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Physics A-level Year 1 > 4.1 - Fluids > Flashcards

4.1 - Fluids Flashcards

1
Q

What is a fluid

A

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

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2
Q

What is density

A

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

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3
Q

What does the density of a substance depend on

A

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

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4
Q

How do we calculate density

A

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

You can rearrange it for other values

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5
Q

What does the equation for working out density work for ?

A

Mixtures, pure substances, and for all states of matter

Thus fluid density is also mass per unit volume

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6
Q

Is density affected by temperature

A

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

Yes

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7
Q

What is upthrust

A

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

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8
Q

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

A

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

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9
Q

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

A

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

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10
Q

Why would an object sink in a fluid

A

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

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11
Q

Why can objects float

A

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

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12
Q

Whats the concept behind a hydrometer

A

The idea of floating at different depths

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13
Q

What’s a hydrometer for

A

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

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14
Q

Give me an excuse for a use of a hydrometer

A

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

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15
Q

What’s a fluid

A

It’s defined as any substance that can flow

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16
Q

Density

A

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

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17
Q

What’s upthrust

A

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

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18
Q

What’s Archimedes principle

A

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

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19
Q

Define a hydrometer

A

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

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20
Q

How can a skier reduce air resistance

A

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

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21
Q

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

A

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

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22
Q

What’s known as a Newtonian fluid

A

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

23
Q

What are streamlines

A

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

24
Q

What happens with fluid velocity with turbulent flow

A

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

25
Q

What does tubulent flow do to drag

A

Increases the drag on a vehicle and so increases fuel consumption

26
Q

What are swirling eddies

A

That circle bit when water runs through a plug hole

27
Q

How can turbulent flow be investigated

A

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

28
Q

Define laminar / streamline flow

A

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

29
Q

What’s turbulent flow

A

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

30
Q

Define streamlines

A

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

31
Q

What is the frictional force in fluids due to

A

Viscosity

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

32
Q

What’s the co efficient of viscosity

A

The fluid dependant factor has that n symbol

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

33
Q

What does viscosity determine

A

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

34
Q

What’s Poiseuille ‘s law

A

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

35
Q

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

A

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

36
Q

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

A

(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

37
Q

How is viscosity related to fluid temperature

A

They are directly related to fluid temperature

38
Q

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

A

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

39
Q

How does coefficient of viscosity change with increase temperature of gases

A

Viscosity increases with temperatures

40
Q

How can we investigate how viscosity changes with temperature

A

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

41
Q

Define viscosity

A

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

42
Q

Define coefficient of viscosity

A

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

43
Q

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

A

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

44
Q

What’s an equilibrium situation

A

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

45
Q

When is terminal velocity applicable

A

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

46
Q

What is Stokes law

A

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

47
Q

What are the conditions for using stokes law

A

It applies to small spheres at low speeds with laminar flow

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

48
Q

How can we work out terminal velocity or viscosity or something

A

Assume an equilibrium position so

Weight =upthrust + stokes force

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

49
Q

How can we investigate terminal velocity with viscosity

A

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

50
Q

What’s viscous drag

A

There is a friction force between a solid and a fluid

The drag

51
Q

Why can it be complicated to calculate viscous drag

A

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

52
Q

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

A

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

53
Q

Define terminal velocity

A

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

Physics A-level Year 1 (13 decks)

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