What happens when the piston in a piston positive displacement pump moves back?

When the piston in a piston positive displacement pump moves back, atmospheric pressure plays a crucial role in facilitating the movement of water. As the piston retracts, it creates a vacuum within the cylinder, resulting in a pressure drop inside the chamber. This lower pressure allows atmospheric pressure, which is higher outside the pump, to push water into the cylinder from the source or reservoir.

This design utilizes the principle of pressure differential, where fluids naturally move from areas of higher pressure to areas of lower pressure. Thus, the correct understanding of the action of the piston moving back is that it creates conditions favorable for water intake, relying on the external atmospheric pressure to fill the newly formed space in the cylinder.