Sources of Electromagnetic Waves

Whether a charge radiates an electromagnetic wave depends entirely on how it moves. A stationary charge produces only a static (unchanging) electric field around it and nothing else. A charge moving with uniform (constant) velocity produces a steady current, which in turn produces a magnetic field around the conductor -- but crucially this magnetic field does not change with time (it only varies with position, i.e. it is space-dependent, not time-dependent), so by Faraday's and Maxwell's equations it cannot induce any further changing field, and no wave is radiated. Only when the charged particle accelerates -- meaning its velocity changes in magnitude or direction -- does it produce a genuinely time-varying magnetic field in addition to its electric field, with both fields now varying with time. Since electromagnetic waves are transverse, the resulting wave's direction of propagation is perpendicular to the plane containing the oscillating electric and magnetic field vectors. Because any oscillatory motion about a mean position is itself a continuous form of accelerated motion (the velocity is constantly reversing direction), an oscillating charge -- such as the spark discharge in Hertz's apparatus, or a vibrating molecular dipole -- is therefore always a source of electromagnetic waves. This is the single unifying principle behind every practical source of electromagnetic radiation discussed later in the unit, from radio antennas (a …