Chapter 01 ELECTRIC CHARGES AND FIELDS
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INTRODUCTION
Discharge of electric charges through our body, which were accumulated due to rubbing of insulating surfaces: spark or hearing a crackle when we take off our synthetic clothes or sweater, particularly in dry weather. This is almost inevitable with ladies garments like a polyester saree. lightning that we see in the sky during thunderstorms We also experience a sensation of an electric shock either while opening the door of a car or holding the iron bar of a bus after sliding from our seat.
ELECTRIC CHARGE
Historically the credit of discovery of the fact that amber rubbed with wool or silk cloth attracts light objects goes to Thales of Miletus, Greece, around 600 BC The name electricity is coined from the Greek word elektron meaning amber. Many such pairs of materials were known which on rubbing could attract light objects like straw, pith balls and bits of papers if two glass rods rubbed with wool or silk cloth are brought close to each other, they repel each other The two strands of wool or two pieces of silk cloth, with which the rods were rubbed, also repel each other. However, the glass rod and wool attracted each other Similarly, two plastic rods rubbed with cat’s fur repelled each other but attracted the fur. On the other hand, the plastic rod attracts the glass rod and repel the silk or wool with which the glass rod is rubbed. The glass rod repels the fur.
We say that the bodies like glass or plastic rods, silk, fur and pith balls are electrified. They acquire an electric charge on rubbing. The experiments on pith balls suggested that there are two kinds of electrification and we find that (i) like charges repel and (ii) unlike charges attract each other.
The property which differentiates the two kinds of charges is called the polarity of charge.
The experiments also demonstrated that the charges are transferred from the rods to the pith balls on contact. It is said that the pith balls are electrified or are charged by contact. The property which differentiates the two kinds of charges is called the polarity of charge.
The charges were named as positive and negative by the American scientist Benjamin Franklin.
Apparatus to detect charge
Gold-leaf electroscope:- It consists of a vertical metal rod housed in a box, with two thin gold leaves attached to its bottom end. When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and they diverge. The degree of divergence is an indicator of the amount of charge.
CONDUCTORS AND INSULATORS
CHARGING BY INDUCTION:-
Example 01 : How can you charge a metal sphere positively without touching it?
BASIC PROPERTIES OF ELECTRIC CHARGE
Additivity of charges
Charge is conserved
Quantization of charge
Example 02 : If 10^9 electrons move out of a body to another body every second, how much time is required to get a total charge of 1 C on the other body?
Example 1.3 How much positive and negative charge is there in a cup of water?
COULOMB’S LAW
Coulomb measured the force between two point charges and found that it varied inversely as the square of the distance between the charges and was directly proportional to the product of the magnitude of the two charges and acted along the line joining the two charges. Thus, if two point charges q1 , q2 are separated by a distance r in vacuum, the magnitude of the force (F) between them is given by