Notes Chapter 4 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

Electricity: Magnetic and Heating Effects

1. Introduction

• Electricity shows two important effects:
  1. Magnetic effect
  2. Heating effect

2. Magnetic Effect of Electric Current

2.1 Discovery

• Activity: A compass needle deflects when placed near a current-carrying wire.
• Current produces magnetic field around the wire.
• Stops when current is switched off.
• Hans Christian Oersted (1820): discovered this effect.

2.2 Electromagnet

• A coil of wire connected to a battery behaves like a magnet.
• Putting an iron core (nail/rod) inside the coil makes it stronger.
• Called an Electromagnet (temporary magnet).

Factors affecting strength:

• Amount of current.
• Number of turns in coil.
• Nature of core (iron makes it stronger).

Uses:

• Electric bell, motors, fans, speakers, telephones, cranes.

2.3 Lifting Electromagnets

• Used in factories/scrap yards.
• Lift heavy iron/steel by switching current ON.
• Release load by switching OFF.

2.4 Earth’s Magnetism

• Earth behaves like a giant magnet.
• Due to liquid iron movement in Earth’s outer core → produces currents → magnetic field.
• Protects Earth from harmful space radiation.
• Birds, animals, and navigators use it for direction.

3. Heating Effect of Electric Current

3.1 Discovery

• Activity: A nichrome wire becomes warm when current flows.
• Reason: wire resists current flow → electrical energy changes into heat energy.

3.2 Factors Affecting Heat Produced

• Type of material.
• Thickness of wire.
• Length of wire.
• Time current flows.
• Amount of current.

3.3 Uses

• Light bulbs: filament glows due to heat.
• Appliances: electric iron, heater, immersion rod, kettle, hair dryer.
• Industry: used in furnaces to melt scrap steel.

3.4 Risks

• Excess heat → energy wastage, socket damage, fire.
• Need proper safety measures and wires.

4. Electric Cells and Batteries

4.1 Voltaic Cell

• Oldest type (also called Galvanic cell).
• Made of two electrodes in a liquid electrolyte.
• Produces current due to chemical reactions.
• Becomes “dead” when chemicals finish.
• Luigi Galvani & Alessandro Volta studied these cells.
• Activity: Lemon with copper and iron electrodes can light an LED.

4.2 Dry Cell

• Most common cell.
• Uses paste instead of liquid electrolyte.
• Structure:
  • Zinc container = negative terminal.
  • Carbon rod in centre = positive terminal.
  • Electrolyte = moist paste around carbon rod.
• Cannot be recharged (single-use).

4.3 Rechargeable Batteries

• Can be used again and again after charging.
• Examples: mobile phones, laptops, inverters, vehicles.
• Lithium-ion batteries are widely used.
• New solid-state batteries being developed (safer, faster, longer life).

4.4 Battery Disposal

• Dead batteries contain harmful chemicals: acid, lead, cadmium, nickel, lithium.
• Must not be thrown in dustbin.
• Need to be recycled in e-waste centres.
• Recycling protects environment and saves resources.

5. Snapshots (Summary)

• Electric current produces magnetic effect (Oersted’s discovery).
• A coil with current behaves as an electromagnet (stronger with iron core).
• Electromagnets are widely used (bells, motors, cranes).
• Earth has a magnetic field due to liquid iron in its core.
• Current produces heat (heating effect).
• Used in bulbs, heaters, irons, furnaces.
• Cells and batteries produce current by chemical reactions.
• Dry cells are single-use, Rechargeable batteries can be reused.
• Recycling batteries is important for environment.