Gauge and Absolute Pressure- Hydraulics

[ElBell]

Homework Statement

I am reading through my introductory course notes, and am given a question I don't understand how to answer.

A rectangular tank contains fresh water inside it to a depth of 0.5m. The tank is 1m long and .7m wide.

a) calculate gauge pressure at the bottom of tank
b) calculate absolute pressure at the bottom of tank

Homework Equations

P = pgh ?

The Attempt at a Solution

(This is only a workbook example, not an assignment question)
Can somebody please explain to me the difference between gauge and absolute pressure, and what the equation is for above?

[I have tried to work out density of water (p) using 1000 x .35 = 350 but I don't think its right).

1000 is density of water, .35 is area of water.

please help! :)

 

[jbsteele]

Gauge pressure is the pressure relative to atmospheric pressure. It is the difference between the absolute pressure and atmospheric pressure. Absolute pressure is the pressure relative to a perfect vacuum. It is the sum of gauge pressure and atmospheric pressure. The equation you need is P = pgh, where P is the pressure, p is the density of the fluid (1000 kg/m3 for water), g is the acceleration due to gravity (9.8 m/s2) and h is the height of the fluid column (0.5 m in this case). So, the gauge pressure at the bottom of the tank is P = 1000 x 9.8 x 0.5 = 4900 Pa. The absolute pressure at the bottom of the tank is P + Patm = 4900 + 101325 = 106225 Pa.