Fluid Statics MCQ with answer and solution

Concept

A simple U-tube manometer has one limb open to the atmosphere, so it measures pressure relative to the atmosphere, which is gauge pressure.

Formula

p_A = ρ₂ g h₂ - ρ₁ g h₁ (gauge)

Step-by-step solution

1. A simple U-tube manometer has one end connected to the pipe. 2. The other end is open to the atmosphere. 3. So the reading shows the difference between pipe pressure and atmospheric pressure. 4. This difference is gauge pressure.

Explanation

One limb of the U-tube is open to the air, so atmospheric pressure acts on it. The manometer shows only the extra pressure above (or below) atmospheric pressure. This is gauge pressure.

Why the other options are wrong

B: Absolute pressure needs a barometer reading added to the gauge pressure. C: Vacuum pressure is only a special case of negative gauge pressure, not the general reading. D: Atmospheric pressure is measured by a barometer, not a U-tube manometer.

Shortcut

Open end to the atmosphere means the reading is gauge pressure.

Common mistake

Students think a manometer gives absolute pressure. It does not, because the atmosphere acts on the open limb.