Contents
Physics Topics can help us understand the behavior of the natural world around us.
What is meant by Electric Charge? What are the different kinds of Electric charges?
In about 600 BC, Greek philosopher Thales of Miletus observed that if amber (hard resin from pine tree) is rubbed with flannel it acquires the property of attracting small pieces of paper or light bodies towards it. In 1600 AD, William Gilbert, a physician to Queen Elizabeth, observed that many other substances behave in the same way.
As for example, a glass rod rubbed with silk exhibits the same phenomenon. Even when we comb our hair, the comb will show the same property. An object which gets such an ability to attract others due to rubbing, is called an elec-trified object and the process is called electrification. Practically, this electrification is due to transfer of electric charge from one object to another.
The electricity produced by rubbing is called frictional electricity. This type of electricity remains confined within the body where it originates and cannot move from one place to another. Hence it is also called statical electricity’.
The word ‘electricity’ is derived probably from electron which is the Greek name of amber.
Electric charge: Gravitational attraction exists between any two particles inside a material. The mass of each particle is the property which is responsible for this attraction. On the other hand, the repulsion between two elements or the attraction between an electron and a proton is due to a force known as an electric force. This force originates from a property of the particles known as electric charge. This is a measurable physical quantity.
- Electric charge is a scalar quantity.
- Its unit in CGS system is esu or statcoulomb (state).
- The unit of charge in SI is coulomb (C).
1 C = 3 × 109 statC - The dimension of electric charge is IT.
Electrification By Rubbing
Experiment: In winter, after combing our hair if we bring the comb near some small pieces of paper, it will attract them. Here, the comb is electrified due to rubbing with hair and it acquires the property of attracting the pieces of paper. Some-times the comb and the hair are so much electrified during rubbing that we can hear a crackling sound due to tiny sparks.
Example:
A metallic chain is often hung from the fuel tank of a motor vehicle and it touches the road below. When the vehicle is in motion, statical electricity is developed due to rub-bing between the fuel and the inner wall of the tank. The hang-ing chain helps to move the electric charges to the ground instantly. In the absence of such a chain, the accumulated charges may inflame the fuel in the tank.
Two Kinds of Electric Charge : Positive and Negative
With the help of a simple experiment it can be proved that the electric charges are of two types — positive charge and negative charge.
Experiment:
A glass rod rubbed with silk is suspended by a silk thread from a support [Fig (a)]. An ebonite rod rubbed with fur is suspended from the same support close to the
glass rod. The two rods will attract each other. So two bodies being electrified differently may attract each other.
The ebonite rod is now replaced with another glass rod, rubbed with silk and is suspended in its position [Figure (b)]. The two rods will repel each other. So two bodies being electrified similarly repel each other.
Inference: From the above experiment we can infer that similar electric charges are produced in two similar rods if they are rubbed with similar substances. Since the two rods repel each other, it is proved that similar charges repel each other. On the other hand, opposite charges may be produced in two different rods if they are rubbed with different substances. Then they attract each other. Here, we can infer that opposite charges attract each other.
The fundamental law of statical electricity: Opposite or unlike charges attract each other and similar or like charges repel each other. Note that both the charges attract an uncharged body.
American scientist Benjamin Franklin called one of them positive charge and the other one negative charge. The reason is that if we give equal amounts of two opposite charges to a body, it will remain uncharged (like 5 – 5 = 0). The convention is that the charge produced in a glass rod rubbed with silk is positive and the charge produced in an ebonite rod rubbed with flannel is negative.
Electrostatic series: This is a list where the substances have been arranged in such a way that if any two of them are rubbed together, the one preceding the other in the list acquires a positive charge and the latter acquires a negative charge. This is known as the electrostatic series.
- Fur
- Flannel
- Sealing wax
- Glass
- Paper
- Silk
- Human body
- Wood
- Metals
- Rubber
- Resin
- Amber
- Sulphur
- Ebonite
From the table it is clear that a particular substance may be either positively charged or negatively charged if it is rubbed with two different substances. For example, a glass rod becomes positively charged if it is rubbed with silk and negatively charged if it is rubbed with flannel.
Repulsion is the Conclusive Test of Electrification
A charged body attracts an uncharged body as well as an oppo-sitely charged body. But if repulsion takes place between the two bodies, it can be concluded that the experimental body must be charged, because repulsion is possible only between two bodies with the same kind of charge. Hence, repulsion is the conclusive test of electrification.