Notes Chapter 4 Science Curiosity Class 8 CBSE Board

Electricity: Magnetic and Heating Effects

Introduction

This chapter explores how electricity creates magnetic and heating effects. We learn about electromagnets, how electric current produces heat, and how cells and batteries generate electricity. These concepts are demonstrated through real-life applications and key observations.

1. Magnetic Effect of Electric Current

When electric current flows through a wire, it creates a magnetic field around it. This is called the magnetic effect of electric current. The magnetic field can affect a compass needle, causing it to deflect.

Key Points:

• Discovery: In 1820, Hans Christian Oersted found that a current-carrying wire deflects a compass needle, showing a link between electricity and magnetism.
• Magnetic Field: The area around a current-carrying wire or magnet where its magnetic effect can be felt is called a magnetic field.
• The magnetic field disappears when the current stops.

2. Electromagnets

A current-carrying coil behaves like a magnet and is called an electromagnet. Adding an iron core makes it stronger.

Key Points:

Electromagnets have two poles (North and South), like a bar magnet.

The strength of an electromagnet depends on:

• The amount of electric current.
• The number of turns in the coil.
• The presence of an iron core.

Reversing the current direction reverses the poles of the electromagnet.

Applications: Used in electric bells, motors, fans, loudspeakers, and lifting cranes.

Real-Life Application: Lifting Electromagnets

• Use: In factories and scrap yards to lift and move heavy metal objects.
• How it Works: A crane operator turns the current ON to lift objects and OFF to release them.

3. Heating Effect of Electric Current

When electric current flows through a conductor, it faces resistance, which converts some electrical energy into heat energy. This is called the heating effect of electric current.

Key Points:

Resistance: Different materials resist current differently. Nichrome has higher resistance than copper, so it generates more heat.

The amount of heat depends on:

• The current strength (more current = more heat).
• The material, thickness, and length of the wire.
• The duration of current flow.

Applications: Used in electric heaters, stoves, irons, kettles, hair dryers, and industrial furnaces.

Problems: Overheating can cause energy loss, damage appliances, or start fires. Safety devices like fuses help prevent this.

4. How Batteries Generate Electricity

A cell or battery generates electricity through chemical reactions between electrodes and an electrolyte.

4.1 Voltaic Cell (Galvanic Cell)

• Structure: Two metal plates (electrodes) dipped in a liquid electrolyte (e.g., weak acid or salt solution) in a container.
• Working: Chemical reactions between the electrodes and electrolyte produce electric current. The current flows from the positive to the negative terminal.
• Limitation: The cell stops working when the chemicals are used up (it becomes “dead”).
• History: Invented by Alessandro Volta, inspired by Luigi Galvani’s experiments with frog legs.

4.2 Dry Cells

• Structure: A zinc container (negative terminal), a carbon rod with a metal cap (positive terminal), and a paste-like electrolyte.
• Use: Common in everyday devices like torches and remotes. They are single-use and must be disposed of when used up.

4.3 Rechargeable Batteries

• Definition: Batteries that can be recharged and reused multiple times.
• Examples: Used in phones, laptops, cameras, inverters, and electric vehicles.
• Common Type: Lithium-ion (Li-ion) batteries, which use metals like lithium and cobalt.
• Future: Solid-state batteries (with solid electrolytes) are being developed for safety, faster charging, and longer life.
• Note: Rechargeable batteries wear out after many cycles, requiring more frequent charging over time.

5. Environmental

• Battery Disposal: Used batteries contain harmful materials like acids, lead, cadmium, nickel, or lithium. Throwing them in regular garbage can cause fires or environmental damage.
• Recycling: Many materials in batteries are valuable and can be reused. Dispose of batteries at e-waste recycling facilities.