Which energy is lost by an oscillating charge?

Question

Let us imagine a charged block connected to a fixed spring in gravity free space and in a complete vacuum (to avoid air drag) . Now if an external agent stretches the spring from its natural length and then releases it , then the block starts accelerating and gains kinetic energy which comes from the stored potential energy. Now since it is left on its own (with no external disturbing i.e. no external work) its kinetic and potential energy should just keep transforming into one another and lost of one must appear in the form of the other.

But this can't be true since the block is similar to an accelerating charge so it must radiate energy but again this can't be the kinetic or potential (for the same reason explained in the above para i.e. they must keep transforming). Then the natural question is :-

Where is this energy coming from ? Is it somehow related to the "mass - energy" ?

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P.S. :- I am just a high school student , so i apologise if the question sounds dumb.

Answer 1

Now since it is left on its own (with no external disturbing i.e. no external work) its kinetic and potential energy should just keep transforming into one another and lost of one must appear in the form of the other.

The mistake is in this part. There is also electromagnetic field energy. Thus there are three forms of energy to be considered for energy conservation. Elastic energy in the spring transforms into both kinetic energy and electromagnetic energy. Then only the kinetic energy transforms back into elastic energy. The electromagnetic energy remains as electromagnetic energy as it radiates away from the spring.