Unit 8: Magnetism

Introduction to Magnetism

Magnetism is one of the most fascinating forces in nature. From the simple compass that helped ancient navigators find their way, to modern applications in electric motors and data storage, magnetism plays a crucial role in our daily lives and technology.

Key Concepts Covered

• Magnetic and non-magnetic materials
• Properties of magnets
• Magnetic poles and their behavior
• Magnetic fields and field lines
• Electromagnets and their applications
• Permanent magnets and their uses
• Magnetic domains and theory of magnetism
• Methods of magnetization and demagnetization

Important Definitions

Key Concepts

Detailed Chapter Content

1. What is Magnetism?

Magnetism is a force that acts at a distance upon magnetic materials. These materials are attracted to magnets and are called magnetic materials.

2. Magnetic and Non-Magnetic Materials

Materials such as iron, nickel, and cobalt are magnetic materials because they are attracted to magnets.

Non-magnetic materials include brass, copper, wood, glass, and plastic as they are not attracted to magnets.

3. Historical Discovery

Over 1000 years ago, the Greeks discovered a rock called lodestone or magnetite that could attract materials containing iron. They also found that if this rock was suspended from a string, it would settle in the north-south direction. This property led to the creation of the compass, which was used for navigation.

4. Identifying Magnets vs Magnetic Materials

To identify whether an object is a magnet or simply a magnetic material:

• Bring one end of the object close to any pole of a suspended bar magnet
• If it is attracted, bring the same end close to the other pole of the suspended magnet
• If attracted again, it is a magnetic material
• If repelled, it is a magnet

The repulsion between like poles is a real test to identify a magnet.

5. Properties of Magnets

Magnets exhibit the following properties:

• Magnetic Poles: Every magnet has north and south poles
• Attraction and Repulsion: Like poles repel, unlike poles attract
• Pole Isolation: Isolated magnetic poles do not exist - if a magnet is broken, each piece becomes a complete magnet with both poles

6. Temporary vs Permanent Magnets

Temporary magnets work only in the presence of a magnetic field and lose their magnetic properties when the field is removed. Examples include paper clips, office pins, and electromagnets.

Permanent magnets retain their magnetic properties forever. Examples include cobalt, alnico, and ferrite magnets.

7. Magnetization

Magnetic materials such as iron or steel can be made into magnets through a process called magnetization. This can be demonstrated using a compass and an iron nail placed in contact with a bar magnet.

8. Magnetic Field and Field Lines

A magnetic field is the region around a magnet where another magnetic object experiences a force. Magnetic field lines can be visualized using iron filings or a small compass.

Field lines appear to originate from the north pole and end on the south pole. The magnetic field is stronger where the lines are close together and weaker where they are far apart.

9. Electromagnets

Electromagnets are temporary magnets formed when an electric current passes through a coil of wire wound around an iron core. The magnetic properties exist only while current flows through the coil.

10. Applications of Magnets

Permanent magnets are used in:

• Electric motors and generators
• Moving coil loudspeakers
• Door catchers and refrigerator seals
• Separating iron objects from mixtures
• Medical applications (removing iron splinters)

Electromagnets are used in:

• Electric bells and telephone receivers
• Magnetic relays and circuit breakers
• Cranes for lifting heavy objects
• Tape recorders and hard disks
• Maglev trains

11. Domain Theory of Magnetism

Magnetism is caused by moving charges. In a bar magnet, magnetism is due to spinning and revolving electrons within atoms.

In ferromagnetic materials like iron, nickel, and cobalt, groups of atoms form regions called magnetic domains where electron spins are naturally aligned parallel to each other.

12. Magnetization and Demagnetization

Methods of magnetization include:

• Stroking method (single touch and double touch)
• Using a solenoid with direct current

Methods of demagnetization include:

• Heating the magnet strongly
• Hammering or vigorous shaking
• Using alternating current in a solenoid

Daily Life Applications

Comparison Tables

Permanent vs Temporary Magnets

Magnetic vs Non-Magnetic Materials

Sample Problems