Charged particles emit electromagnetic radiation when they are accelerated because of a fundamental principle in electrodynamics called "acceleration radiation" or "bremsstrahlung" (German for "braking radiation"). This phenomenon is a consequence of the interaction between charged particles and electromagnetic fields.
According to classical electrodynamics, when a charged particle accelerates or changes its velocity, it creates disturbances in the surrounding electromagnetic field. These disturbances propagate outward as electromagnetic waves, which we perceive as radiation. The emission of this radiation carries away energy and momentum from the charged particle, causing it to lose energy and decelerate.
The process can be understood through Maxwell's equations, which describe the behavior of electromagnetic fields. When a charged particle accelerates, its changing velocity induces time-varying electric and magnetic fields around it. These changing fields propagate as electromagnetic waves, carrying energy and momentum away from the particle.
The emitted electromagnetic radiation can have various frequencies, depending on the nature of the acceleration. If the acceleration is random or caused by thermal motion, the emitted radiation spans a wide range of frequencies, resulting in a continuous spectrum known as thermal or blackbody radiation. This is the type of radiation emitted by objects at finite temperatures, such as hot metals.
In the case of charged particles moving in curved paths or undergoing oscillatory motion, they emit radiation at specific frequencies determined by their acceleration characteristics. This radiation can manifest as discrete spectral lines or as a broad range of frequencies depending on the specific conditions.
It's important to note that the description above applies to classical electrodynamics. In the realm of quantum mechanics, the behavior of accelerated charged particles and the emission of radiation are described by quantum electrodynamics (QED). QED provides a more comprehensive and accurate description of these phenomena, incorporating the quantum nature of both particles and electromagnetic fields.
In summary, charged particles emit electromagnetic radiation when accelerated because of the interaction between their changing velocities and the surrounding electromagnetic fields. This process, known as acceleration radiation or bremsstrahlung, is a fundamental aspect of electrodynamics.