Electric charge-Force and Field, Physics tutorial

Introduction to Electric charge:

Electric charge is the fundamental property of matter carried by a few elementary particles. Electric charge, which can be negative or positive, takes place in discrete natural units and is neither created nor destroyed.

Types of charge:

The ancient Greeks introduced that amber if rubbed with silk get the property of attracting light objects like pieces of chaff. William Gilbert introduced that other substances represent the similar effect, and that the magnitude of the effect is generally proportional to the area of the surface rubbed. He was then led to the idea of a charge of electricity.

Du Fay in the year 1745 discovered that there are two types of electricity. Two ebonite rods if rubbed with fur apply a force of repulsion on one other. The two glass rod rubbed with silk as well repels one another. Though, an ebonite rod that has been rubbed by means of fur attracts a glass rod that has been rubbed with silk.

Any substance rubbed by a different substance acquires a charge of electricity, and is found both to repel charged ebonite and attract charged glass, or vice-versa. As the two types of electricity can neutralize each other effect, one is termed as positive and the other negative. Note that the choice as to which is positive was purely arbitrary. Glass rubbed having silk is stated to have a positive charge and ebonite rubbed by fur a negative charge.

The origin of the positive and negative charge of proton and electron. The law of force between charges might be stated as follows: like charges repel, different charges attract.

We are familiar that an atom comprises of a positively charged nucleus having negatively charged electrons around it. The nucleus is made up of electron and proton. The neutron is neutral (without charge) that the proton and electron have equivalent however opposite charges (negative and positive).

The proton and neutron in the nucleus are held altogether very tightly through a nuclear force. Therefore strong is the nuclear force that the protons are not capable to move away from the nucleus. On the other hand, the force holding electron to the atomic nucleus is much weaker than the nuclear force. Therefore the electrons might move away from the atom.

If two different materials are rubbed altogether, electrons get transferred fairly simple from one material to the other. As a few materials tend to hold their electrons more strongly than others, the direction of transfer of electrons based on the materials. For instance, if a plastic ruler is rubbed by a woolen cloth, electrons flow from wool to plastic, in such a way that it carries total negative charges while the wool, having a deficit of electrons, carries a positive charge of equivalent magnitude.

This procedure of charging the bodies by means of rubbing them altogether is termed as charging by friction. In any case, we must note that friction in reality has nothing to do with the charging process. It would come out that friction is just borrowed to depict the rubbing procedure.

Unit of Charge:

In the System International (SI), electric charge is measured in coulombs (C) that is stated in terms of ampere. A coulomb is the quantity of charge flowing per second via a conductor in which there is a steady current of 1A.

Conservation of Charge:

The Law of conservation of charge illustrates that the total charge of an isolated system remains constant.

When a system starts out with an equivalent number of negative and positive charges, there is nothing we can do to make an excess of one type of charge in that system unless we bring in charge from exterior the system (or eliminate few charge from the system). Similarly, when something begins out having a certain net charge, state +100 e, it will for all time contain +100 e except it is allowed to interact with somewhat external to it.

Charge can be created and destroyed, however merely in positive-negative pairs.

Quantization of charge:

The smallest charge which is possible to acquire is that of an electron or proton. The magnitude of this charge is represented by 'e'. A charge smaller than 'e' has not been found. If one finds out the amount of charge on any charged body (example: a charge sphere) or any charged particle (example: α - particle) or any ion, its charge is for all time found to be an integral multiple of 'e', that is e, 2e, 3e and so forth. No charge will be a fractional multiple of 'e' such as 0.7e or 2.5e. This is true for both negative and positive charges and is represented as:

q = ne 

Here 'n' is a positive or negative integer

Coulomb's law:

Knowledge of the forces which exist between charge particles is essential for a good understanding of the structure of the atom and of matter. The magnitude of the forces between charged spheres was first examined quantitatively in the year 1785 by Charles Coulomb, a French scientist. He noticed that the electrostatic force between the two spheres is proportional to the product of the charges and is inversely proportional to the square of their distance spaced out.

Statement: The electrostatic force 'F' between the two point electric charges 'q1' and 'q2' is directly proportional to the product of the charges and inversely proportional to the square of the distance 'r' between the two charges. The force acts all along the line joining the charges.

Coulomb's law might be defined in mathematical terms as:

F α (q1 q2)/r2

F = K [(q1 q2)/r2]

K = 1/4πε

Here the constant 'ε' based on the material surrounding the charges and is termed as permittivity.

K = 9 x 109 N.m2/C2 = Coulomb's constant

Principle of superposition:

Electric fields formed by various sources, example: by two or more point charges, simply add altogether as vectors. Likewise magnetic fields formed by various sources, example: by two or more current-carrying wires, as well add altogether as vectors. This superposition principle exerts to all electric and magnetic fields, comprising those including electromagnetic waves created by various sources. When the vectors 'E' point approximately in the similar direction at a given instant of time, the effect of adding the vectors will be a sum which is bigger than its parts: this is termed as constructive interference. If on the other hand, the vectors 'E' point approximately in opposite directions, the result will be smaller than its parts, which are termed as destructive interference.

Superposition works for other kinds of waves as well. For illustration, if small-amplitude waves on the surface of a liquid pass one other, the superposition of two wave crests passing each other make an extra-high crest; whereas a crest passing a trough makes a flat spot. (Large-amplitude waves in the ocean are a more complicated story: when two of them meet, they can cause each other to break.)

The Electric Field:

An electric field is an area where an electric charge undergoes a force, just as a football field is an area where the game is played. When a very small, positive point charge 'q' is place at any point in an electric field and it experiences a force 'F', then the field strength 'ε' as well termed as the E-field at that point is stated by the equation.

ε = f/q or F = qE

The magnitude of 'E' is the force per unit charge and its direction is that of 'F' (that is, the direction of the force that acts on a positive charge).

Therefore 'E' is a vector.

Calculating Electric Field:

The electric field is a vector field: at each and every point in space, there is a vector corresponding to the electric field. The force 'F' experienced through a particle 'q' in electric field E is:

F = qE

By joining the equation with Coulomb's Law, we can as well compute the magnitude of the electric field made by a charge 'q' at any point in space. Simply replace Coulomb's Law in for F, and we get:

E = Kq/r2

Field lines:

The electric field can be symbolized by means of electric field lines or lines of force. The lines are drawn in such a way that:

a) The field line at a point (or the tangent to it if it is curved) provides the direction of 'E' at the point. This is the direction in which a positive charge would accelerate.

b) The number of lines per unit cross-section area is proportional to 'E'.

We must note that the field line is imaginary; the illustration serves just the helpful purpose of allowing us to know the general characteristics of the electric field in the whole area at a glance.

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