An earthquake is a sudden movement of the earth’s trembling of the earth crust which lasts for a very short time. It is caused by a disturbance deep inside the earth due to a movement of tectonic plates.
The magnitude of the intensity of an earthquake is measured through an instrument called the seismograph.
Earthquakes are caused due to shifting of tectonic plates with respect to one another. The region that lie on the boundaries of these plates, called seismic zones, are more prone to earthquakes than other regions on the earth’s surface.
There are three types of tectonic movement:
• Plates collide with each other
• Plates move away from each other
• Plates slide along each other
The layers of the earth are:
• Inner core
• Outer core
The uppermost part of the earth is the crust. It is different as it is fragmented and divided into plates which move.
The power of an earthquake is expressed on a scale called Richter scale which is not a linear scale. It is a logarithmic scale which means an increase of 2 in magnitude of an earthquake results in 100 times more destruction. Hence, an earthquake of magnitude 7 is a hundred times stronger than earthquake of magnitude 5.
When a glass rod is rubbed with silk, there is a transfer of negative charge from glass to silk and hence the glass rod acquires a positive charge.
Electroscope is a device that helps us to determine whether an object carries charge or not.
An uncharged object contains an equal number if positive and negative charges. A charged object if brought near an uncharged one repels the like charges in the latter and attracts the unlike ones, thus attracting the uncharged object.
For example, a balloon which has been negatively charged by rubbing it with a piece of cloth sticks to a wall because of an attractive force. This is because the negative in the balloon are attracted to the positive ones while the negative charges in the wall are repelled away.
Earthing is the process of neutralising a charged object by transferring its charge to the surface of the earth.
Lightning takes place when positive and negative charges get built up within a cloud or in different clouds. Due to movement of water droplets and ice particles within the clouds, the clouds get positive charges at the top and negative at the bottom. These charges become very strong and eventually the difference in positive and negative charges overcomes the effect of the insulating atmosphere between them, and the two charges are neutralised with a strong electric current that produces bright light and the sound of thunder. The lightning can occur within a cloud, between different clouds, or between a cloud and the earth.
The following precautions should be taken if you are outdoors during a thunderstorm:
• Move away from open areas, to somewhere with lots of trees or tall objects
• Do not touch these trees or tall objects, as it is possible that the lightning could use them as a way of reaching the earth’s charge
• Do not use an umbrella
• If in an open field, do not lie on the ground. Instead, sit on your haunches and make yourself as small a target as possible.