[List of Quiz Problems]     Deutsche Seite 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:  Enable JavaScript to get the solution.   [List of my Quiz Problems] Last modified :