The pressure due to a column of liquid is found using the equation pressure = height of column × density of liquid × gravitational field strength, or P = hρg. Pressure is in pascals (Pa), height in metres (m), density in kilograms per cubic metre (kg/m3) and gravitational field strength in newtons per kilogram (N/kg). Here the height of the column is the depth below the surface. Use g = 10 N/kg.
Worked example: water has a density of 1000 kg/m3. At a depth of 20 m the pressure due to the water is 20 × 1000 × 10 = 200000 Pa. Oil with a density of 800 kg/m3 at a depth of 3 m gives 3 × 800 × 10 = 24000 Pa.
To find the difference in pressure between two depths, use the difference in depth. A diver moves from 5 m to 15 m in water. The difference in depth is 10 m, so the pressure rises by 10 × 1000 × 10 = 100000 Pa. The equation can be rearranged to find the depth: height = pressure ÷ (density × g). Doubling the height of the column doubles the pressure, and a liquid with a lower density gives a lower pressure at the same depth.