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