Hertz produce radio wave when he made an electric spark.
Why is that spark neccessary?
It isn't "necessary"-it was just convenient with the equipment he had. what he needed to do was produce a strong electro-magnetic wave. The spark was just a "side effect".
I understand spark was just a "side effect".
I need to make clear one question first.
Is there a electromagnetic field already(in the setup)?
Then why can and the process of 'making a spark' produce a strong electro-magnetic wave?
The spark itself isn't what produces the wave, it's the oscillating current flow through the LRC circuit. The spark gap acts as a sort of cork stop to the circuit, allowing the capacitor to build up until the potential difference across the spark gap is high enough to form a spark. Once it does this, the LRC circuit is completed and the capacitor is free to discharge, dissipating energy in the form of radio waves in the process.
Also, the high voltage spark is a pulse with generates alot of harmonics. Hertz devised a resonator which he could tune into one of these harmonics and attenuate the rest which resulted in a more cleaner electromagnetic wave.
Why can the oscillating current discharge energy?
Why is the energy in the form of radio waves?
Because accelerated charges radiate (in the classical theory). I won't get into the whole mess about consistency with the equivalence principle. The type of radiation (in this case radio) depends upon the frequency of oscillation.
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