Overview of Electric charge:
If you rub a hard rubber pen or comb on a sheet of paper if it is free to move easily. The paper and rubber then give evidence of a static electric charge. The work of rubbing resulted in separating electrons and protons to produce a charge of excess electrons on the surface of rubber and a charge of of excess protons on the paper.
Because paper and rubber are dielectric materials, they hold their extra electrons or protons. As a result , the paper and rubber are no longer neutral, but each has an electric charge. The resultant electric charges provide the force of attraction between the rubber and the paper. This mechanical force of attraction or repulsion between charges is the fundamental method by which electricity makes itself evident.
Any charge is an example of static electricity because the electrons or protons are not in motion. There are many examples. When we walk across a wool rug, your body becomes charge with an excess of electrons. Similarly, silk, fur, and glass can be rubbed to produce a static charge. This effect is more evident in dry weather, because a moist dielectric does not hold its charge so well. Also, plastic materials can be charged easily, which is why thin, lightweight plastics seem to stick to everything.
The charge of many billions of electrons or protons is necessary for common applications of electricity. Therefore, it is convenient to define a particle unit called the coulomb (C) as equal to the charge of 6.25 X 1018 electrons or protons stored in a dielectric ( see figure a , b ). The analysis of static charges and their forces is called electrostatic.
The symbol for electric charge is Q or q, standing for quantity, For instance, a charge of 6.25 X 10 18 electrons is stated as Q = 1C. This unit is named of ter Charles A Coulomb (1736 – 1806), a French physicist, who measures the force between charges.
Negative and Positive Polarities of Electric Charge:
Historically, the negative polarity has been assigned to the static charge produced on rubber, amber, and resinous materials in general. Positive polarity refers to the static charge produced on glass and other vitreous materials. On this basis, the electrons in all atoms are basic particles of negative charge because their polarity is the same as the charge on rubber. Protons have positive charge because the polarity is the same as the charge on glass.
Electric Charges of Opposite Polarities:
If two small charged bodies of light weight are mounted so that they are free to move easily and are placed close to each other, one can be attracted to the other when the two charges have opposite polarity ( see diagram (c ) . In terms of electrons and protons, they tend to be attracted to each other by the force of attraction between opposite charges. Furthermore, the weight of an electron is only about 1/1840 the weight of a proton. As a result, the force of attraction tends to make electrons move to protons.
Electric Charges of Same Polarities:
In Diagram d and e , it is shown that when the two bodies have an equal amount of charge with the same polarity , they repel each other. The two negative charges repel in diagram ( d ), while two positive charges of the same value repel each other as in diagram ( e).
Polarity of an Electric Charge:
An electron charge must have either negative or positive polarity, labeled – Q or + Q, with an excess of either electrons or protons. A neutral condition is considered zero charge. On this basis, consider the following examples, remembering that the electron is the basic particle of charge and the proton has exactly the same amount, although of opposite polarity.
Example 1: A neutral dielectric has added to it 12.5 × 1018 electrons. What is charge in coulomb?
Answer: This number of electrons is double the charge of 1 C. Therefore, -Q = 2 C
Example 2: A dielectric has a positive charge of 12.5 × 1018 protons. What is its charge in coulomb?
Answer: This is the same amount of charge as in example 1 but positive. Therefore, +Q = 2C
Note that we consider the electrons moving, rather than the heavier protons. However, a loss of a give number of electrons is equivalent to a gain of the same number of protons.
Charge of an Electron:
Fundamentally, the quantity of an charge is measured by its force of attraction or repulsion. The extremely small force of an electron or proton was measured by Milliken in experiments done from 1908 to 1917. Very briefly, the method consisted of measuring the charge on vaporized droplets of oil, by balancing the gravitational force against an electrical force that could be measured very precisely.
A small drop of oil sprayed from an atomizer becomes charged by friction. Furthermore, the charges can be increased or decreased slightly by radiation. These very small changes in the amount of chare were measured. The Three smallest values were 0.16 × 10-18 C, 0.32 × 10-18 C, and 0.48 × 10-18 C. These values are multiples of 0.16. In fact, all the charges measured were multiples of 0.16 × 10-18 C. Therefore, we conclude that 0.16 × 10-18 C is the basic charge from which all other values are derived. This ultimate charge of 0.16 × 10-18 C is the charge of 1 electron or 1 proton, then
1 electron or Q = 0.16 × 10-18 C
The reciprocal of 0.16 × 10-18 gives the number of electrons or protons in 1 C. Then
1 C = 6.25 × 10-18 electrons
Note that the factor 6.25 equals exactly 1/0.16 and the1018 is the reciprocal of 10-18
The Electric Field of a Static Charge:
The ability of an electric charge to attract or repel another charge is actually a physical force. To help visualize this effect, lines of force are used, as shown in Diagram ( f ). All the lines form the electric field. The lines and the field are imaginary, since they cannot be seen. Just as the field of the force of gravity is not visible, however, the resulting physical effects prove the field is there.
Each line of force directed outward to indicate repulsion of another charge in the field with the same polarity as Q, either positive or negative. The lines are shorter further away from Q to indicate that the force decreases inversely as the square of the distance. The larger the charge, the greater is the force. These relations describe coulomb’s law of electrostatics.
The electric field in the dielectric between two plates with opposite charges is the basic for the ability of a capacitor to store electric charge.