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The equation PV = nRT represents the ideal gas law, where P is the pressure of the gas, V is the volume, n is the number of moles, R is the universal gas constant, and T is the temperature. Gibbs's free energy formula
Maxwell's equations in curved spacetime, commonly used in high-energy and gravitational physics, are compatible with general relativity. [ note 2 ] In fact, Albert Einstein developed special and general relativity to accommodate the invariant speed of light, a consequence of Maxwell's equations, with the principle that only relative movement ...
The electromagnetic wave equation is a second-order partial differential equation that describes the propagation of electromagnetic waves through a medium or in a vacuum. It is a three-dimensional form of the wave equation. The homogeneous form of the equation, written in terms of either the electric field E or the magnetic field B, takes the form:
These equations say respectively: a photon has zero rest mass; the photon energy is hν = hc|k| (k is the wave vector, c is speed of light); its electromagnetic momentum is ħk [ħ = h/(2π)]; the polarization μ = ±1 is the eigenvalue of the z-component of the photon spin.
Both differential equations have the form of the general wave equation for waves propagating with speed , where is a function of time and location, which gives the amplitude of the wave at some time at a certain location: = This is also written as: = where denotes the so-called d'Alembert operator, which in Cartesian coordinates is given as
This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave. Maxwell's equations predicted an infinite range of frequencies of electromagnetic waves, all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum.
Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames , but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.
Intuitively the wave envelope is the "global profile" of the wave, which "contains" changing "local profiles inside the global profile". Each propagates at generally different speeds determined by the important function called the dispersion relation .