CBSE CLASS VII NOTE ON SOME NATURAL PHENOMENA BY LHAKPA WANGYAL

SOME NATURAL PHENOMENA             

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Natural phenomena can be harmful and destructive. These cause lots of damage to human life and property.

Some natural disasters or calamities are storms, cyclones, earthquakes, lightning, landslide, tsunami, cloud burst, hurricane, forest fire, etc.

Lightning is an electric spark on a huge scale caused by accumulation of charges in the cloud.

In 600 BC, Greeks knew that when amber is rubbed with fur, it attracted hair.

In 1752, Benjamin Franklin, an American scientist showed lightning as static charge same as electric sparks produced with crackling sound while taking off woolen or polyester clothes, especially in the night.

Static charge is acquired by rubbing an object with another object. The object acquiring charge after rubbing is termed charged object.

Static charge may be positive or negative.

Like charges repel and unlike charges attract each other.

Convention for static charge:  Charge acquired by a glass rod when rubbed with silk is positive.  Straw rubbed with polythene is negatively charged Electroscope is used to test whether object is charged or not.

Loss of charge from an object or body is known as discharge Earthing is a process of transferring of charge from a charged object to the earth.

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

LIGHTNING During development of thunderstorm, vigorous air currents move upward while the water droplets move downward causing separation of charges.

Positive charges accumulate near the upper edges of clouds and negative charges collect near the lower edges.

There is gathering of positive charges near the ground.

When the magnitude of the accumulated charges becomes very large, there produce streaks of bright light and sound due to meeting of negative and positive charges. The process is termed electric discharge.

Electric discharge can occur between two or more clouds or between clouds and earth.

Scientists have not yet understood the entire detail of lightning.

Meteorology department can predict developing of thunderstorm. If a thunderstorm occurs there is possibility of lightning and cyclones.

LIGHTNING SAFETY During lightning and thunderstorm, one should stay in safe places (house or building).

If in you are in car, doors and windows should be closed.

DO’S AND DON’TS DURING A THUNDERSTORM OUT SIDE

 Unsafe places which don’t protect us from lightning strokes are: motor bike, tractors, construction machineries, tall trees, open fields, shelters in parks, elevated places, under umbrella  In forest, take shelter under shorter trees.  If you are in open field, try to avoid threes, metallic objects, poles and squat low to ground instead lying on the ground.

INSIDE

 During lightning, avoid bathing and contact with telephone cords, electrical wires, metal pipes.  It is safer to use mobile phones or cordless phones. You shouldn’t use wired phone during lightning.  Unplug electrical appliances like computers, TV, etc during lightning.

LIGHTNING CONDUCTOR  Lightning conductor is device used to protect buildings from effect of lightning. Installation of metallic rod taller than building during its construction exposing one end of the rod in air and the other is buried deep in the ground. The rod provides easy route for transfer of electric charge to the ground.  Electrical wires, water pipes in buildings also protect us to some extent.

EARTHQUAKES  Earthquake is a sudden shaking or trembling of the earth lasting for short period of time caused by disturbance deep inside earth’s crust.  Earthquake is difficult to predict accurately. It is occurring all the time all over the earth.  Earthquake cause immense damage to buildings, bridges, dams and people. It can cause floods, landslides and tsunamis.

WHAT CAUSES EARTHQUAKES?  Outer most layer of earth crust is fragmented. Each fragment is called plate. These plates are in continual motion.

When the plates brush past one another or a plate goes under another due to collision, they cause disturbance in the earth crust. This disturbances cause earthquake on the surface of the earth.  Tremors can be caused by volcanic erupts, meteor hits the earth or by nuclear explosion is carried out.  Boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic zones or fault zones.  Seismic zones of India are Kashmir, western and central Himalayas, whole of north-east, Rann of kutch, Rajasthan and Indo-Gangetic plane.  Power of earthquake is expressed in terms of a magnitude on a scale called Richter scale. Destructive earthquake have magnitudes higher than 7 on the Richter scale.  Richter scale is not a linear scale.  The tremors produce waves on the surface of the earth are termed seismic waves.  Seismic waves are recorded by an instrument called seismograph.

MOVEMENT OF TECTONIC PLATES  Seismograph is a simple device with a vibrating rod or a pendulum attached to a pen which record seismic waves on a paper on a moving drum.  Seismologist can estimate the power of the destruction due to earthquake with the help of seismograph.

PROTECTION AGAINST EARTHQUAKES  People living in the seismic zones should be prepared before an earthquake strikes.  It is advised to construct buildings which are quake safe.  Tips for constructing buildings in fault zones;  Use of timber and mud are better than using heavy construction materials.  Roofs should be made by light structures.  Cupboards should be fixed to the walls so that these didn’t fall.  Should avoid hanging wall clocks, photo frames, water heaters, etc so that they don’t fall on the people.  Every building should have fire fighting equipments.  Central Building Research Institute, Roorkee has developed quake proof houses.

PROTECTIVE MEASURE WHEN EARTHQUAKE STRIKES  If you are inside home  Take shelter under table and stay until the shaking stops.  Stay away from tall and heavy objects that may fall on you.  If you are on bed, don’t get up. Protect your head with a pillow.  If you are outdoors  Find a clear spot, away from buildings, trees, overhead power lines. Drop to ground.  If you are in car or bus, don’t come out until the tremor stops. Ask driver to drive slowly to a clear spot.