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Where is the missing energy ?

A capacitor (capacity C) is charged at the voltage U

and then disconnected from the power supply. It contains the electric charge Q = C·U and the energy W = 0.5·C·U2.

Now, the capacitor is connected (parallel) to the same uncharged capacitor (total capacity now C+C=2·C), and the voltage will drop to U1 = Q / (2·C) = U / 2.

Each of the capacitors has the energy W1 = 0.5·C·(U/2)2 = W / 4 and both capacitors together have the energy W2 = 2·W1 = W / 2, which is half of the energy W at start, even if the wires are very thick and have very little resistance.

Where is the missing energy ???

The voltage doesn't drop to U1=U/2

The is an error in the calculation.

The calculation is correct.
My explanation for the loss of half the energy:

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