I'm having a hard time understanding air flow caused by pressure differentials. Given the following contextual image, crafted by myself: Bare with me here, these are two interconnected boxes open to the Earth's atmosphere. The orange arrow represents an air pump, such as a vacuum cleaner's electric motor, which fills this connection's space completely. When off, atmospheric pressure is observed at every point. Now turn the pump on. I can clearly predict based on common sense that an air flow will be generated, following this path: atmosphere->box 1->pump->box 2->atmosphere. However, I realize I don't understand much of what's happening. I would suppose box 1 would experience a sudden drop of pressure, while a rise of pressure would occur in box 2, dragging air in and out... But how does this happen? The initial location of the air flow is the atmosphere, and so is its final location. The pressure differential between the atmosphere and the atmosphere is 0... Summarizing into questions: 1- How is their an air flow if the net pressure differential is 0? 2- Is the pressure drop in box 1 the same in every single point of its interior? Same goes for box 2. 3- If so, is it exactly the same or just a very good approximation? 4- If it is exactly the same, why can't the atmosphere be considered an extension of the box and also suffer the pressure drop/increase? (Supposing one of the boxes is now sealed, leading to question 5) 5- What happens with respect to air flow and pressure changes as the opening areas on each box is decreased/increased? What if one of them or both of them are sealed? I'm very confused, and I suspect this might be some basic mechanics that I'm just not following... In such case, I apologize. Thanks in advance!