Physics · Ch 13 — Electromagnetic Waves and Communication System
Sources of EM waves
Sources of EM waves
Maxwell's theory makes a precise statement about what produces an EM wave: accelerated electric charges radiate EM waves. Consider a charge that oscillates back and forth with some fixed frequency. As it accelerates and decelerates, it sets up an oscillating electric field around it. That oscillating electric field is itself a source of an oscillating magnetic field, and the oscillating magnetic field in turn regenerates an oscillating electric field -- the two fields continuously recreate each other. This self-sustaining exchange between varying electric and magnetic fields is what allows the disturbance to propagate outward through space as a wave, rather than staying localised near the charge.
Because of this mechanism, an EM wave is formally defined as a wave caused by the acceleration of charged particles, made up of electric and magnetic fields that vibrate sinusoidally at right angles both to each other and to the direction the wave travels -- which is why EM waves are said to be transverse waves. A crucial feature of the source mechanism is frequency matching: an oscillating charge always emits an EM wave with exactly the same frequency as the charge's own oscillation. …
What this figure shows. A three-dimensional schematic showing an electromagnetic wave travelling along the z-axis. Two sinusoidal curves are drawn along the propagation direction: one lying in the x-z plane, representing the time-varying electric field oscillating along the x-axis, and one lying in the y-z plane, representing the time-varying magnetic field oscillating along the y-axis. The two curves are drawn perpendicular to each other and both perpendicular to the z-axis, visually illustrating that E, B, and the direction of propagation form a mutually perpendicular (transverse) set, exactly as described in the surrounding text. No numeric values or scales are printed on the figure; i …