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.

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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.

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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.

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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.

 

Electricity 

Electricity is defined as a form of energy created by the flow of tiny charged particles called electrons. 

 

House wiring

House wiring is the mechanism for dividing electricity and making it available in appropriate places of the home.

 

Electric circuit

An electric circuit is a closed conducting path that allows electricity to pass across it.

 

Electric fuse

Electric fuse is an electric component used to protect circuits from high currents and to ensure circuit protection.

 

MCB (Miniature Circuit Breaker)

MCB is a type of fuse that is used to safeguard circuits against excessive current. It is an automatic switch that cut off current in abnormal conditions such as overload and short circuits.

 

Advantages of an MCB over a fuse

Ø An MCB can be used again after the fault is removed, whereas a fuse needs to be replaced each time it blows.

Ø An MCB provides better safety because it trips immediately during overloads or short circuits.

Ø It is very easy to operate an MCB, as you just need to switch it on or off without touching any wires.

Ø An MCB helps in identifying the faulty circuit quickly because its switch goes down when there is a problem.

Ø An MCB reduces the risk of fire since it prevents sparking and overheating better than a fuse.

Ø An MCB is more durable and gives more reliable protection compared to a fuse.

 

Main switch

The main switch is a device in a house that controls the entire electricity supply.

 

Advantages of main switch

1. The main switch allows us to turn off the electricity of the entire house at once during emergencies.

2. It helps protect people from electric shocks by quickly cutting off the power when needed.

3. It makes electrical repair and maintenance work safer by stopping the flow of current to all circuits.

4. It prevents electrical accidents during short circuits or overloads by disconnecting the supply.

5. It gives easy control over the whole electrical system of the house from one place.

6. It can disconnect both the live and neutral wires at the same time.

 

Electric switch

An electric switch is a device that is used to open or close an electric current in an electric circuit.

 

Electric light bulb

An electric light bulb is a device that gives out light using electricity.

Types of electric light bulb:

ILB (Incandescent Light Bulb):

• It is the traditional bulb that produces light by heating a thin tungsten filament.
• It consumes more electricity and has a shorter life.
• Commonly used in homes for general lighting.

CFL (Compact Fluorescent Lamp):

• A smaller and energy-saving version of fluorescent tubes.
• Produces bright light and uses less electricity than an ILB.
• Often used in homes, offices, and classrooms.

HL (Halogen Lamp):

• A type of incandescent bulb filled with halogen gas.
• Produces very bright light and is more efficient than a regular ILB.
• Commonly used in car headlights and spotlights.

LED (Light Emitting Diode):

• Uses semiconductors to produce light.
• Very energy-efficient, long-lasting, and produces less heat.
• Used in homes, streets, electronic devices, and decorative lights.

 

Distribution Board
A distribution board is an important part of house wiring that receives electricity from the main supply and distributes it safely to different circuits in a house.

 

Electric meter

An electric meter is a device that is used to measure the electrical energy consumed at home, building, or other electrically powered devices.

 

Current transformer (CT) meter

A CT meter is a type of electric meter used to measure high electric currents safely. CT meters are commonly used in industries, big buildings, and places where electricity consumption is high.

 

Note: A unit of electricity is defined as the consumption of 1kW of electric power in our hour.

 

Electric Wire:
An electric wire is a conductor used to carry electric current from one place to another. It is usually made of copper or aluminum because they are good conductors of electricity.

Types of electric wires:

Live Wire (Red or Brown)

• The live wire carries electric current from the power source to the electrical appliances.
• It is the wire through which electricity enters the device.
• Touching the live wire can cause an electric shock, so it is very dangerous.

Neutral Wire (Blue or Black)

• The neutral wire carries electric current back to the power source.
• It completes the electrical circuit.
• It is connected to the ground at the power station, so it is relatively safer than the live wire.

Earth Wire (Green or Yellow-Green)

• The earth wire provides a safe path for electric current in case of a fault.
• It protects people from electric shocks and prevents damage to appliances.
• It is connected directly to the ground.

 

Electric Phase

An electric phase refers to the distribution of alternating current (AC) in one or more wires of a power supply system. The number of phases determines how electricity is supplied and used.

1. Single Phase:
• In a single-phase system, electricity flows through one live wire and one neutral wire.
• It is commonly used in homes, small offices, and shops.
• Suitable for low-power appliances like bulbs, fans, and TVs.
2. Three Phase:
• In a three-phase system, electricity flows through three live wires and one neutral wire.
• It is used in industries, factories, and large buildings.
• Suitable for high-power machines and heavy equipment.