Some Natural Phenomena Summary Class 8 NCERT Summary Chapter 12

Introduction

• Objects get charged when rubbed with another material.

Charges

• Charges are of two types:
  (i) Positive charge
  (ii) Negative charge
• Like charges repel each other.
• Opposite charges attract each other.
• Charge generated by rubbing is static-charge.

Electroscope

• Electroscope is used to detect whether an object is charged or not.
• A simple electroscope cannot detect the nature of charge.
• Pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscopes.

Transfer of electric charges

• Electric charge can be transferred from a charged object to another through a conductor.
• Earthing is a process of transferring charge from a charged object to the earth.

Discharging

• Removing charge from a charged body is known as discharging.
• Buildings are provided with earthing so that in case of leakage of an electrical charge, people inside the building are not affected, and the charge is transferred to the ground safely.

Discharge of charge

• The static charge in the clouds is the cause of lightning.

• Generally lower portion of clouds get negatively charged and positive charge is induced on the objects below the cloud.
• When a huge amount of charge bulids up, the insulating property of air breaks down which results in discharging of charge to the earth surface.
• The electrical discharge that causes lightning may take place between two clouds or between a cloud and the Earth.

Earthquake

• It is the sudden shaking or trembling of the earth’s surface because of disturbance deep inside the earth’s crust.
• It cannot be predicted.

Causes of Earthquake

• The uppermost layer of the earth is called crust. Crust is fragmented and each fragment is called plate.
• Movements of earth’s plates (there may be collision or a brushing pass between two plates along their boundary).
• Plate boundaries are called seismic zones or fault zones.
• Earthquake may occur because of volcanic eruption.
• Kashmir, Western and central Himalayas, North-East India, Rann of Kutch, Gangetic plains, and some parts of South India are earthquake prone.