Why don engineers place turbines in series in dams to use the water again and again ?”
There is a limited amount of potential energy in the water behind a dam. You cannot just set up an endless number of turbines to generate endless energy. Once the water passes through the first turbine, its energy is mostly spent. If another identical turbine were set up in the exhaust of the first turbine, it would require the same amount of force as the first turbine. Water would build up in front of that second turbine until enough force had built up to turn the second turbine. If the water is exerting that force on the second turbine, it is also exerting that force on the first turbine in the opposite direction. That first turbine will not generate as much energy, since the pressure differential between the water entering into it, and the water on the other side of it is nearly equal. It is the same reason why the don’t just build multiple dams in a row. The water in between the turbines (and dams) pushes both ways. Energy can be generated when the turbine is the border between lot
