Study guide asked/stated: A 100 microF capacitor has 100 V across it terminals, A 100 ohm resistor is placed across these terminals. How much energy is lost when the voltage is dropped to 50V? I answered with w= 1/2 CV^2, 1/2*100*10E-6*100^2 = .5 J Book says w = 25 * 10-6 * 10^4. = .25 J (next step is to calculate energy at 50V). Where did the 25 come from? Hope this is the right forum (and not already asked and answered).