# Pascals principle

Can someone explain why pressure is transmitted equally to all parts of an enclosed fluid, as stated in pascals principle ? Why doesn't the 'pressure increases with height' rule play a part ? Is it because the volume of the liquid will always stay the same? But then so what ?

sophiecentaur
Gold Member
2020 Award
Can someone explain why pressure is transmitted equally to all parts of an enclosed fluid, as stated in pascals principle ? Why doesn't the 'pressure increases with height' rule play a part ? Is it because the volume of the liquid will always stay the same? But then so what ?

These are two separate issues and they appear very regularly here. I don't think there is a problem if you deal with them one at a time and there is no conflict. Take a volume of fluid way out into microgravity conditions. The pressure will be transmitted evenly throughout. Why? Because if it were not equal everywhere, the fluid would be constantly flowing around and the pressure would relate to the actual shape of the container (not a reasonable suggestion).

Once you get down on Earth, there are other forces at work and the principle only applies, strictly, in an infinitessimally small volume. The weight of the fluid above will be causing the pressure and it's quite reasonable to explain this in terms of the density, height of column above and g.

If you take an imaginary surface, between one region and another in a contained fluid, the pressure must be the same in each direction, when equilibrium has been reached - just the same as pressure on the walls of the container.

I still don't get how they link.

Chestermiller
Mentor
The pressure varies with position within the enclosure, but at any given location, it is measured to be the same in all directions. If you swim to the bottom of a swimming pool, the pressure on your ear drum is the same whether your head is erect, or whether your ear is pointing toward the bottom of the pool, or whether you ear is pointing toward the surface. A parameter that does not depend on direction at a given spatial location is said to be isotropic, meaning independent of direction.

Oh so you are saying that pressure at same level is the same , but at different levels in the enclosed fluid the ' pressure increases with height' rule still applies?

Nugatory
Mentor
Oh so you are saying that pressure at same level is the same , but at different levels in the enclosed fluid the ' pressure increases with height' rule still applies?

yes.

So the pressure exerted by the master piston is the pressure that acts on the slave piston to produce a force to lift the load right ? Without the force applied at the master piston nothing would happen right ?
But what happens if there is loquid transfer? what will be the force exerted at the slave piston, and how do we calculate it ?