Physics · Mechanical Properties Of Fluids · NEET
Yes, for a liquid open to the atmosphere. Start from absolute pressure P = P0 + rho g h. Gauge pressure is what is left after you subtract the atmosphere: Pg = P - P0 = rho g h. So at depth h in an open tank, gauge pressure = rho g h and does NOT contain P0. This is the pressure that pushes on a diver's ears extra, over and above the air already pressing everywhere.
A flat tyre still has air inside at atmospheric pressure, so its absolute pressure is about 1x10^5 Pa (1 atm), not zero. But a tyre gauge measures gauge pressure = inside pressure minus outside atmospheric pressure. When inside equals outside, that difference is zero. The gauge only shows the EXTRA pressure you pumped in, which is why a properly inflated tyre might read '2 atm gauge' meaning 3 atm absolute.
Absolute pressure is bigger whenever gauge pressure is positive, because absolute = gauge + atmospheric. Absolute pressure can never be negative (the lowest possible is a perfect vacuum, zero). Gauge pressure CAN be negative when the pressure is below atmospheric, for example inside a suction pipe. So: Absolute >= 0 always; Gauge can be positive, zero, or negative.
Add P0 only when the question asks for the TOTAL or ABSOLUTE pressure, or when the container is closed and the top is not open air. If the question asks for gauge pressure, or the force due to the liquid alone, or a manometer height difference, use only rho g h and do NOT add P0. Read the word 'absolute' or 'total' carefully, this is exactly where NEET sets its trap.
Use P0 = 1.01x10^5 Pa (about 1 atm) unless the question gives another value. Sometimes NEET gives 1x10^5 Pa to keep the numbers clean, as in the 2026 submarine question. It also equals 76 cm of mercury or 760 torr. Always use the value stated in the problem; if none is given, 1.01x10^5 Pa is standard.
A submarine is designed to withstand an absolute pressure of 100 atm. How deep can it go below the water surface? (density of water = 1000 kg per m^3, 1 atm = 1x10^5 Pa, g = 10 m per s^2)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Gauge pressure Pg = P - P0 = rho g h for a liquid column of depth h, where rho is the liquid density, g is gravity, and P0 is atmospheric pressure.
Absolute pressure P = P0 + rho g h. It is the total pressure at the point, measured from a perfect vacuum, and equals atmospheric pressure plus the pressure of the liquid above.
Yes. If the pressure at a point is below atmospheric (for example in a suction or partial vacuum), the gauge pressure is negative. Absolute pressure, however, can never be negative.
Most everyday instruments (tyre gauge, blood pressure cuff, Bourdon gauge) read gauge pressure, because they compare the pressure inside against the surrounding atmosphere.
They differ by atmospheric pressure: Absolute pressure = Gauge pressure + Atmospheric pressure. So Absolute is always greater than Gauge whenever Gauge is positive.
