What is in liquid?

Answer 1

The question is a bit vague, but two things come into play for pressure in a liquid: external pressure and hydrostatic pressure. For a liquid where the top surface is in contact with a gas, like a glass of water sitting out on the table, the external pressure would be the pressure of the gas. For the glass of water in this example, the pressure of the gas is just the ambient atmospheric pressure. In a pressure cooker with hot gas and hot liquid confined in a fixed volume, the pressure of the gas will probably reach the pressure the relief valve is set to. Hydrostatic pressure comes from the weight of the liquid above the liquid at any point in the liquid. As an example, if you go 10 meters down under the surface of the water in a swimming pool, you will feel the pressure created by the weight of the water above you. As a formula, P.H. (hydrostatic pressure) = (gravitational acceleration)x(density of liquid)x(depth of liquid). To get total pressure at any point in a liquid, you add the external pressure and the hydrostatic pressure. In situations like a hydraulic line, you would add the pressure exerted by the piston (external pressure) to the hydrostatic pressure from changes in the height of the hydraulic line. By the way, if the line goes UP, the effective depth is NEGATIVE and the hydrostatic pressure term is also negative, so you would have less pressure at the top of the hydraulic line than you did down at the level of the piston.