Chapter 4 • Class 8 Science
⚡ Electricity:
Magnetic & Heating Effects
Magnetic & Heating Effects
Curiosity — Textbook of Science for Grade 8
📋 Table of Contents
4.1 Magnetic Effect of Electric Current
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Fig: Compass deflects
when current flows
when current flows
When electric current flows through a wire, place a magnetic compass beneath it.
- Switch ON → compass needle deflects (moves away)
- Switch OFF → compass needle returns to original position
- This shows current-carrying wire has a magnetic effect
📌 Key Definition
Magnetic Field → The region around a magnet or current-carrying wire where its magnetic effect can be felt (shown by deflection of compass needle).
⚡ Core Concept
Magnetic Effect of Electric Current → When electric current flows through a conductor (wire), it produces a magnetic field around it. The field disappears when current stops.
Hans Christian Oersted (1777–1851)
In 1820, while giving a demonstration, he noticed that whenever an electrical circuit was closed or opened, the needle of a magnetic compass nearby deflected. He was the first to discover the link between electricity and magnetism! ⚡🧲
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4.1.1 Electromagnets
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Fig: Coil of wire
connected to cell
connected to cell
When a coil of wire carries electric current, it behaves like a magnet!
- Wrap insulated wire tightly around an iron nail
- Connect ends to a battery → nail picks up iron clips!
- Disconnect battery → clips fall off!
- This nail + coil = Electromagnet 🎉
📌 Key Definition
Electromagnet → A current-carrying coil that behaves as a magnet. Most electromagnets have an iron core to make them stronger. It is a temporary magnet — works only when current flows!
🔑 Properties of Electromagnet
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Fig: Cylindrical coil of wire (solenoid)
Remember! Stronger Electromagnet?
More cells (more current) → Stronger magnetMore turns of coil → Stronger magnet
Iron core inside coil → Much stronger magnet!
Earth is a Giant Magnet!
Deep inside Earth, liquid iron moves and creates electric currents → generates Earth's magnetic field. Birds and fish use this to navigate! 🐦🐟
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4.1.2 Lifting Electromagnets
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Fig: Crane with lifting
electromagnet
electromagnet
- Very strong electromagnets hung to cranes
- Current ON → lifts heavy iron/steel objects
- Current OFF → magnetic field disappears → objects released
- Used in factories & scrap yards to sort heavy metals
Practical Applications of Magnetic Effect: Electromagnets, Electric bells, Motors, Fans, Loudspeakers, MRI machines!
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4.2 Heating Effect of Electric Current
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Fig: Nichrome wire
heating experiment
heating experiment
When current flows through a conductor, it faces resistance → electrical energy converts to heat energy.
- Touch nichrome wire before current → feels normal
- Switch ON for 30 sec, then OFF → wire feels warm!
- This is the Heating Effect of Electric Current
📌 Key Definition
Heating Effect of Electric Current → When electric current passes through a conductor, it gets heated. This is because the conductor offers resistance to the flow of current, converting electrical energy into heat energy.
📊 Heat Generated Depends On:
- Material of the wire (nichrome has high resistance)
- Thickness of the wire (thinner → more resistance → more heat)
- Length of the wire (longer → more resistance)
- Amount of current (more current → more heat)
- Duration for which current flows
🏠 Household Appliances Using Heating Effect
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Fig: Common heating
appliances
appliances
All these work on the heating effect. They contain a coil or rod of wire called a heating element:
- 🌡️ Electric Room Heater
- 🍳 Electric Stove
- ☕ Electric Kettle
- 👔 Electric Iron
- 🚿 Water Heating Immersion Rod
- 💨 Hair Dryer
- 💡 Incandescent Lamp (filament heated)
Why Nichrome for Heating Elements?
Nichrome wire offers higher resistance than copper wire of same size → generates MORE heat for same current. Perfect for heaters! 🔥
Industrial Use!
Steel manufacturing industries use high-temperature electric furnaces to melt and recycle scrap steel using the heating effect of electric current.
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4.3 How Does a Battery Generate Electricity?
⚗️ 4.3.1 Voltaic Cell (Galvanic Cell)
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Fig: Simple Voltaic Cell
One of the earliest electric cells, named after Alessandro Volta & Luigi Galvani.
- Two metal rods (electrodes) of different materials
- Placed in a liquid called electrolyte (weak acid or salt solution)
- Chemical reaction between rods & electrolyte → produces electricity
- Chemicals get used up → cell becomes "dead" ☠️
- Dead Voltaic cell cannot be recharged
Lemon Battery! (Activity 4.6)
You can make a Voltaic cell using lemons! Copper wire = positive electrode, Iron nail = negative electrode, Lemon juice = electrolyte. Multiple lemons in series can glow an LED! 💡
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Fig: Lemon battery — a DIY Voltaic cell!
🔦 4.3.2 Dry Cells
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Fig: Dry cell &
its internal structure
its internal structure
Most commonly used electric cell today. Called "dry" because electrolyte is a thick moist paste (not liquid).
- Zinc container → negative terminal (−)
- Carbon rod with metal cap → positive terminal (+)
- Carbon rod surrounded by paste-like electrolyte
- Single use only → cannot be recharged! Dispose after use.
♻️ 4.3.3 Rechargeable Batteries
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Fig: Common rechargeable
batteries
batteries
- Can be recharged and reused many times
- Prevents wastage, saves money
- Used in: Laptops, phones, cameras, inverters, electric vehicles
- After many charge-discharge cycles → they wear out slowly
- Most common today: Lithium-ion (Li-ion) battery
Future: Solid-State Batteries!
Scientists are working on batteries that replace liquid/paste electrolytes with solid materials. These will be safer, charge faster and last longer! 🚀
Battery Disposal = Important!
Even "dead" batteries contain harmful chemicals (lead, cadmium, lithium). Always dispose at e-waste recycling centres — don't throw in regular garbage! 🌍
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📸 Quick Revision Snapshots
Electric current flowing through a conductor produces a magnetic field around it → called the magnetic effect of electric current.
A current-carrying coil that behaves as a magnet = Electromagnet. Iron core makes it stronger.
Strength of electromagnet ↑ by: more current, more turns of coil, or iron core.
Lifting electromagnets are used in cranes at factories & scrap yards to lift heavy iron/steel objects.
Current through a conductor → generates heat = Heating Effect of Electric Current. Used in heaters, irons, kettles, etc.
A cell/battery generates electric current due to chemical reactions inside it.
Rechargeable batteries can be recharged & reused multiple times. Most common = Li-ion battery.
✨ Notes by @edugrown — Happy Learning! ✨