Electricity - Electric charges

Physics is the detailed study of nature based on the observations from the nature.

In that way, a long time before 600 BC it was observed that certain objects such as rods of amber, on rubbing with cat's fur attracts light objects like feathers.

And the other famous observation which observed for long time from our birth is the lightning.

Many stories were told in various civilizations to explain this lightning.

But as a physicist, we should only concern about real experimental observations.

To get a clear vision of any observation, we are trying to make some experiments on that observation.

Similarly in the above case,

            We rubbed many things with one another and tested whether it gives shock or not.

And the experimental work was done by the great Thales of Miletus and later by William Gilbert.

After many experiments, it was observed the electric shock only when we rub two things such as rods of amber rubbed with cat’s fur, glass with silk, hard rubber rubbed with fur, and comb through hair, etc.

This type of contact electrification in which certain materials become electrically charged after they come into contact with another different material through friction is called as the triboelectric effect (also known as triboelectric charging).  They also noted that if they rubbed the amber for long enough, they could even get an electric spark to jump.

After rubbing when you take the two different rubbed objects near to each other, they attracted each other. Instead of two different objects, it you take the two parts of same object it repels each other.

It means that something is there inside these objects which make them to attract or repel. And that something else is called as Electric charges.

The continuous experiments showed that same objects repelled each other. e.g. Amber repelled amber, glass repelled glass and ebonite repelled ebonite. Due to symmetrical argument the charge on one amber piece is the same as the charge on the other amber piece. So we concluded that same charges (similar charges) repel each other.

It was also observed that different objects like glass and ebonite attracted each other. We know that the charge on the glass will not be there in ebonite as we already know that like charges repel each other. So it must be two different charges.

From careful experiments it was concluded that there are only two kinds of charges because all these charged objects showed only two qualities that is attracting or repelling. Same kind of charges repels each other and different kind of objects attracts each other.

E.g. When amber rubbed with cat’s fur,

Amber repels another amber piece – Same kind of charges repel each other  

Amber attracts fur – Different kind of charges attracts each other

similarly when Ebonite rubbed with fur,

Ebonite repels another ebonite piece - Same kind of charges repel each other  

Ebonite attracts fur - Different kind of charges attracts each other

and Glass with silk,

Glass repels another Glass piece - Same kind of charges repel each other 

Glass attracts silk - Different kind of charges attracts each other

but also Ebonite attracts Glass - Different kind of charges attracts each other

So it says that the charge on ebonite is different from the charge on the glass. And Ebonite repelled silk - Same kind of charges repel each other.

And so we concluded that the charge on ebonite is different from the charge on the glass and the charge on the ebonite is same as the charge on the silk.

First we named the charge on the glass as the positive charge. Being the same it implies that the charge on the fur is same as the glass that is positive.

It was named the charge on the ebonite as negative charge and also the charge on the silk. 
Only these two kinds of charges were found yet. When two objects rubbed with each other, the gaining of positive or negative charge is determined by humans and it was ordered in the following triboelectric series.

  
Triboelectric Series:

                    Order from  Most positively charged  {+}   to 
                                        Most Negatively Charged {-} 

Polyurethane foam {+}
Hair, oily skin
Nylon, dry skin
Glass
Acrylic, Lucite
Leather
Rabbit's fur
Quartz
Mica
Lead
Cat's fur
Silk
Aluminium
Paper (Small positive charge)
Cotton
Wool (No charge)

 Place where the change of sign happens with the materials {0}

Steel (No charge)
Wood (Small negative charge)
Amber
Sealing wax
Polystyrene
Rubber balloon
Resins
Hard rubber
Nickel, Copper
Sulfur
Brass, Silver
Gold, Platinum
Acetate, Rayon
Synthetic rubber
Polyester
Styrene and polystyrene
Orlon
Plastic wrap
Polyethylene (like Scotch tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon
Silicone rubber
Ebonite    {−}

Our final conclusions are,

•  There are two charges in nature. And it was called the positive and the negative electric charges.
•   Like charges repel each other and unlike charges attracts each other.
•   Positive repels positive.
•   Negative repels negative.
•   Positive attracts negative.
•   This classification of positive and negative charges was termed by American scientist, Benjamin Franklin who is the one of the Founding Fathers of the United States of America.
•   It was assumed that the charge on the Glass is positive when rubbed with silk. And obviously the charge on the silk is negative as they attracted each other.  

Thus we found two new electric charges which gave us a start to examine Electricity.