I am a high school student and I am little confused in manometers, My teacher told me that we cannot equate pressure even at same horizontal level if different liquids are there, but he doesn't give the reason, can anyone explain why we cannot equate the pressure in different liquids , if the pressure is different here, then why the liquid in not flowing? it should be the same no matter which liquids are there.

• If the pressure is different, the liquid is not flowing because the pressure difference is balanced by a difference in gravitational forces. One fluid weighs more than the other. Commented Nov 14, 2020 at 12:05
• I am not able to think of visually ,what you are saying? could you please elaborate. Commented Nov 14, 2020 at 13:10
• Think of a U tube with fluid A to the left and fluid B to the right, and where they meet below at the midpoint of the bottom of the U. Say you are looking at the same elevation above the bottom, and no fluid is above this elevation in either leg of the U. Commented Nov 14, 2020 at 13:59
• Commented Nov 20, 2020 at 4:15
• @Chet_Miller sir "If the pressure is different, the liquid is not flowing because the pressure difference is balanced by a difference in gravitational forces. One fluid weighs more than the other" can u give a answer where its shown using fbd of full water body taking both fluids and then showing net forcs zero Sir? Commented Dec 8, 2021 at 15:15

Good question, Liquid manometers measure differential pressure by balancing the weight of a liquid between two pressures.But you need to see the whole scenario$$-$$ $$P-P_0=\rho gh$$ so the change in pressure is due to the different mass of the liquid taken and that outweigh from the gravitational pull by earth. So there is no net flow of liquid..

• This formula applies to an ideal gas. The drawings in the OP clearly show liquids so the formula for hidrostatic pressure should be used.
– nasu
Commented Nov 14, 2020 at 16:23
• inquisitive soumyadip, i don't understant your this line "change in pressure is due to the different mass of the liquid taken and that outweigh from the gravitational pull by earth. So there is no net flow of liquid.." please elaborate? Commented Nov 15, 2020 at 8:34
• @ArunBhardwaj Originally the text referred to a different formula (mentioned by nasu) which included mass. The formula has been changed without changing the text. That is why the text makes less sense than it did originally..(Click on 'edited...' to see earlier versions of the answer.) Commented Nov 20, 2020 at 3:14