Basic Fluid Characteristics

Question 1

Calculate pressure with surface tension

Answer 1

p = 2σ/r

p = internal pressure
σ = surface tension
r = radius

Question 2

what is poise

Answer 2

poise is the unit of dynamic viscosity

1 poise = 0.1 Pa

Question 3

Why are droplets formed when water is placed on a surface

Answer 3

Due to surface tension which has a property that wants to minimise the surface area of the fluid
(stopping it from spreading out)

Along side the fact that water molecules attract each other, meaning each molecule feels a net force directed towards the centre
(forming a beam/semi circle on the surface)

Question 4

Define adhesive force

Answer 4

attractive forces between molecules of different types

Question 5

Define cohesive forces

Answer 5

attractive forces between molecules of the same type

Question 6

Why is it when a capillary tube is placed into a beaker of water, the water inside the tube rises above the original fluid level

Answer 6

This is due to the fact that in water the adhesive forces that attract the water molecules to the glass are greater than the cohesive forces among the water molecules, forming a concave meniscus. The amount the water rises is dependent on the diameter of the tube.

(the molecules are attracted more to the glass than other molecules)

Question 7

Why is it when a capillary tube is placed into a beaker of mercury, the mercury inside the tube drops below the original fluid level

Answer 7

This is due to the fact that in mercury the cohesive forces among the mercury atoms are greater than the adhesive forces that attract the mercury atoms to the glass, forming a convex meniscus.

(attracted to itself more than to the glass)

Question 8

Define vapour pressure

Answer 8

A measure of the tendency of a material to change into a gaseous state. Vapour pressure is the internal pressure of the system when dynamic equilibrium has been reached, condensation rate = evaporation rate.
(If pressure drops below VP the fluid becomes gas)

Question 9

Pascal’s law

Answer 9

any pressure applied at one point in a closed fluid system is evenly distributed and transmitted to all other points within the system (push on side of the system and the whole system moves)

Question 10

how does a hydraulic lift operate

Answer 10

F1/A1 = F2/A2 therefore if A1 is much smaller than A2, then F2 will have to be proportionally bigger than F1 (A1:A2 = F1:F2 ratios).

This principle is used in a hydraulic lift to lift an object with a much larger force (F2) than is actually applied (F1). This is done by making one end of the closed system filled with fluid considerably smaller (A1) than the opposite end (A2)

Question 11

factors contribute to the onset of cavitation

Answer 11

When the local pressure drops below the vapour pressure the surrounding fluid vapourises

Question 12

What is Bernoullis principle

Answer 12

States that as the speed of a fluid increases, it’s pressure decreases and vice versa

Question 13

Bulk Modulus equation

Answer 13

Bulk stress (F/A)
————————
Bulk strain (ΔV/V0)

—>

Pressure
—————
ΔV
—
V0

The same as Young’s modulus except instead on length for strain you use volume

Question 14

What is surface tension

Answer 14

The tendency of a liquid to minimise its surface area, so that the liquid can be a lower energy state by having less water molecules at its surface (unbalanced forces). The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules