Download File - The Physics Doctor

A –Level Physics:
Fluids: Fluid motion + Viscosity
Objectives:
51.
• a. be able to use the equation for viscous drag (Stokes’s Law)
F = 6𝜋ηrv.
• b. understand that this equation applies only to small spherical
objects moving at low speeds with laminar flow (or in the
absence of turbulent flow) and that viscosity is temperature
dependent
Additional skills gained:
• Application of knowledge to wider contexts
Starter: Fermi Question
Try to arrange the densities of the
fluids (Highest at the top). Remember
fluids can be both liquid or gas!
Water, Ethanol, Carbon Dioxide, Air, Hydrogen, Mercury, Helium
Complex Questions
A 500ml flask containing carbon dioxide gas has a mass of
180.39g, a pump is then used to remove 80% of the gas.
The mass of the flask and gas is now 179.60g. Calculate
the density of carbon dioxide.
Complex Questions
Calculate the density of the hot air in a balloon floating
at a fixed height close to the ground. The density of the
cold air is 1.4kgm-3. The total mass of the balloon’s fabric
gondola, fuel, burners and occupants is 700kg and it’s
volume is 2500m3
Moving Liquids
Fluids can be said to move in either
laminar flow or turbulent flow.
The movement velocity of a fluid (gas or liquid) can be represented by
streamlines, which are arrowed lines showing the path taken by small regions
of fluid.
In laminar flow, like slow
moving water, adjacent
layers do not cross over
Laminar
one another as there is
flow
no abrupt changes in
speed or direction
Laminar
flow
However in turbulent
flow the fluid swirls
around in vortices or
eddy currents so
streamlines are no longer
continuous
Visualising Flow
Describe the flow of the gas over
these two objects (a) a car (b) a
block
Car:
• Laminar flow of gas
over the top of the
car as the
streamlines stayed
continuous due to
gradual change of
direction
• At back of the car,
the streamlines
crossed into one
another causing
turbulent flow.
Context 1: How
does a plane fly?
You have 5mins as a pair to come
up with an idea of HOW a plane
gains LIFT.
Flow in context
Draw a neat copy of this diagram,
adding labels where appropriate
Below is an image of an aerofoil (i.e. like the wing of a plane), as
no aerofoil is perfect, the laminar flow that goes over the wing
naturally produces turbulent flow at the end. The better the
wing design, the less turbulence it produces!
The air moving over the aerofoil is faster than the air underneath.
This is because of a circulation of some air around the aerofoil
(clockwise).
Flow in context
Describe the flow of the gas over
these two objects (a) a car (b) a
block
As the air moves faster over the top, this means that there is a
difference in pressure on the wing. This means that there is more
force applied to the bottom of the wing, resulting in uplift (and
drag)
https://www.youtube.com/watch?v=aFO4PBolwFg
Context 2:
Pipeworks
Write a small list of factors that
would affect flow through a pipe
In low speed fluids or
high viscosity fluids,
flow tends to be
laminar through a
pipe.
Sometimes in the food industry, when
molten sugar or chocolate is transferred by
pipe, unwanted air bubbles can form. What
could have caused this during the flow and
how could it be stopped?
There is a parabolic
distribution of
velocities with the
fastest moving stream
in the centre.
If the fluid moves too
quickly, eddies can
start to form
(turbulence)
Viscosity
Viscosity is defined as ‘the magnitude of internal friction within a
fluid’ a.k.a ‘how sticky it is’ and as such how resistant it is to flow
Come up with a short
list of liquids with
high viscosity at the
top and low viscosity
at the bottom
Viscosity
η
Greek symbol: Eta
F
Coefficient of viscosity
U
When a sphere moves slowly through
a liquid, the relative movement of the
liquid around the sphere is laminar
As the molecules its passing through
will stick to the surface as it travels, a
viscous drag (F) is created
This force was shown to be related
to the radius of the sphere, the
velocity of the sphere and the
coefficient of viscosity ( η )
W
Viscosity
η
Greek symbol: Eta
Coefficient of viscosity
(Nsm-2)
F
U
This relationship is called
Stoke’s law
Radius of
sphere (m)
Drag force (N)
Velocity of
sphere (ms-1)
W
Bernoulli’s Principle
No spin
Spin
Measuring Viscosity using Stoke’s
Law
Read page 181 of the text book and make notes on how to
measure viscosity using Stoke’s law
Start reading the literature for your next
core practical (Core practical 4) and
complete the tasks.
Objectives:
51.
• a. be able to use the equation for viscous drag (Stokes’s Law)
F = 6𝜋ηrv.
• b. understand that this equation applies only to small spherical
objects moving at low speeds with laminar flow (or in the
absence of turbulent flow) and that viscosity is temperature
dependent
Additional skills gained:
• Application of knowledge to wider contexts