare wire joints
Exposed copper joints in the wall with no insulation tape.
📋 Competency-Based Questions
Meera's family got a new electric kettle. Within a week, the kitchen fuse blew out. Her father replaced the fuse — but instead of using another fuse, he used a thick copper wire "so it won't blow out again." Three days later, the socket in the kitchen melted and caught a small fire.
Q1. L2 Understand Why is nichrome, and not copper, used as the heating element in a kettle?
Answer: B. High resistance produces enough heat; high melting point prevents the coil from melting.
Q2. L1 Remember Name the metal used for filaments in incandescent bulbs and give its melting point. (Short Answer)
Answer: Tungsten, melting point approximately 3400°C.
Q3. L4 Analyse Meera's father replaced the fuse with copper wire. Explain in 2–3 sentences why this caused the fire. (3 marks)
Answer: Copper has a very high melting point, so it did not melt even when the current became dangerously high. With no fuse to break the circuit, the excess current continued flowing through the wiring, heating the insulation and socket until they melted and caught fire. The correct action is to use a fuse of the rated amperes.
Q4. L3 Apply Fill in the blank: An MCB is an electrical safety device that _______ automatically when current exceeds a safe limit.
Answer: trips (switches OFF).
Q5. L5 Evaluate HOT: Filament bulbs are being replaced by LED bulbs in most Indian homes. Give two scientific reasons why LEDs are better. (3 marks)
Hint: (1) Filament bulbs waste about 90% of the energy as heat, while LEDs convert most of it into light — so LEDs are much more energy-efficient. (2) LEDs last 20–40 times longer than filament bulbs because there is no delicate filament to burn out — less e-waste and lower electricity bills.
🔗 Assertion–Reason Questions
Assertion (A): Copper wire is used for household wiring but not as heating element.
Reason (R): Copper has very low resistance, so it does not heat up significantly when current flows.
Answer: A. Low resistance makes copper ideal for carrying current without wasting energy as heat.
Assertion (A): Tungsten filament is kept in an atmosphere of inert gas.
Reason (R): At very high temperatures, tungsten would react with oxygen and burn out quickly.
Answer: A. Inert gases like argon prevent combustion, letting the filament last much longer.
Assertion (A): A fuse is made of a material with high melting point.
Reason (R): A fuse must melt quickly when excess current flows.
Answer: D. A is false — fuses use a tin-lead alloy with a low melting point. R is true: fast melting is exactly why the alloy is chosen.
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