Important Notes for CBSE Class 8 Science Chapter 15 – Some Natural Phenomena. Some Natural Phenomena Class 8 Notes and Study Material Download in PDF form.
Some Natural Phenomena Notes | Some Natural Phenomena Class 8 Notes | Some Natural Phenomena Quick Revision | Some Natural Phenomena Class 8 Quick Revision Notes | Science Class 8 Ch 15 Notes | NCERT Class 8 Science Chapter 15 Notes PDF.
Some Natural Phenomena Class 8 Notes.
Some Natural Phenomena Class 8 Notes
- Two naturally occurring destructive phenomena: lightning and earthquakes
1.) Static charge:
- The type of electrical charge produced when two materials (either positively charged or negatively charged) are rubbed together is called static electrical charge.
- This mainly occurs due to transfer of electrons.
- This type of charge does not move.
Examples:
- When amber is rubbed with fur, it attracts light objects such as hair
- When a plastic refill is rubbed with polythene, it acquires a small electric charge.
- When a plastic comb is rubbed with dry hair, it acquires a small charge.
- CHARGED OBJECT: These are objects who have acquired a charge, on account of rubbing
- There are two types of charges:
- Positive charge
- Negative charge
- Another important property of electric charge is that like charges repel each other whereas unlike charges attract each other. Following are some examples to understand this property:
a) A charged balloon is repelled by another charged balloon. Similarly, a charged refill is repelled by a charge refill. But on the contrary, a charged balloon is attracted by a charged refill.
b) It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. The silk will be having a negative charge.
- Charge transfer: Electrical charge has the capability to get transferred to another charged object via a metallic conductor.
- Electroscope: A device used to test whether an object is possessing any charge or not.
- Discharging: The process by which an object loses the charge present on it.
- Earthing: When a charged object transfers its charge to the earth it is called earthing. It is important as it protects the buildings from shock in case of current leakage.
2.) Lightning:
- Lightning is caused by the accumulation of charges in the clouds.
- Usually there is separation of charge as the air currents move upward and water droplets move downwards.
- Due to this process positive charge is accumulated near the ground as well as near the top edge of the cloud. The negative charge accumulates near the bottom edge.
- As the magnitude of the charges on the clouds increase’s,air (normally a bad conductor of electricity) allows the flow of electric charge.
- As the two opposite charges meet, they produce streaks of light and sound.
- Electric discharge: the transfer of charge from cloud to another or to the earth on account of separation of charge is called electric discharge.
- Preventive measures:
- When present outside: Open vehicles, cars, fields, tall trees, shelters arenot safe. Carrying umbrellas during thunderstorm’s is not advisable. If a person is in a forest take shelter under small trees. In open fields stay away from large trees, poles and other metal objects. Never lie on the ground, instead take a squat position with your hands on the knee and head between hands.
- When present inside: avoid using wired telephones, unplug electrical appliances like televisions and computers. Avoid bathing
- Lightning Conductors:
A device which protects buildings from lightning. It is made up of a metal rod which is taller than the building and is installed in the building during construction.
It provides earthing for the building.
3.) Earthquakes:
- Earthquakes is sudden trembling and shaking of the tectonicplates due to disturbance deep inside the crust.
- It causes loss of life and property on a large scale.
- It lasts for a very short period of time.
- Crust: the uppermost layer of the earth. It is fragmented. And each fragment is called a plate.
- Such tremors can lead to other disasters like volcanic eruptions, meteor hits and underground nuclear explosion.
- Fault zone: it is also known as seismic zone. It the area where earthquakes are most prevalent. They are the boundaries of plates on the crust.
- Richter scale: the scale used to measure the magnitude of the earth quake.
- A destructive earthquakehas magnitudes higher than 7
- Seismograph: device which records seismic waves.
- Seismic waves: The tremors produce waves on the surface of the earth are called seismic waves.
- Safety measures against earthquakes:
- If you are at home:
- Take shelter under a table and staythere till the shaking stops.
- Stay away from tall and heavy objects
3.If you are in bed, do not get up and protect your head with a pillow.
- If you are outdoors
- Find a clear spot, away frombuildings, trees and overhead powerlines. Drop to the ground.
- If you are in a car or a bus, do notcome out. Ask the driver to driveslowly to a clear spot. Do not comeout till the tremors stop.
- Buildings in seismic regions should be constructed in such a way so as to withstand major tremors.