- Mar 8, 2004
Michael Blue;93296 said:How can a pump provide force without using energy?
Some pumps, like a reciprocating pump, can provide a force against backflow without using energy, because they include one-way valves, and there are other things, like any solid object, that can generate force without using energy-- if I set a book on a table, I don't have to have the table plugged in to the wall to keep the book from falling through to the floor. But the table is also not going to lift the book higher into the air: that requires work, which requires energy.
But that's not what I'm trying to get at. The pump is using energy to move the water through the pipe, which pressurizes the water. That pressure is there whether it's in a closed pipe, or whether it's in an open pipe with a column of water pushing down, because the column of water is supplying the same force role as the pressure-containment of the pipe was in the closed system.
I'm not actually saying that the pump is doing anything different in the two (closed and vented) systems, I'm saying that adding the column of water removes the exact same amount of strain energy in the pipe; they're equal forces containing the pressure generated by the pump, so the pump is generating the same flow against the same pressure in both situations. And the water height in the pipe will naturally seek the level where this is the case, if the vent pipe is high enough to accommodate it, anyway.