4.1 - Fluids Flashcards
What is a fluid
Any substance that can flow
Normally this means any gas or liquid, but solids made up of tiny particles can sometimes behave as fluids eg sand through an hourglass
A key property of a fluid is density
What is density
A measure of mass per unit volume of a substance - technically called “volumic mass”
What does the density of a substance depend on
The mass of the particles the substance is made from and how closely the particles are packed
How do we calculate density
Density (kg m^-3) = mass (kg) divided by volume (m^3)
You can rearrange it for other values
What does the equation for working out density work for ?
Mixtures, pure substances, and for all states of matter
Thus fluid density is also mass per unit volume
Is density affected by temperature
As objects expand when they get hotter - the volume changes with temp so the density must be affected
Yes
What is upthrust
When an object is submerged in a fluid it feels an upwards force caused by the fluid pressure
What is the size of the force of upthrust equal to in fluids
The size of the force is equal to the weight of the fluid that has been displaced by the object
What is the name of the principle that the size of force is equal to the weight of fluid that’s been displaced in upthrust
Archimedes principle
Thus if the object is completely submerged, the mass of fluid displaced is equal to the volume of the object multiplied by the density of the fluid
The weight is then found using
W = mg
Why would an object sink in a fluid
If the object is denser than the fluid as the upthrust to prevent sinking is only equal to the weight of the fluid displaced - so if the object is denser there will be a bigger force downwards so the objects sinks
If upthrust less than objects weight - sinks
Why can objects float
The instant an object touches the surface there’s no upthrust as no fluid has been displaced - as the object sinks deeper, it displaces an increasing volume so increasing the upthrust
A point may be reached when the upthrust and weight are balanced exaclty - the object will stop sinking
The object will have to sink until is has displaced its own fluid of weight
Whats the concept behind a hydrometer
The idea of floating at different depths
What’s a hydrometer for
An instrument used to determine the density of a fluid. The device has a constant weight so it will sink lower in fluids of less density
This is BECUASE a greater volume of a less dense fluid must be displaced to balance the weight of the hydrometer
Scale markings on the narrow stem of the hydrometer indicate the density of liquid
Give me an excuse for a use of a hydrometer
Comparing the density of alcoholic drinks with that of water gives and indication of the proportion of alcohol they contain
The lower the density, the greater the alcohol content as alcohol has a lower density that water it’s mixed with. This has long been th basis for taxation of alcohol - more alcohol greater the tax
What’s a fluid
It’s defined as any substance that can flow
Density
It’s a measure of the mass per unit volume of a substance
What’s upthrust
An upwards force on an object caused by the object displacing fluid
What’s Archimedes principle
States that the upthrust on an object is equal to the weight of the fluid displaced
Define a hydrometer
It’s an instrument used to determine the density of a fluid
How can a skier reduce air resistance
Crouching their body by presenting a smaller area to air resistance - making speed faster and the force slowing ylu down is reduced
When in general would laminar flow (streamline flow) switch to turbulent flow
In general laminar flow occurs at lower speeds, and will chance to turbulent flow as the fluid velocity increases past a certain value
The velocity at which this changerover occurs will vary depending upon the fluid in question and the shape of the area through which it is flowing
What’s known as a Newtonian fluid
If a Liquid follows Newton’s formulae, as most common liduids do then it is
Eg water moving through a pipe with laminar flow will move fastest in the centre and slowest where it’s in contact with the pipe and friction - the bit in between the water touching th pipe and centre will get friction from the slower water but not as much so the water in the centre moves fastest xoxoxo
What are streamlines
The lines of laminar fluid flow
At any point of any of these streamlines, the velocity of the flow will be constant over time
Water at any point in a uniform pipe will always move in the same direction and at the same speed
Direction and speed may be different in different places but at any given place direction and speed must stay constant
What happens with fluid velocity with turbulent flow
The fluid velocity in any given place changes over time
The flow becomes chaotic and swirling eddies form
A poorly designed car would cause turbulent flow of air over it
What does tubulent flow do to drag
Increases the drag on a vehicle and so increases fuel consumption
What are swirling eddies
That circle bit when water runs through a plug hole
How can turbulent flow be investigated
Osborne Reynolds in 1833 first demonstrated an experiment showing coloured water flowing in a glass tube
You can set up a similar experiment to show turbulence cuased by faster fluid flow, or by different shapes of obstacles
- at most speeds, a smooth, curved obstacle will produce less turbulence than a squarer
Define laminar / streamline flow
Where a fluid moves with uniform lines in which velocity is constant over time
What’s turbulent flow
Where fluid velocity in a particular place changes over time, often in an unpredictable manner
Define streamlines
Lines of laminar flow in which the velocity is content over time
What is the frictional force in fluids due to
Viscosity
If the frictional force caused by movement through the fluid is very small, we say the viscosity is low
What’s the co efficient of viscosity
The fluid dependant factor has that n symbol
Eg it would be more difficult to walk through a pool of treacl than water
What does viscosity determine
Determines the friction force acting within a fluid
It has a direct effect on the rate of flow of the fluid
Eg fastest rate of flow would have lower viscosity
What’s Poiseuille ‘s law
The rate of flow of fluid through a pipe is inversely proportional to the viscosity of the fluid
This is help full for developing things like tooth paste and chocolate sauce
How can one investigate how fluid flow rate depends on the fluids viscosity
By doing an experiment similar to one Poiseuille did in the mid ninteenth century
Using a constant pressure, water forced through a narrow pipe will flow at a certain rate inversely proportional to its viscosity
By varying height of water tank, yku can record measures of head of pressure, height against flow rate
The gradient of the best fit line will allow you to calculate viscosity
In Poiseuilles experiment, how can we calulcate the gradient of the line of graph ( gradient is rate of flow) this will later help us determine viscosity
(Pi x density x gravity x radius^4 ) / (8 x viscosity (n) x l (length of tubing)
Units of rage of flow (Q) = cm^3/s
How is viscosity related to fluid temperature
They are directly related to fluid temperature
How does the coeffiecnt of viscosity change as temp increases with liquid
In general, liquids have a lower coeffient of viscosity at higher temperatures
How does coefficient of viscosity change with increase temperature of gases
Viscosity increases with temperatures
How can we investigate how viscosity changes with temperature
Use a resealable tin or bottle half full of test fluid such as golden syrup, temp of the liquid is varied using a water bath, viscosity of liquid will affect rate the tin or bottle rolls down a fixed ramp
Define viscosity
Essentially the idea of how resistant a fluid is to flowing (a complete definition is beyond the scope of this book)- lol wot hahah
Define coefficient of viscosity
Numerical value given to a fluid to indicate how much it resist flow
We say gravity is 9.81 ms^-2 and yes objects accelerate at this in a vacuum but why might this not be true
We need to take into account other forces acting on it, combine these to find a resultant
Use Newton’s second law a= sum of forces / m
To calculate resting acceleration
Eg for a skydiver we need to include weight, upthrust caused by the object being in the fluid air and the viscous drag varies with speed through the fluid and speed is constantly changing as a result of acceleration
What’s an equilibrium situation
Usually we consider the equilibrium situation, in which weight exaclty balances the sum of upthrust and drag - meaning the falling volcity remains constant - this constant velocity is the terminal velocity
When is terminal velocity applicable
The phrase is only applicable to objects falling under gravity. For similar situations horizontally, eg a car using a constant thrust force, an alternative phrase such as “maximum velocity” would be more appropriate
Odd flex but okay
What is Stokes law
The equation for viscous drag
F = 6 x pi x radius x coefficsjnt of viscosity x velocity
R = metres V = ms^-1 n= pa s
Drag force is proportional to the radius of the sphere and directly proportional to the velocity
What are the conditions for using stokes law
It applies to small spheres at low speeds with laminar flow
Laminar flow bit not in textbook but that wot miss said ?!
How can we work out terminal velocity or viscosity or something
Assume an equilibrium position so
Weight =upthrust + stokes force
Then we substitute everything in and cancel and re arrange and then calculate
How can we investigate terminal velocity with viscosity
Yo can investigate the viscosity of a liquid by allowing various differently sized spheres to fall through it and then measure their terminal velocity and radii at different points
You can plot a graph of terminal velocity against the square radius r^2 and hence calculate the viscosity of the water
What’s viscous drag
There is a friction force between a solid and a fluid
The drag
Why can it be complicated to calculate viscous drag
Can be very complicated for large objects, fast objects and irregularly shaped objects, as the turbulent flow creates an unpredictable situation
Generally the largwr the object .. the ….. speed
The larger objects fall faster generally as long as the objects without a parachute
Define terminal velocity
It’s the velocity of a falling object when it’s weight is balanced by the sum of drag and upthrust acting on it
