Electric Charge

We all expriance a spark of electricity in our daily life work. Rubbing our lather shoes across a wollen or silk carpet and touch a metal doorknob, we know the result that we can be zapped by  a spark of small electricity. This phenomenan we can understand by stuidy of a physics branch called Electrostatics. Electrostatics is the word comes from the Greek word that is the combination of electron, which means "amber” and static means stasnory. Amber is a tree and it was well known to the ancients that if we rub an amber rod with a piece of cloth, the amber attracts small pieces of dry leaves or paper.

Experiments show that there are exactly two kinds of electric charges :

• Negative charge    
• Positive charge

There are two types of charges known as positive and negative charges. All objects normally contain equal amount of positive and negative charges and are therefore, electrically neutral.

If an object has the same amount of positive and negative charge the NET CHARGE on the object is the sum of the two kinds of charge and adds to zero, and we say the object is electrically neutral. So if an object has a number of protons equal to the number of electrons its net charge is zero. (All atoms have a net charge of zero, but ions can have a positive or negative charge depending on whether they have more protons or electrons.) Since the electrons are more mobile than the protons (which are buried in the nucleus of the atom) we transfer electrons to a neutral object to make it have a net negative charge. When we do that the neutral object from which we transferred the electrons becomes positively charged. Note that the total electric charge of the two bodies together does not change.This is known as the conservation of charge. We cannot create or destroy electric charge, we simply transfer charges (electrons) from one object to another to change the NET charge of objects.

The ancient Greeks discovered that electric charge could be transferred between two objects by rubbing them together. In the diagram below, rubbing plastic and fur together results in electrons from the fur being rubbed off onto the plastic, leaving the fur positively charged and the plastic negatively charged. When glass and silk are rubbed together which one has the electrons rubbed off?

Plastic rubbed with fur becomes negatively charged,  glass rubbed with silk becomes positively charged.

When we comb dry hair, the comb gets charged and can pick small pieces of paper brought near it. When we rub a glass rod with silk cloth or a piece of ebonite rod with woolen material, the charge acquired by a glass rod rubbed with silk is called a positive charge and that on ebonite rod, is called a negative charge.

Although the charges (on the electrons) are tightly bound to the atoms in an insulator they are free to move slightly within the atom. This is called polarization. If a plastic comb is rubbed on fur (or your dry hair) electrons will be rubbed off the hair onto the plastic comb. The plastic comb becomes charged negatively. If the comb is brought close to a neutral insulator, like a piece of dry paper, it will repel the negatively charged electrons in the atoms causing them to moving away slightly, leaving the protons without an electron closer to the comb. Since opposite charges attract and the positive charges are closer to the comb than the negative charges, the piece of paper is attracted to the comb. This effect is used to remove soot and ashes from smoke going up industrial chimneys. The inventor became rich as a result of his patent.

• A charged comb attracts a piece of paper.
• Part (b) of the diagram shows the same result (attraction of the paper) even if we had a positively charged comb.
• Glass rod and ebonite rod will attract each other while two glass rods as well as two ebonite rods will repel each other.