Class 8 Unit-8 (Electricity & Magnetism)

 

Unit-8 (Electricity and Magnetism)

Magnet

• A magnet is a material that can attract certain metals, mainly iron, nickel, and cobalt.
• Magnets are widely used in daily life, such as in compasses, electric motors, speakers, and magnetic locks.

 

Properties of a Magnet

1. A magnet attracts magnetic materials like iron, nickel, and cobalt.

2. A magnet repels or attracts other magnets depending on the poles.

3. Magnet has two poles:

o North Pole (N): Points toward Earth's north.

o South Pole (S): Points toward Earth's south.

4. Magnetic effects are strongest at the poles.

5. A magnet loses its magnetism if heated, hammered, or cut improperly.

 

Types of Magnet:

1. Natural Magnets

1. Definition: Magnets that are found naturally in the earth.
2. Example: Lodestone.
3. Characteristics:

ü They are naturally magnetic without human intervention.

ü Usually, they are irregular in shape.

ü They can attract iron, cobalt, and nickel.

2. Artificial Magnets

• Definition: Magnets made by humans using iron or steel.
• Example: Bar magnets, horseshoe magnets.
• Characteristics:

ü Can be made in different shapes like bar, horseshoe, or cylindrical.

ü Stronger and more reliable than natural magnets.

ü Used in compasses, electric motors, and magnetic locks.

 

 

Methods of Making Artificial Magnets

1. Single Touch Method

• How it works:
• Take a bar of soft iron or steel.
• Rub one pole of a strong magnet along the length of the bar in one direction only.
• Do not rub back and forth.
• Result: The iron or steel becomes a magnet.
• Example: Making a bar magnet in a science lab.
• Note: Only one pole of the magnet is used, hence called single touch.

---

2. Double Touch Method

• How it works:
• Take a bar of iron or steel.
• Rub both poles of a strong magnet along each end of the bar simultaneously, moving from the ends toward the center.
• Result: A stronger magnet is produced than by single touch.
• Note: Both poles of the magnet are used, so it’s called double touch.

---

3. Electric Method (Electromagnetic Method)

• How it works:
• Take a soft iron rod and wrap it with a coil of insulated copper wire.
• Pass electric current through the coil.
• The soft iron rod becomes a magnet while the current flows.
• Result: A temporary electromagnet is formed.
• Use: Electric bells, cranes for lifting iron, electric motors.
• Strength Control: The magnetism can be increased by:

1.   Increasing the current.

2.   Increasing the number of turns of the coil.

3.   Using a soft iron core.

 

Uses of Magnets

1. Magnets are used in compasses to help people find directions.
2. Electromagnets are used in industries to lift heavy iron and steel objects easily.
3. Magnets are used in many electric devices such as motors, generators, and speakers.
4. Strong magnets are used in MRI machines to see inside the human body.
5. Magnets are found in daily life items like refrigerator magnets, magnetic locks, and toys.
6. Magnetic tapes and hard drives use magnets to store information.
7. Magnets are used to separate iron filings from sand or other materials.
8. Electromagnetic cranes help in moving large amounts of iron scrap in scrapyards.
9. ATM cards, credit cards, and ID cards use magnets for storing information.
10. Magnets are used in science experiments to demonstrate magnetic effects.

 

Demagnetization

The process of destroying the magnetic properties of magnet is called demagnetization.

There are several methods of demagnetization:

1.      Rough handling

2.      Hammering the magnet

3.      Heating the magnet

4.      Passing alternating current

5.      Keeping the poles together

6.      Natural loss

 

Conservation of magnetic properties

Keeping the magnetic strength for a long time in a magnet is called the conservation of magnetism.

Here are some methods of preservation of magnetism:

1.      Keep in dry place

2.      Store away from high temperature regions.

3.      Do not let electric current flow though it.

4.      If the magnet is not in use, always keep it in the keeper.  

 

Geomagnetism (Terrestrial magnetism)

Geomagnetism means the magnetism of the Earth.
It refers to the fact that the Earth itself acts like a giant magnet.

---

1. Earth as a Magnet

• The Earth has two magnetic poles:
• Magnetic North Pole
• Magnetic South Pole
• These poles are near, but not exactly at, the Earth’s geographical poles.
• The Earth’s magnetic field behaves just like that of a bar magnet placed inside the Earth with its south pole near the geographic north and north pole near the geographic south.

 

Evidence of terrestrial magnetism includes:

1. A freely suspended magnet aligns north–south.
2. Existence of magnetic poles on Earth.
3. Compass needle always points north–south.
4. Magnetic declination observed worldwide.
5. Effect on electric currents and satellites.

 

Applications of terrestrial magnetism include

1. Navigation using compass,
2. Finding directions and surveying,
3. Protecting Earth from solar radiation,
4. Helping satellites in space,
5. Animal migration guidance,
6. Scientific research on Earth’s structure.