Class 8, Science, Chapter-12, Lecture-1, Lightning (Notes)
CHARGE:
The property of static particle of matter due to which it can attract or repel another static particle having a similar property is termed as Charge.
Charged Object:
An object which has a property due to which it can attract or repel another static particle having a similar property is termed as a Charged Object.
Example:
A glass rod becomes charged when it is rubbed with silk cloth.
An ebonite rod becomes charged when it is rubbed with fur.
A comb becomes charged when it is rubbed with dry hair.
Two Types of Charges:
- Positive Charge
A glass rod becomes positively charged when it is rubbed with silk cloth.
A ballpoint pen refill becomes positively charged when it is rubbed with polythene sheet.
A clear plastic scale becomes positively charged when it is rubbed with dry hair.
- Negative Charge
A rubber balloon becomes negatively charged when it is rubbed with woollen cloth.
A comb becomes negatively charged when it is rubbed with dry hair.
An ebonite rod becomes negatively charged when it is rubbed with fur.
Properties of Charges:
- Like charges repel each other
Example:
Two balloons negatively charge by rubbing with woollen cloth repel each other.
Two ballpoint pens positively charge by rubbing with polythene sheet repel each other. - Opposite charges attract each other
Example:
A balloon negatively charge by rubbing with woollen cloth and a ballpoint pen positively charge by rubbing with polythene sheet attract each other.
Transfer of Charges:
Electric charges can be transferred from one object to another through metal conductors or graphite conductors.
ELECTROSCOPE:
A scientific device used to detect the presence and magnitude of electric charge on a body is called an electroscope.
Working of Electroscope:
An electroscope consists of two aluminium or gold foil strips hanged on a metallic wire inside a closed glass jar.
One end of the wire remains outside the jar through a hole in the insulated lid.
When a charged object comes in contact with the outer end of wire, charges are transferred thorough the conducting wire to the foil strips.
Both the foil strips get like charges and repel each other. Divergence of the strips indicate presence of charge and the degree of divergence indicates the magnitude of charge.
EARTHING:
The process of transferring of charge through a conductor from a charged object to the earth is called earthing.
LIGHTNING:
A bright flash of light seen in the sky between the clouds or between clouds and the earth is called a lightning.
THUNDER:
The loud noise heard from the sky after a flash of lightning is termed as thunder.
MECHANISM OF LIGHTNING:
During development of thunderstorm, air currents move upward whereas water droplets move downward.
The friction between the air and the droplets cause charging of water droplets.
Lighter and smaller droplets get negatively charged and accumulate towards the upper edge of the cloud.
Heavier and larger droplets get positively charged and accumulate towards the lower edge of the cloud.
Surface of the earth develops positive charge due to induction.
With accumulation of large number of unlike charges, the air becomes a good conductor and lets charges flow through it.
The flow of charges is followed by release of heat, light and sound due to friction with the air.
LIGHTNING CONDUCTOR:
A metallic rod installed along the walls of a building with one end kept in open air and the other buried in the ground to provide easy route for transfer of charges during lightning so as to protect the building is called a lightning conductor.
Do’s and Don’ts During a Thunderstorm:
- Don’t stay in open fields, rush to nearby building or take shelter under a shorter tree.
- Don’t carry umbrella.
- Don’t stay near poles and metal objects.
- Don’t lie on an open ground. Squat low and place hand on your knees with you head between the hands to make yourself the smallest target.
- Don’t use wired phones.
- Don’t bathe during thunderstorm.
- Unplug electrical appliances.
- Remain inside moving vehicles with doors and windows shut.
Flash of lightning is seen first but the sound of thunder is heard a little later.
Reason:
Thunder and Lightning both occur at the same time but light travels at 3 x 108 ms-1 which is much faster than sound travelling at 344 ms-1.