4.1 - Fluids

Question 1

What is a fluid

Answer 1

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

Question 2

What is density

Answer 2

A measure of mass per unit volume of a substance - technically called “volumic mass”

Question 3

What does the density of a substance depend on

Answer 3

The mass of the particles the substance is made from and how closely the particles are packed

Question 4

How do we calculate density

Answer 4

Density (kg m^-3) = mass (kg) divided by volume (m^3)

You can rearrange it for other values

Question 5

What does the equation for working out density work for ?

Answer 5

Mixtures, pure substances, and for all states of matter

Thus fluid density is also mass per unit volume

Question 6

Is density affected by temperature

Answer 6

As objects expand when they get hotter - the volume changes with temp so the density must be affected

Yes

Question 7

What is upthrust

Answer 7

When an object is submerged in a fluid it feels an upwards force caused by the fluid pressure

Question 8

What is the size of the force of upthrust equal to in fluids

Answer 8

The size of the force is equal to the weight of the fluid that has been displaced by the object

Question 9

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

Answer 9

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

Question 10

Why would an object sink in a fluid

Answer 10

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

Question 11

Why can objects float

Answer 11

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

Question 12

Whats the concept behind a hydrometer

Answer 12

The idea of floating at different depths

Question 13

What’s a hydrometer for

Answer 13

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

Question 14

Give me an excuse for a use of a hydrometer

Answer 14

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

Question 15

What’s a fluid

Answer 15

It’s defined as any substance that can flow

Question 16

Density

Answer 16

It’s a measure of the mass per unit volume of a substance

Question 17

What’s upthrust

Answer 17

An upwards force on an object caused by the object displacing fluid

Question 18

What’s Archimedes principle

Answer 18

States that the upthrust on an object is equal to the weight of the fluid displaced

Question 19

Define a hydrometer

Answer 19

It’s an instrument used to determine the density of a fluid

Question 20

How can a skier reduce air resistance

Answer 20

Crouching their body by presenting a smaller area to air resistance - making speed faster and the force slowing ylu down is reduced

Question 21

When in general would laminar flow (streamline flow) switch to turbulent flow

Answer 21

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

Question 22

What’s known as a Newtonian fluid

Answer 22

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

Question 23

What are streamlines

Answer 23

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

Question 24

What happens with fluid velocity with turbulent flow

Answer 24

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

Question 25

What does tubulent flow do to drag

Answer 25

Increases the drag on a vehicle and so increases fuel consumption

Question 26

What are swirling eddies

Answer 26

That circle bit when water runs through a plug hole

Question 27

How can turbulent flow be investigated

Answer 27

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

Question 28

Define laminar / streamline flow

Answer 28

Where a fluid moves with uniform lines in which velocity is constant over time

Question 29

What’s turbulent flow

Answer 29

Where fluid velocity in a particular place changes over time, often in an unpredictable manner

Question 30

Define streamlines

Answer 30

Lines of laminar flow in which the velocity is content over time

Question 31

What is the frictional force in fluids due to

Answer 31

Viscosity

If the frictional force caused by movement through the fluid is very small, we say the viscosity is low

Question 32

What’s the co efficient of viscosity

Answer 32

The fluid dependant factor has that n symbol

Eg it would be more difficult to walk through a pool of treacl than water

Question 33

What does viscosity determine

Answer 33

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

Question 34

What’s Poiseuille ‘s law

Answer 34

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

Question 35

How can one investigate how fluid flow rate depends on the fluids viscosity

Answer 35

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

Question 36

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

Answer 36

(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

Question 37

How is viscosity related to fluid temperature

Answer 37

They are directly related to fluid temperature

Question 38

How does the coeffiecnt of viscosity change as temp increases with liquid

Answer 38

In general, liquids have a lower coeffient of viscosity at higher temperatures

Question 39

How does coefficient of viscosity change with increase temperature of gases

Answer 39

Viscosity increases with temperatures

Question 40

How can we investigate how viscosity changes with temperature

Answer 40

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

Question 41

Define viscosity

Answer 41

Essentially the idea of how resistant a fluid is to flowing (a complete definition is beyond the scope of this book)- lol wot hahah

Question 42

Define coefficient of viscosity

Answer 42

Numerical value given to a fluid to indicate how much it resist flow

Question 43

We say gravity is 9.81 ms^-2 and yes objects accelerate at this in a vacuum but why might this not be true

Answer 43

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

Question 44

What’s an equilibrium situation

Answer 44

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

Question 45

When is terminal velocity applicable

Answer 45

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

Question 46

What is Stokes law

Answer 46

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

Question 47

What are the conditions for using stokes law

Answer 47

It applies to small spheres at low speeds with laminar flow

Laminar flow bit not in textbook but that wot miss said ?!

Question 48

How can we work out terminal velocity or viscosity or something

Answer 48

Assume an equilibrium position so

Weight =upthrust + stokes force

Then we substitute everything in and cancel and re arrange and then calculate

Question 49

How can we investigate terminal velocity with viscosity

Answer 49

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

Question 50

What’s viscous drag

Answer 50

There is a friction force between a solid and a fluid

The drag

Question 51

Why can it be complicated to calculate viscous drag

Answer 51

Can be very complicated for large objects, fast objects and irregularly shaped objects, as the turbulent flow creates an unpredictable situation

Question 52

Generally the largwr the object .. the ….. speed

Answer 52

The larger objects fall faster generally as long as the objects without a parachute

Question 53

Define terminal velocity

Answer 53

It’s the velocity of a falling object when it’s weight is balanced by the sum of drag and upthrust acting on it