14. Fluids

Question 1

What is Archimedes’ principle?

Answer 1

When an object is immersed in a fluid, the fluid exerts an upward force on the object equal to the weight of the fluid displaces by the object

Question 2

Buoyant force of displaced fluid

Answer 2

Buoyant force is defines as equal in magnitude to the weight of the displaced fluid

Question 3

A boat carrying a large boulder and the boulder is thrown overboard. What happens to the level of water in the lake?

Answer 3

• inside the boat: the boulder displaces its weight in water
• when thrown overboard: it only displaces its volume in water
• so the water level of the lake with respect to the shore falls

Question 4

What is the formula for Buoyant force and its relaiton to volume?

Answer 4

Fb = p(fluid) x g x V (object submerged)

p - roh, density

Question 5

What is surface tension

Answer 5

• The surface tension of a liquid results from an imbalance of intermolecular attractive forces
• A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.
• A molecule at the surface of a liquid experiences only net inward cohesive forces.

Question 6

Surface energy define

Answer 6

The energy needed to increase the surface of a fluid by and area ∆A

Question 7

What allows the curved surface of bubbles to be maintained?

Answer 7

• For small bubbles, the internal pressure is larger than the external pressure
• the pressure difference maintains the curved surface

Question 8

Formula for transmural pressure

Answer 8

∆p = 4σ/r
σ = surface tension
r = radius

Question 9

Define transmural pressure

Answer 9

Difference in pressure between two sides of a wall or equivalent separator.

Question 10

What would happen to the radius; if we use a surfactant that reduces to 50% the surface tension of a bubble membrane (and we obtain the the same transmural pressure)?

Answer 10

∆p = 4σ/r

• ∆p is the same
• surface tension 2σ
• to obtain the same ∆p, need radius to reduce to half the previous value