Liquid column
The taller the liquid column (with narrow base), the larger the amount of liquid contained, the greater the weight of the liquid to exert pressure
The amount of pressure in the same liquid column is different at different depths.
The greater the depth, the greater the weight of the liquid above it, the greater the pressure.
Pressure in a liquid
Liquids are difficult to compress because the particles are very closely packed together. The particles hitting the container and any objects in the liquid causes pressure. Pressure in a liquid acts equally in all directions.
The pressure depends on the forces exerted on a particular area. A big force concentrated in a small area giver a higher pressure.
The pressure in liquids is due to its weight. E.g. a tall vessel with water has side tubes fitted at different heights. Water flows out furthest from outlet 3, followed by outlet 2 and then outlet 1. This shows that liquid pressure increases with depth.
To determine the pressure at a certain depth of a liquid consider a column of liquid of height h, base area A, and density ρ.
The volume of a liquid is given by, V = Ah
The mass of the liquid is given by, m = ρV
The weight of the liquid column is given by: W = mg; = ρ(V)g; = ρ(Ah)g
The pressure at the base of the liquid column is : p = W/ A; = ρAhg/ A ; p = hρg
This equations shows that the pressure in a liquid depends on the depth and density of the liquid.
Pressure due to a liquid column = height of column × density of the liquid × gravitational field strength
The pressure in a liquid acts equally in all directions. If any level of water is higher than the other, the excess pressure difference will force it, to drop.
Manometer
The manometer is used to measure gas pressure. The manometer consists of a U-tube containing a column of liquid, which can be mercury, water or oil.
In its simplest form the manometer is a U-tube about half filled with liquid. With both ends of the tube open, the liquid is at the same height in each leg.
When positive pressure is applied to one leg, the liquid is forced down in that leg and up in the other. The difference in height, “h,” which is the sum of the readings above and below zero, indicates the pressure.
When a vacuum is applied to one leg, the liquid rises in that leg and falls in the other. The difference in height, “h,” which is the sum of the readings above and below zero, indicates the amount of vacuum.