Fluid Properties

Question 1

What defines a Fluid?

Answer 1

A fluid may be liquid, vapor, or gas. It has no
permanent shape but takes up the shape of a
containing vessel or channel or is shaped by external
forces.

Fluids are readily deformable, and flow.
▪ Liquids have a fixed volume
▪ Gases may expand to fill container they are in

Question 2

Molecules in a fluid

Answer 2

A fluid consists of atoms/molecules in random motion
and in continual collision with the surroundings.

Question 3

Shear in Solids

Answer 3

For a solid, application of a shear stress causes a deformation which, if modest
(elastic), is not permanent and solid regains original position.

Question 4

Shear in Fluids

Answer 4

For a fluid, continuous deformation takes place as the layers of molecules slide
over each other. Deformation continues until the force is removed.

A fluid is a substance for which a shear stress tends to
produce unlimited, continuous deformation.

Question 5

Density Formula

Answer 5

Units = kg/m^3

Question 6

What is pressure

Answer 6

▪ In a fluid (liquid or gas) molecules are in continuous motion
▪ There is an amount of energy associated with this motion

Question 7

How does molecular motion create force?

Answer 7

When the molecules interact with each other there is an equal and opposite
reaction – therefore no net force
▪ When they interact with walls there is a resultant force perpendicular to the
surface
▪ Each impact force is small but there are lots of them!

Question 8

Pressure and force equation

Answer 8

▪ Force acting on small area 𝛿A is 𝛿F
𝑝 =𝛿𝐹/𝛿A

▪ If we want the total force we have to sum all the elemental forces
𝐹 = Σ𝛿F

IF pressure is constant then this integrates to:
F=pA

(Only if pressure is constant)

Question 9

Scalar and Vectors

Answer 9

Force is a vector and Pressure is a scalar

Question 10

Are all fluids compressible?

Answer 10

Yes, especially gases

Question 11

What is Surface Tension

Answer 11

▪ In a liquid the molecules are bound
to each other by forces of molecular
attraction

▪ At a boundary between two fluids this
gives rise to “surface tension”

▪ Surface tension usually has the
symbol y or σ and has units N/m

Question 12

A perfect Gas?

Answer 12

No real gas is perfect, although many behave nearly so, luckily air is
one.
For a perfect gas the molecules behave as perfectly elastic small
spheres, influencing each other only when they collide, and so small
that they occupy a tiny proportion of the volume of space in which they
move

Question 13

Perfect Gas Law

Answer 13

Universal gas constant, 8.31445 J/mol K