Electromagnetic & Mechanic

turn led to Maxwell's equation. In this picture electromagnetic field is supposed to surround every electric charge and when the charge moves, the propagation of disturbance in the field was described by waves. The finite velocity of propagation of electromagnetic field was identified with the speed of light. In 1905,

Einstein proposed special theory of relativity which recognized the speed of light , as having a special significance in being the maximum velocity attainable by any physical entity. This led to a revision of the ideas of spacetime. Therefore, the special relativity explained why the Coulomb's law failed to describe all electromagnetism. The interaction in Coulomb's law is instantaneous: i. e the interaction propagated from one particle to another with infinite velocity. This conclusion led people to wonder whether the Newton's laws of gravitation was also wrong. Once we accept special relativity, we must object to Newton's laws of gravitation on the same grounds that apply to Coulomb's laws. In 1915, Einstein proposed a radical revision of the law of gravitation which is known as general theory of relativity.

Einstein approached the subject of gravitation by comparing the properties of gravitation with those of electromagnetic field. From the broad similarity of Newton's and Coulomb's laws, we might expect gravitation to be a field in the Maxwellian sense. However, the electromagnetic field is

capable of being switched on or off while ( in the absence of antigravity ), the gravitational field in a given region of spacetime is more permanent. Einstein expressed this result by relating gravitation to the geometrical properties of spacetime. He postulated that the presence of gravitation means the spacetime geometry is non-Euclidean. Starting from the principle of relativity and finiteness of

the velocity of propagation of interaction emerges Einstein's relativity in terms of Lorentz invariance. From the principle of equivalence of the motion between the noninertial and inertial system in a gravitational field will emerge the notion of curved space time, the general theory

of relativity.)

In classical physics, "particles" and "fields" are two different concepts: they are characterized by discreteness and continuity respectively. A system may contain a vast number of particles - such as the molecules in a gas, but as long as they are denumerable, the basic theory is classical dynamics. But if the variables of the system are not denumerable - such as electric field or velocity field of a fluid, the

basic theory is called field theory. The fundamental theory in classical dynamics is the Newtonian dynamics which can be expressed in different mathematical forms, such as Lagrangian, Hamiltonian and the variational form.

The phenomena and the laws of fields are expressed in partial differential equations such as those of fluid dynamics and electromagnetic fields. But these equations can also be expressed in the form of Lagrange or Hamilton's equations which can be derived using a variational principle. Therefore variational methods unify the description of particle and waves.

The quantum theory of radiation of Einstein in 1905, led us to seek a mathematical theory to describe the quantum properties of continuum fields. This is analogous to the quantization of atomic