The electric device which blocks DC but allows AC?

CAPACITOR

Explanation:

An interesting property of capacitors is that they will block DC after they become charged, but will (for the most part) allow AC to flow through. When a capacitor is fully discharged, DC can flow through it freely, but as it is doing so, the cap is gradually becoming charged. Finally, when it has reached its storage limit, the cap will not allow any more electricity to flow through it, and will act as a blocker on the circuit. This can be observed if you simply wire a capacitor in series with a simple circuit connecting a battery to a light. When the circuit first comes on, the light will turn on, but after some time (when the cap becomes fully charged) it will turn off. Exactly how long it takes this to happen depends on the capacitance of the cap; With very low-capacitance caps, it will probably happen faster than your eye can perceive, but it still will happen. If you remove the battery and simply leave the cap connected to the light, the light will turn on again as the cap flows into itself, acting like a battery, and the light will stay on until the cap fully discharges. So a cap blocks DC; If you think about it for a moment, you should be able to understand why they allow AC to pass through: AC keeps reversing its polarity. As long as the AC switches fast enough to prevent the cap from becoming fully charged in any direction, then the cap will partially charge in one direction, and as the AC polarity reverses, the cap will start to discharge, then charge in the opposite direction. Very low-capacitance caps may partially block the AC because they become fully loaded before the AC cycle is complete, however. TOO many Words for an explanation yes it block DC and it acts as a storage of energy. charged or not charged it will block dc IF THE DC changes then more or less it will pass this levels because varying DC is AC after all,