CBSE Class 8 Science Chapter 12 Some Natural Phenomena Notes

CBSE Class 8 Science Chapter 12 Some Natural Phenomena: Lightning

A short burst of fire, which occurs due to the sudden flow of electric current between two objects, is known as an electric spark.

It can be observed in our daily life in many situations, when a plug is loose in its socket, when the wires become loose on an electric pole, etc. In nature, lightning is caused by the accumulation of charges in the clouds.

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Initially, people are afraid of lightning and consider it the wrath of God. But we need not be scared of lightning; we should take precautions to protect ourselves from the sparks.

” Cbse Class 8 Science Chapter 12 Notes “

CBSE Class 8 Science Chapter 12 Charging By Rubbing

When an object is rubbed with an appropriate object (like polythene, dry hair), it acquires a small electric charge, said to be a charged object. So, we can say that an electric charge can be produced by rubbing one object with another object of a specific kind. An object with no electric charge is said to be an uncharged object.

Some common examples of charging by rubbing are

1. When a plastic refill is rubbed with polythene, it acquires a small electric charge.
2. When a plastic comb is rubbed with dry hair, it acquires a small electric charge

” Some Natural Phenomena Class 8 “

CBSE Class 8 Science Chapter 12 Types Of Charges And Their Interaction

When two objects are rubbed together, then both the objects get charged with opposite charges, i.e. positive charge (+ sign) and negative charge (- sign) Example; When we rub a glass rod with a silk cloth, then the charge acquired by a glass rod is positive and silk cloth gets a negative charge.

The nature of the object determines which object gets a positive charge and which one gets a negative charge.

Depending on the type of electric charge on the objects, they will either attract or repel each other.

Like charges repel each other. It means, if both bodies have the same type of charges, they will repel each other.

Unlike charges attract each other. It means, if both bodies have different types of charges, they will attract each other.

” Lightning And Thunder Explanation “

The electrical charges generated by rubbing are known as static charges. Example: by rubbing a plastic comb with dry hair. Static charges do not move themselves. If these charges move, then they constitute electric current.

Transfer of Charges

Electric charge can he transferred from a charged object to an uncharged object, it is called transfer of charge. On the basis ot transfer of charges, we can construct a device called an electroscope. It is a device that is used to test whether an object is carrying a charge or not.

Detection of Charge with Electroscope

To detect the electric charge on a body, we touch the metal top ot the electroscope with it and notice the aluminium leaves.

1. If the body is electrically charged, then the leaves diverge.
2. If the body is electrically uncharged, then the leaves do not diverge.

” Static Electricity Class 8 “

An electroscope proves that electric charge can be transferred from a charged object to another through a metal conductor.

Earthing

The process of transferring an electric charge from a charged object to the earth is known as earthing. When a body loses charge to the earth or any other body, then it is said to be discharged.

Note: Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electric current.

CBSE Class 8 Science Chapter 12 Story Of Lightning

1. Lightning is bright streaks or flashes of light seen in the dark cloud sky. Due to the transfer of charges from cloud to cloud or from cloud to the earth. It is usually followed by a loud cracking sound known as thunder.
2. During the development of a thunderstorm, there is always an upward movement of air while the water droplets move downward.
3. These vigorous movements cause the separation of charges. Therefore, the top of the cloud becomes positively charged while the bottom of the cloud becomes negatively charged.
4. The quantity of opposite electric charges on the top and bottom of storm clouds becomes very large, so the air, which is normally a poor conductor of electricity, will no longer be able to resist their flow. Therefore, the electric charges start flowing at high speed.
5. When both the positive and negative charges of the cloud meet, they produce an intense spark of electricity. We observe this electric spark as the flash of lightning in the sky. This process is also called an electric discharge.
6. The process of electric discharge can occur between two or more clouds or between clouds and the Earth.

” Charging By Rubbing Class 8 “

CBSE Class 8 Science Chapter 12 Lightning Safety

During thunderstorms and lightning, a house or a building is always a safe place to be in. Any open place is not a safe place.

If you are travelling by car or by bus, then you are safe inside with the windows shut. The following precautionary measures should be taken to protect ourselves from lightning strikes during a thunderstorm.

” Types Of Charges And Their Interaction “

CBSE Class 8 Science Chapter 12 Lightning Safety Outside the House

Dos During a Thunderstorm

1. We should stay away from tall trees.
2. If we are in a park or forest, we should take shelter under shorter trees.
3. If we are in a closed car, we should stay inside with the windows and doors closed.
4. Squat on the Hoot. Thin position makes us the smallest target to he struck.

” Earthquake Causes And Effects “

Don’ts During a Thunderstorm

1. Do not stand in open ground during a lightning storm,
2. Do not carry an umbrella as its metal rod is a good conductor of electric charge.
3. Always stay away from poles or other metallic objects.

Lightning Safety Inside the House

Dos During a Thunderstorm

1. We should switch OFF the TV as antennas fixed on the roof of a building are prone to lightning strikes,
2. We should avoid touching the metallic pipes and electric equipment connected by electric wires in the house.
3. We should unplug electrical appliances like the refrigerator, computers, etc.

Don’ts During a Thunderstorm

1. We should not use a wired telephone,
2. We should not take a bath as water is a good conductor of electricity.

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor.

” Richter Scale And Earthquake Measurement “

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor. end of the lightning or is fixed to a vital plate, which is deep into the pound, is shown in the figure.

Lightning Conductor Protects the Building From Thunderstorms

1. If the lightning strikes on building, it will first strike the top end of the lightning conductor.
2. The lightning conductor provides  an easy route for the runnier of electrical charge to discharge to ground,
3. The electrical energy of lightning, which passes through a tin metal strip, gel discharged safely into the ground, and the hurled metal plate.
4. Since the lightning charge does not pass through the Imilding, no damage is caused.

” Cbse Class 8 Science Notes “

CBSE Class 8 Science Chapter 12 Earthquake

A sudden shaking or trembling of the earth that lasts for a very short time is an earthquake. It can cause immense damage to buildings, bridges, dams, and people.

A sudden shaking of the earth causes tremors, and these tremors produce waves on the surface of the earth. Tremors can be caused by to volcanic eruption or a meteor hitting the Earth.

Causes of Earthquakes

The tremors are caused by the disturbances inside the Earth. Before learning about the causes, we should know that the Earth consists of three layers.

1. Core The innermost part of the Earth is called its core.
2. Mantle The central region of the Earth is called the mantle.
3. Crust The outermost layer of the Earth is called the crust.

” Electric Spark And Its Causes “

The outermost layer ofthe most is fragmented, due to which the Earth’s crust is divided into many plates. These plates are in Continuous motion

When these plates collide with one another or brush past one another then they cause disturbance in the Earth’s crust. This disturbance results from an earthquake on the Earth’s surface.

The place inside the Earth’s crust where the earthquake is generated is called the focus of the earthquake.

Seismic or Fault Zones

As earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. These weak zones are also known as seismic or fault zones. In India, Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic plain are the most threatened areas.

” Like Charges Repel Each Other “

Richter Scale

It is a scale that is used to measure the intensity or magnitude of an earthquake. Destructive earthquakes have magnitudes higher than 7 on the Richter scale.

Seismograph

The seismic waves are those waves that are produced by tremors on the Earth’s surface. These waves are recorded by an instrument called the seismograph.

• This instrument is simply a vibrating rod or a pendulum that starts vibrating when tremors occur. A pen is attached to the vibrating system, and this pen records the seismic waves on paper that moves under it.
• By studying waves, scientists can construct a complete map of the earthquake, as shown in the figure. They can also calculate its power to cause destruction.

” Electroscope And Charge Detection “

Protections Against Earthquake

Some of the important precautions that can be taken by people living in seismic zones are as follows

1. All the quake-proof houses should be constructed using modern building technology. For which, we should consult a qualified architect and structural engineers.
2. Even in highly seismic areas, it is advisable to use mud or timber for building houses, as these are better than heavy materials, which keep roofs as light as possible.
3. There should be a tight fixation of cupboards and shelves to the wall, so that they do not fall when shaking occurs.
4. The heavy mirrors, photo frames, water heaters, etc., should be mounted to the wall at low levels, so that in the event of an earthquake, they do not fall on the people.
5. Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have firefighting equipment in working order.

In the event of an earthquake, follow these steps to protect yourself.

” Earthquake Safety Measures “

If you are at home

1. Try to get a shelter under a table and stay there till the shaking stops.
2. Stay away from tall and heavy objects as they may fall on you.
3. If you are in bed. Then do not get up. Immediately, protect your head with a pillow.

If you are outside your home

1. Observe a clear spot away from buildings, trees, and overhead power lines. Drop to the ground.
2. Besides this, if we are in a car or a bus, we should not get out. The car should be driven slowly to a clear spot.