Maxwell’s equations tell us that a current that varies in time generates a varying magnetic field, and hence in turn a varying electric field, and so on: the field detaches from the source and propagates. But what is the precise condition for this to happen?
Principle — Source of an EM wave
An accelerated charge (i.e. a current that varies in time) emits an electromagnetic wave that propagates in vacuum at speed and carries energy away from the source. Charges at rest or in uniform rectilinear motion do not radiate.
Hertz’s experiment (1888) was the first experimental confirmation of the EM waves predicted by Maxwell. Hertz used a spark-gap oscillator: two small spheres separated by a thin gap, subjected to high voltage. The spark of dielectric breakdown made the charge oscillate back and forth over a short segment of wire, generating an EM wave. Several metres away, a loop of wire — the receiving antenna — showed a secondary spark, in phase with that of the source. Electromagnetic waves in empty space, reproduced and detected in the laboratory.
Historical context — From Hertz to Marconi
Hertz publicly declared that his waves would never have any practical application: it was pure confirmation of Maxwell. Seven years later, Guglielmo Marconi, twenty-one years old and without formal academic training, made them the basis of wireless telegraphy. In 1895 he was already transmitting Morse signals over km on the family estate at Pontecchio (Bologna); in 1901 he crossed the Atlantic with a station at Poldhu (Cornwall) and a receiver at Signal Hill (Newfoundland). In 1909 he won the Nobel Prize in Physics (Simonyi 2012). It is a classic example of how an “academic confirmation” can, within a few years, turn into mass technology.
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Topics: Electromagnetic waves Concepts: Electromagnetic wave
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