Fluid flow and Stoke's law

Question 1

What is Laminar flow?

Answer 1

-When it flows in the same direction
-The layers do not cross one another
-Occurs when the fluid travel is slow

Question 2

What is turbulent flow?

Answer 2

-No regular flow pattern
-Layers may cross
-Contain eddy currents, which are parts which move in a circulating manner
-Occurs in fast flowing fluids

Question 3

What is viscosity?

Answer 3

-The resistance of a fluid when it is flowing
-Measured in Pa/s
-Depends on the temperature

Question 4

How does temperature affect viscosity?

Answer 4

Liquids:
-Induce higher temperature which reduces viscosity
-Has more internal energy creating quicker particles and less bonds so less compact and less thick
Gases:
-Induce higher temperatures which increases viscosity
-Has more internal energy which creates quicker particles, more collision so they are normally closer together and bunched, making it more thick

Question 5

What is Stoke’s Law?

Answer 5

-Drag = 6 x pi x viscosity x radius x velocity of sphere
-Drag = N
-Viscosity = Pa/s
-Radius = m
-Velocity of sphere = m/s

Question 6

-What has to be the case to be able to use stoke’s law?

Answer 6

-Has to be small, spherical and have laminar

Question 7

What does stoke’s law calculate?

Answer 7

-Objects moving in a fluid which produces friction between the object and the fluid

Question 8

How do you derive an expression for terminal velocity of a sphere falling through a liquid?

Answer 8

Equations Needed:
Weight = mass (m) x gravity (g)
Drag = 6πrηv
Mass = Density (p)x Volume (V)
Volume = 4/3πr^3

1. Weight = Upthrust + Drag (Initial Formula)
2. msg= mfg + 6πrηv (Subsitute in)
3. Vpsg = Vpfg + 6πrηv (Subsitute mass out)
4. 6πrηv = Vpsg - Vpfg (Get drag onto one side)
5. 6πrηv = Vg (ps-pf) (Factorise Vg out)
6. 6πrηv = 4/3πr^3g(ps-pf) (Subsitute volume)
7. v = (4/3nr^3g(ps-pf))/6πrη (Divide to get velocity by itself)
8. v = (2r^2g(ps-pf)/9η (Simplify)

Question 9

What do you measure in the practical, measuring terminal velocity of a sphere falling through a liquid?

Answer 9

-The mass of the ball bearing (Scales)
-Diameter of Ball Bearing (Ruler, callipers, or micrometer)
-Volume of Liquid (Measuring cylinder)
-Mass of Liquid (Scales)
-Distance Ball Dropped (Ruler)
-Time Taken To Fall (Stopwatch)

Question 10

What do you calculate in the practical, measuring terminal velocity of a sphere falling through a liquid?

Answer 10

-Volume of Ball Bearing (4/3πr^3)
-Density of Ball Bearing (Mass/Volume)
-Terminal Velocity of Ball Bearing ((2r^2g(ps-pf)/9η
-Radius of Ball Bearing (1/2diameter)
-Density of Fluid (Mass/Volume)

Question 11

What are some tips to do in the practical, measuring the terminal velocity of a sphere falling through a liquid?

Answer 11

-Keep the Ball Bearing centre (Reduces friction to the wall of the cylinder)
-Start timer halfway down (For when it reaches terminal velocity)
-Repeat and calculate a mean (Reduce Random errors)