A cylinder with a piston to seal it starts empty. The are of the piston give a force of about 100 pounds. Lift the piston till there is 100 liters of vacuum. Now 2 cases: 1) release the piston, it slams down almost instantly. Since 1 atmosphere is about 10 newtons per centimeter, and 1 liter is 10 centimeter liters, 1 atmosphere-liter is 100 newton meters or joules. 100 liters makes for 10,000 joules of energy, as it hits the bottom. So loud bang, lots of heat. 2) Here is the real question. What happens if instead a hole is opened in the piston, letting air rush in. Ideal gas law says if volume increase/decreases work happens, but opening volume to connect, should create no work. In fact, opening a compressed bottle of air it cools of, as far as that air goes, it is expanding against a wall of other gas. Not precise i know. Either way, the atmosphere is lifted 100 liters, then dropped. Do that with a giant can of sand, drop it, bang. Open it, sand trickles down. Energy does not go the same place. Initially, the air expanding to fill the vacuum should cool slightly, then more air from outside the room, 100 liters of it, comes in to the room to equalize pressure. So, where does the 10,000 newtons go? Does the air in the room gain 10,000 joules of heat? or is the 10,000 joules distributed through the atmosphere, as would happen with the sand example?