Electric charge is a characteristic that accompanies fundamental particles, wherever they exist.
According to William Gilbert, charge is something possessed by material objects that make it possible for them to exert electrical forces and to resin to electrical force.
The three most common elementary particles are electrons, protons and neutrons having masses
Me = 9, 1040 x 10-31 kg;
Mp = 1, 67262 x 10-27 kg and
Mn = 1, 67 493 x 10 – 27kg
Because of their mass, theses particles attract one another by gravitational forces. Thus an electron attracts another electron at a distance of 1 cm, with a gravitational force
F = Gm1m2 / r2 = 6.67 x 10-11 x (9.1 x 10 – 31)2 / (10-2)2
F = 5.5 x 10-67 N
However, an electron repels another electron at a distance of 1 cm with a force = 2.3 x 10-24N.this force is called electric force. We observe that electric force is very large compared to the gravitational force. The electrons must have some additional property (other than their mass). Which is responsible for the electric force between them? This additional property of electron, which gives rise to electric force between tow electrons, is called electric charge. Just as masses are responsible for the gravitational force. Charges are responsible for the electric force.
Two protons placed at a distance of 1 cm also repel each other with the same force = 2.3 x 10-24N. it shows that protons also have charge which in magnitude, must be equal to charge on electron. Two neuters placed at a distance of 1 cm attract each other with a force = 1.9 x 10-60N, which’s the gravitational force dot to their masses. However, electric force between two neutrons is zero. Therefore, neutrons have no charge though they have mass.
Two kinds if charges
Let us look at the some simple facts, which were established from years of efforts and carful experiments.
(i) A glass rod rubbed with a piece of silk brought close to a suspended glass rod rubbed with silk repels the latter as shown in fig 1. 1 (a).
(ii) The two pieces of silk cloth with which the glass rods were rubbed also repel each other. However, cash glass rod attracts the silk piece with which it was rubbed.
(iii) Two ebonite/amber rods rubbed with cat’s fur repelled rachother as shown in fig.1.1 (b). however each cenobite rod attracted the cal’s fur
(iv) An ebonite rod rubbed with cat’s fur attracted a glass rod rubbed with a piece of silk as shown in fig. 1.1(c) however the charged glass rods repelled the cat’s fur.
(v) When a glass rod rubbed with a piece of silk is made to touch tow small pith balls (or polystyrene balls) suspended by silk/nylon threads, the balls repel each other, as shown in fig. 1.1(d).
(vi) Similarly, when an ebonite/ember rod rubbed with cat’s fur is made to touch two small pith balls suspended by silk/nylon threads, the balls repel each other .
(vii) However, az pith ball (or polystyrene ball) touched with charged glass rod attracts another pith bal touched with charged ebonite rod
A careful analysis of these facts observed leads us to the following conclusion:
(i) The bodies like glass rods ebonite/amber rods. Silk fur etc. acquires electric charge on rubbing.
(ii) The nith balls or polystyrene balls acquire electric charge by actual contact with a charged body.
(iii) The charge developed on glass rod rubbed with silk is different from the charge developed on ebonite rod fobbed with fur. This is because a charged gals rod repels another charged glass rod, but attracts a charged ebonite rod. Hence, there are two kinds of charges. When a glass rod is rubbed with silk, the rod acquires one kind of charge and the silk acquires second kind of charge. This is true for any pair of objects that are rubbed to be electrified.
Further, like charges repel each other and unlike cares attract each other.
(iv) When an electrified glass rod is brought in contact with the silk piece, with which it was rubbed, they no longer attract each other. They also do not attract other light objects as they did on being electrified. It means the charges acquired on rubbing are lost when the two oppositely charges bodies are brought in contact with each other. It leads us to the conclusion that charges acquired b the objects must be equal and opposite. That is why they neutralize or nullify each other’s effect. An American scientist Benjamin Franklin named the two kinds of charges as positive and negative. By convention charge acquired by glass rod or cat’s fur is called positive. And the charge acquired by ebonite/amber rod or silk cloth is called negative.
Hence, there are two kinds of charges. The property which differentiates the kinds of charges is called the polarity of charge.
If a body possesss an electric charge, it is said to be charged or electrified, when it has o charge, it is said to be neutral.
Further, note that only rubbed area of non conducting body gees charged, and this charge does not move to other parts of the body, the charge is states on rubbed portion only
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