Electromagnetic field
The empirical investigation of electromagnetism is at least as old as the ancient Greek philosopher, mathematician and scientist Thales of Miletus, who around 600 BCE described his experiments rubbing fur of animals on various materials such as amber creating static electricity.
Michael Faraday visualized this in terms of the charges interacting via the electric field.
[8] Faraday then made the seminal observation that time-varying magnetic fields could induce electric currents in 1831.
In 1861, James Clerk Maxwell synthesized all the work to date on electrical and magnetic phenomena into a single mathematical theory, from which he then deduced that light is an electromagnetic wave.
Maxwell's continuous field theory was very successful until evidence supporting the atomic model of matter emerged.
Beginning in 1877, Hendrik Lorentz developed an atomic model of electromagnetism and in 1897 J. J. Thomson completed experiments that defined the electron.
The electrical generator and motor were invented using only the empirical findings like Faraday's and Ampere's laws combined with practical experience.
[9] With the advent of special relativity, physical laws became amenable to the formalism of tensors.
Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws.
is the vacuum permeability, and J is the current density vector, also a function of time and position.
The Lorentz force law governs the interaction of the electromagnetic field with charged matter.
[10] The Maxwell equations simplify when the charge density at each point in space does not change over time and all electric currents likewise remain constant.
All of the time derivatives vanish from the equations, leaving two expressions that involve the electric field,
These expressions are the basic equations of electrostatics, which focuses on situations where electrical charges do not move, and magnetostatics, the corresponding area of magnetic phenomena.
For example, suppose that a laboratory contains a long straight wire that carries an electrical current.
In the rest frame of the laboratory, there is no electric field to explain the test charge being pulled towards or pushed away from the wire.
So, an observer in the laboratory rest frame concludes that a magnetic field must be present.
and J are zero, the electric and magnetic fields satisfy these electromagnetic wave equations:[15][16] James Clerk Maxwell was the first to obtain this relationship by his completion of Maxwell's equations with the addition of a displacement current term to Ampere's circuital law.
Such radiation can occur across a wide range of frequencies called the electromagnetic spectrum, including radio waves, microwave, infrared, visible light, ultraviolet light, X-rays, and gamma rays.
A notable application of visible light is that this type of energy from the Sun powers all life on Earth that either makes or uses oxygen.
Changing electric dipole fields, as such, are used commercially as near-fields mainly as a source of dielectric heating.
[17] On the other hand, radiation from other parts of the electromagnetic spectrum, such as ultraviolet light[18] and gamma rays,[19] are known to cause significant harm in some circumstances.
