If pressure is scalar, how can it have a direction?

[MDJH]

... and if it doesn't have a direction, then how do you describe the difference between the pressure on one side of a surface and the pressure on the other side? (ie. Let's say there was a force imbalance from one side of the surface to the other... does that mean the pressure has a direction?)

[insane_alien]

pressure doesn't have a direction BUT a pressure gradient DOES have a direction.

[MDJH]

Ah... sort of like voltage and voltage gradient in that case?

[insane_alien]

yes, that is one of the reasons why pressure is often used as an analogue of voltage( for example, the hydraulic analogy of electricity).

[Bignose]

As another answer, temperature is also a scalar. One can easily imagine a temperature difference from one side of an object to another.

[AlphaSheeppig]

Well, pressure is a scalar because it acts equally in all directions. If you consider a tiny bit of fluid next to the wall, the fluid exerts some pressure against the wall, but it also pushes the rest of the fluid around it with exactly the same pressure... How would you define the direction for fluid that's in the middle of a container and not next to a wall?

 

In the maths we deal with this problem by representing the area the pressure acts on as a vector perpendicular to and pointing away from the fluid to get the force out as a vector.

[Cropduster23]

Also,

 

pV=NkT

 

Hot sexy scalar-on-scalar action.

[Farsight]

See the stress and pressure page by Philip Candela. When we assign directionality to pressure we call it stress. Both stress and pressure are measured in Pascals. The difference between them is merely one of definition.