So I have been studying the Bernoulli Equation and still do

So I have been studying the Bernoulli Equation and still do not understand some concept behind it. Why is it that sometimes P1 and P2 are zero, and sometimes they are not? How we know if p1 and p2 are zero?

Solution

P1 and P2 are the pressure at point 1 and point 2 respectively.

P1 and P2 are zero only if the points are exposed to atmospheric pressure. We consider atmospheric pressure as zero if we take the gauge pressure system. If we take the absolute pressure system, it is not zero.

So, normally we take gauge pressure system. Thats why p1 and p2 at that point are zero.

In any other point where other than atmospheric pressure is acting then, p1 and p2 are non zeros.