Some Natural Phenomena Class 8 Notes Science Chapter 12

Introduction and Lightning

Lightning is a natural event that has intrigued humans for a long time. Many have studied what causes lightning and how it occurs. Benjamin Franklin discovered that there is an electric discharge between clouds that creates a spark. This electric spark between the clouds and the ground is what we see as lightning. His famous kite experiment demonstrated this concept. Here is how lightning occurs:

• The formation of clouds happens when water vapour in the atmosphere condenses.
• This condensation charges the particles within the clouds.
• Negative charges gather at the bottom of the cloud, while positive charges accumulate at the top.
• The increasing charge in the cloud generates a positive charge on the ground nearby.
• As the charge builds up, the negative charge from the cloud seeks a path to the ground, creating what we know as lightning.

An Electroscope

An electroscope is a device that detects the type of charge on a body. Like charges repel and unlike charges attract each other. This principle is used in an electroscope. An electroscope consists of:

• A glass jar fitted with a cork lid.

• A metallic wire passing through the cork lid.

• Two metallic strips at the bottom of the wire.

• A metal disc connected to the upper end of the wire.
  

A. When a positively charged body is touched to the metal disc:

• The charge is transferred to the metallic strips through the wire.

• The strips diverge from each other on gaining a like charge.

B. If a negatively charged object is brought into contact with the disc:

• The strips converge towards each other.

• This indicates the unlike charge on the body.

C. If a positively charged body is brought in contact with the metal disc again:

• The divergence of the metal strips increases.

• This indicates the like charge on the body.

D. If the metal disc of the electroscope is touched with the hand:

• It loses its charge to the ground.

• The transfer of charge occurs through the human body.

• This process is called earthing.

Lightning and Safety

Lightning results from the build-up of charges in clouds and can cause significant damage to life and property. Therefore, it is crucial to take protective measures during lightning storms. Safe places to seek shelter from lightning include:

• Staying indoors and avoiding open areas.
• Not sheltering under trees or in open spaces if outside.
• If in a vehicle, remaining inside and keeping doors and windows closed.
• Tall buildings often have lightning rods, which are metal rods on the roofs connected to a thick copper wire buried underground. If lightning strikes, it is attracted to the rod, allowing the electrical discharge to be transferred safely to the ground.
• The weather department can provide warnings about developing thunderstorms.

Earthquakes

• A natural phenomenon that cannot be predicted is an earthquake. The earth consists of three major layers, called the crust, the mantle and the core. The core is further divided into the inner core and the outer core. 
• The mantle consists of semi-solid material above which the crust floats. The crust consists of oceans and continents. The crust is divided into several parts, called tectonic plates. The regions where one tectonic plate slides against another are referred to as fault zones, and these are the regions where an earthquake is likely to occur. Hence, these zones are referred to as seismic zones.
• The place in the interior of the earth where an earthquake occurs is the focus, and the region on the surface of the earth that is the closest to focus is likely to experience the largest damage. This region is called the epicentre of the earthquake.

• The instrument that measures the severity of an earthquake is a seismograph. It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on a sheet like a graph paper. The severity of an earthquake is measured on the Richter scale. A major earthquake measures 7 or more on the Richter scale.