Chapter 4 โข Class 8 Science
โก Electricity:
Magnetic & Heating Effects
Magnetic & Heating Effects
Curiosity โ Textbook of Science for Grade 8
๐ Table of Contents
4.1 Magnetic Effect of Electric Current
@edugrown
Fig: Compass deflects
when current flows
when current flows
When electric current flows through a wire, place a magnetic compass beneath it.
- Switch ON โ compass needle deflects (moves away)
- Switch OFF โ compass needle returns to original position
- This shows current-carrying wire has a magnetic effect
๐ Key Definition
Magnetic Field โ The region around a magnet or current-carrying wire where its magnetic effect can be felt (shown by deflection of compass needle).
โก Core Concept
Magnetic Effect of Electric Current โ When electric current flows through a conductor (wire), it produces a magnetic field around it. The field disappears when current stops.
Hans Christian Oersted (1777โ1851)
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! โก๐งฒ
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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 โ nail picks up iron clips!
- Disconnect battery โ clips fall off!
- This nail + coil = Electromagnet ๐
๐ Key Definition
Electromagnet โ A current-carrying coil that behaves as a magnet. Most electromagnets have an iron core to make them stronger. It is a temporary magnet โ works only when current flows!
๐ Properties of Electromagnet
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Fig: Cylindrical coil of wire (solenoid)
Remember! Stronger Electromagnet?
More cells (more current) โ Stronger magnetMore turns of coil โ Stronger magnet
Iron core inside coil โ Much stronger magnet!
Earth is a Giant Magnet!
Deep inside Earth, liquid iron moves and creates electric currents โ generates Earth's magnetic field. Birds and fish use this to navigate! ๐ฆ๐
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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 โ lifts heavy iron/steel objects
- Current OFF โ magnetic field disappears โ 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 โ electrical energy converts to heat energy.
- Touch nichrome wire before current โ feels normal
- Switch ON for 30 sec, then OFF โ wire feels warm!
- This is the Heating Effect of Electric Current
๐ Key Definition
Heating Effect of Electric Current โ 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.
๐ Heat Generated Depends On:
- Material of the wire (nichrome has high resistance)
- Thickness of the wire (thinner โ more resistance โ more heat)
- Length of the wire (longer โ more resistance)
- Amount of current (more current โ more heat)
- Duration for which current flows
๐ Household Appliances Using Heating Effect
@edugrown
Fig: Common heating
appliances
appliances
All these work on the heating effect. They contain a coil or rod of wire called a heating element:
- ๐ก๏ธ Electric Room Heater
- ๐ณ Electric Stove
- โ Electric Kettle
- ๐ Electric Iron
- ๐ฟ Water Heating Immersion Rod
- ๐จ Hair Dryer
- ๐ก Incandescent Lamp (filament heated)
Why Nichrome for Heating Elements?
Nichrome wire offers higher resistance than copper wire of same size โ generates MORE heat for same current. Perfect for heaters! ๐ฅ
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?
โ๏ธ 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 โ produces electricity
- Chemicals get used up โ cell becomes "dead" โ ๏ธ
- 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! ๐ก
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Fig: Lemon battery โ a DIY Voltaic cell!
๐ฆ 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 โ negative terminal (โ)
- Carbon rod with metal cap โ positive terminal (+)
- Carbon rod surrounded by paste-like electrolyte
- Single use only โ cannot be recharged! Dispose after use.
โป๏ธ 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 โ 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! ๐
Battery Disposal = Important!
Even "dead" batteries contain harmful chemicals (lead, cadmium, lithium). Always dispose at e-waste recycling centres โ don't throw in regular garbage! ๐
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๐ธ Quick Revision Snapshots
Electric current flowing through a conductor produces a magnetic field around it โ 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 โ 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 โ 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.
โจ Notes by @edugrown โ Happy Learning! โจ