Why Pressure is a scalar quantity?

[adabistanesoophia]

Hi,

1. As we know that pressure is force per unit area but why it is said that pressure is a scalar quantity because force is used and force is a vector quantity.

2. Whether stress is also a scalar quantity?

3. What is basic difference between pressure and stress?

Regards,

Muhammad Rizwan Khalil

 

[Celt67]

Very interesting question.

Pressure as a scalar quantity is debatable. I think that it is a scalar quantity because it is per unit area. There is no direction involved as such.

I'm not sure about stress.

I think pressure deals with liquids and stress is for solids. I could be wrong.

 

[berkeman]

Pressure does have a direction. What is that direction with respect to a surface if the air is not moving, for example. What about when you have air going over an airfoil? What are the components of the pressure then?

 

[Celt67]

I'm way too tired to get into a discussion like this or make any useful contribution! This is something physics professors like to chat about, or so I've heard!

 

[f(x)]

Well, pressure is a scalar for any point inside a gas=>assume a point inside an ideal gas chamber. Pressure from all sides shall be equal (for steady state)Thus you cannot predict the direction.
However on surfaces, solid liquid and gases, pressure has a definite direction perpendicular to the surface.
EDITI found an interesting link which might be helpful

However, could some1 please elaborate on pressure inside a liquid...does that also not have a direction? (i mean, pressure varies with depth, so at one height, will all the pressure act at a particular point or will the direction be indeterminate ?)

 

[berkeman]

Boy, that NASA article had me worried there for a minute, with them saying pressure is definitely a scalar quantity. But then I saw what I was referring to, which is what they call the "pressure force", which definitely is a vector:

Turning to the larger scale, the pressure is a state variable of a gas, like the temperature and the density. The change in pressure during any process is governed by the laws of thermodynamics. You can explore the effects of pressure on other gas variables at the animated gas lab. Although pressure itself is a scalar, we can define a pressure force to be equal to the pressure (force/area) times the surface area in a direction perpendicular to the surface. The pressure force is a vector quantity.

So I learned from this thread and that link to be more precise in the way I talk about pressure and "pressure force". Thanks for the link, f(x).

 

[adabistanesoophia]

How its does have a direction in solids but not in gases?