Some experiments have shown that there are certain pairs of materials which on rubbing could attract light objects like straw, pith balls and bits of paper.
These objects are said to be electrified.
Some of these pairs are :
Amber with wool or silk cloth ( Thales of Miletus, Greece )
Glass rod rubbed with silk and glass rod rubbed with wool repel each other
Glass rod and wool attract each other
Comb rubbed with dry hair attracts bits of paper
CONCLUSIONS from various Experiments
There are two kinds of charges (electrification of the same entity) :
Like charges repel each other.
Unlike charges attract each other.
The property which differentiates the two kinds of charges is polarity.
The magnitude of both the charges is same 1.6* 10^-19 C (e).
GOLD LEAF ELECTROSCOPE
(For detection of charge)
Degree of divergence is an indication of amount of charge.
BASIC PROPERTIES OF ELECTRIC CHARGE
1. Additive Nature
Charges add up like real numbers.
For a system of 3 charges q1, q2, q3
Q(net) = q1+q2+q3
Let q1= +3c, q2= -2c, q3= +6c
Q(net) = (+3c)+(-2c)+(6c)= 7c
Total charge of an isolated system is always conserved.
According to the above law,
Q1 + Q2 = Q3 + Q4
Charge particles may be created or destroyed but charge cannot be created or destroyed.
All free charges are integral multiple of a basic unit of charge denoted by e.
Q (any body ) = ne
Where n = any integer
At macroscopic level, quantization of charge can be ignored and we can assume that charge can take continuous values.
CONDUCTORS AND INSULATORS
Substances which allow electricity to pass through them easily.
They have free electrons insidethem.
For Example: metals, human body, animal body
Substances which offer high resistance to the passage of electricity through them.
Most electrons inside them are bound.
For Example: plastics, glass, nylon, wood (non-metals)
History of electric charges
Conclusions of various experiments
Gold Leaf Electroscope Experiment
Conductors and Insulators
Basic properties of electric charge :
Conservation of charge
Quantization of charge