# Why is pressure different at different points in the same level when the water is flowing?

In fig.a below, water is to flow out from the yellow tank. But the flow is stopped because of the mercury in the green manometer. So the water is stationary. In this situation, the pressure at both the points A and B will be the same, which is $${\rho gh}$$. Where $${\rho}$$ is the density of water. In fig.b, the mercury is removed from the manometer. So water flows out. In this situation, pressure at A is not equal to pressure at B. Even though A and B are at the same level. Can we give a simple explanation for such a pressure difference? I saw the basics of Bernoulli’s equation. But it does not give the reason. Thanks.

• A pressure difference is required for a flow to exist. Commented Oct 14, 2020 at 13:27
• What happens when the mercury is removed from the manometer tube? The resisting pressure that was stopping the water from flowing through that tube is now gone. So now there is a pressure difference between point B and the manometer tube, which causes water to flow through the manometer tube. Once the flow begins, pressure at A and B change from their initial values; pressure at B will lie between that at A and the manometer tube.
– Deep
Commented Oct 15, 2020 at 3:03