Notes of chapter: Some Natural Phenomena are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 8.
Lightning in cloud is accumulation of charges in the clouds and produces heat which vibrates and expands the air of surrounding. As the result of it we can hear thunder sound after a flash of light.
(i) The ancient Greeks in 600 B.C. knew that hair is attracted by fur rubbed on amber (resin).
(ii) In 1752, Benjamin Franklin, an American scientist realized that spark on woolen clothes are the same phenomena which is known by the Greeks.
(3) Charged objects are those objects which get ions after rubbing with each other.
Eg:- (i) Rub plastic scale on hair. Plastic scale gets charged and can attract small pieces of paper.
(ii) Ball pen refill gets charged after rubbing on polythene. It can attract small pieces of paper.
(4) There are two types of charges one is called positive charge and other is known as negative charge.
(i) Rub an inflated balloon with woollen cloth. Rub a ball pen refill with a polythene sheet. Now, bring balloon near to refill. Balloon will be attracted by charged refill. Therefore, both have different charged particles.
(ii) Rub two inflated balloons with woolen cloth. Bring them near. They will repel each other. Therefore, both have like charge particles.
(5) We can transfer the charge from one body to another body.
Place a cardboard piece on the mouth of the glass bottle. Pierce a hole in the card board. Take two aluminium foil piece and hang them on the paper clip. Put the aluminum foil sheets in the glass bottle. Rub a ball pen refill with polythene. Now, bring it near to paper clip. Aluminium foil sheet will repel each other. It can happen only because charge of refill transfers to paper sheet and through paper sheet aluminium foil sheet gets charged.
The device which can test whether an object is carrying charge or not is called electroscope.
The process of transferring of charge from a charged object to the earth is called earthing.
Eg:- In above example , if we touch charged aluminum foil sheets, the charge will transfer to the earth through us.
Electric discharge is a process in which negative and positive charges meet and produce streaks of bright light and sound.
At the time of thunderstorm, upward movement of air and downward movement of water droplets cause separation of charges. The positive particles collect at the upper end of the cloud and negative charge collects at the lower end of the cloud. The positive charge accumulates near the ground. When accumulation of the charges becomes very large, the air which is poor conductor of electricity can not resist them. The negative and positive charges meet and produce streaks of light and sound. This whole process is called electric discharge.
The process of electric discharge can happen between cloud and cloud or cloud and earth.
(9) Safety rules during lightning-
(i) Open vehicles like motor bikes, scooters, tractors and open fields are not safe.
(ii) Carrying an umbrella is not safe because metal is good conductor of electricity.
(iii) If you are in a forest, take shelter under short trees.. Lightening always find nearest and shortest path to flow to ground. So, tall trees are the nearest and shortest path for electricity to reach ground. Trees are good conductor of electricity. Therefore, it can be possible that electricity will go through the trunk and branches may fall down. Electricity will flow through you if you are sitting under the tree.
(iv) Keep away yourself from tall trees, metal objects and poles in an open field at the time of lightning. Never lie on the ground as our body is a good conductor of electricity. In this case sit in squatting position, place your head between your hands that are resting on your knees.
(v) Use only mobile phones and cordless phones.
(vi) Never take bath during lightning to avoid contact with running water.
(vii) Electrical appliances should be unplugged. Electrical lights can remain on.
Lightning conductor or lightning rod is a metallic rod installed in the walls of the building at the time of construction to protect the structure from light strikes.
The one end of the metallic rod is kept out in the air and the other end is deep under the ground. The electric charges of the lightning transfer to ground through the metallic device.
An earthquake is a sudden shaking or trembling of the earth lasts for a very short time period, caused by the disturbance of the earth’s crust and result in a great destruction.
(i)Earthquakes can cause damage to buildings, bridges, dams and human lives etc. It can cause also tsunamis, landslides and floods etc.
(ii) A tsunamis occurred in the Indian Ocean on 26 December 2004.
(iii) The earth has four layers, crust, mantle, outer core and inner core.
The crust is the outer most layer. It is fragmented in seven plates. These plates slide. When they collide or brush past(touch) each other, cause disturbance in the earth’s crust. It is called earthquake.
(iv) The seismic zones or fault zones are weak zones (boundaries of the plates inside the earth) where earthquakes are more likely to occur.
Eg:- In India, the Kashmir, Western and Central Himalayas, the whole of North – East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plane and some area of South India are come under seismic zone.
The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale.
The magnitude more than 7 is considered as a destructive earthquake.
Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.
(vi) Seismic waves-
Seismic waves are those waves produced by the tremors on the surface of the earth.
The instrument which record movement of the seismic waves is known as seismograph.
It is a very simple instrument having a pendulum, a pen attached to it and a piece of paper attached on a rotating drum. The pendulum begins to vibrate when tremor occurs. The pen records movement of seismic waves. Scientists study it and make a map of earthquake.
(12) Precautions taken at the time of construction of building in seismic zone-
(i) Consult qualified architects and structural engineers.
(ii) Use of mud and timber is good in construction. Roofs of the houses should be of light material. It will cause less damage.
(iii) Fix cupboards and shelves on the walls.
(iv) Building should have fire extinguishers. It will be very helpful if buildings catch fire at e time of earthquake.
(13) Precautions to protect yourself at the time of earthquake-
(i) At the home-
(a)Take shelter under a table and stay there till the shacking stops.
(b) Stay away from tall and heavy objects that may fall on you.
(c) If you are in bed, do not get up. Protect your head with pillow.
(ii) Outside the home-
(a) Find a clear spot on the ground away from the buildings, trees and overhead power lines.
(b) Do not come out from a car. Move car slowly and try to reach a safe place.