Module 2: Solids & Fluids Flashcards
Describe molecule movement in solids (3)
Molecules oscillate about fixed centres
Vibration is small
Fixed centres form a repeated spacial pattern (long-range order)
Describe movement of molecules in liquids (5)
Centres of molecules are free to move
Material take shape if container
Intermolecular distances are slightly larger than solids
Amplitudes of vibration are slightly larger than solids
Regularity of structure is shown only between close molecules (short-range order)
Describe the movement of gas molecules (4)
Great kinetic energy
Widely separated
Experience small attractive forces
Move in straight lines until they collide
When is an object said to behave elastically/ be elastic?
When it returns to its original shape after being subjected to equal and opposite forces which change its size/ shape
Stress = ?
F/A
Units of stress
N m^-2 or Pascal Pa
Describe stress
The effect of a force’s uniform distribution over an object
Describe strain
The relative response of a material to stress
Tensile strain =
ΔL/L(subscript o)
Units of strain
No units
Young’s modulus
Stress over strain
Units of Young’s modulus
Nm^-2 / Pa
What does and does not determine Young’s modulus?
Depends on material of object
Does not depend on size, shape or volume
What is Young’s modulus a measure of? What does a higher Young’s modulus mean?
Stretchiness
A higher Young’s modulus means the material is less elastic
Shear strain
Deformation by slippage across a plane parallel to imposed stress
Symbol for shear modulus
S (or G)
Symbol for young’s modulus
γ
Symbol for bulk modulus
B
When does bulk strain occur?
When a volume is subject to an increase in pressure, it experiences a change in volume/ volume strain/ bulk strain
Why is bulk modulus negative?
The volume shrinks
Describe a stress-strain curve for a ductile material
Steep gradient for elastic area: minimal change in strain for a large amount of stress
Fracture occurs shortly after the elastic area stops
Describe the stress-strain curve for a soft tissue
Strain increase dramatically per increase in stress
J shaped curve
ρ = ?
ρ = mv
Density
Units for density
kg cm^-3
Pressure
A kind of stress that acts in all directions
How does atmospheric pressure arise?
The motion of molecules in the atmosphere exerts a force on the body
P = ?
P = F/A
Pressure
How does fluid exert its pressure across its volume? (3)
Equally in all directions
Acts perpendicular to the surface in contact with it
The same at equal depths in a container, regardless of shape
How can the volume of a liquid be changed?
By applying pressure
What does the equation describe:
P2-P1 = -pg (y2-y1)
Difference in pressure at two different points
Absolute pressure
Pressure relative to a vacuum
Atmospheric pressure + any extra
Gauge pressure
Measured relative to atmospheric pressure
Only any extra pressure
Minimum absolute pressure?
0 Pa
How to calculate gauge pressure?
Subtract atmospheric pressure from total pressure
How to calculate absolute pressure?
Add atmospheric pressure and gauge pressure
Manometer
Device which measures gauge pressure
What does gauge pressure depend on?
Density of fluid
Gravitational acceleration
Height
Archimedes principle (object in fluid)
When a body is immersed in a fluid, the fluid exerts an upward force on the body which is equal to the weight of fluid displaced by the body
Basically, buoyant force = weight of displaced fluid
When does a body come to rest in a fluid, and what is this state called?
When the weight of the body = buoyant force
(mg = pVg)
- p refers to density of fluid
Equilibrium
When will a body sink in a liquid?
When it’s weight force is larger than the buoyant force
Cohesion
Attraction between like substances
Adhesion
Attraction between unlike substances
Because there are attractive forces between liquid molecules, ______?
A net force pulls at the molecules at the top of the liquid (surface tension)
How does surface tension act on surface area of bubbles and balloons?
Minimises it
A sphere gives the smallest surface area possible for a volume
The smaller the bubble, the ______ the pressure
Larger
Do molecules move to where pressure is lowest or highest? What does this mean for bubbles?
Lowest
Small bubbles get smaller (molecules move inwards)
Large bubbles get larger (molecules move outwards)
In a bubble, what is the equation for pressure?
P = 4γ / r
γ = surface tension
Capillarity
Balance between cohesive and adhesive (between liquid and small pore or tube) forces
How do we achieve a negative meniscus?
Adhesive forces outweighs cohesive forces
How do we achieve a positive meniscus?
Cohesive forces outweigh adhesive forces
Incompressible fluid
A fluid which has constant density throughout
Viscosity
Internal friction in the fluid
Laminar flow, and what fluid characteristic does it suggest?
Layers of fluid slide smoothly past each other in a steady state pattern.
Usually fluids have lower viscosities like honey
Turbulent flow, and when does it occur?
Irregular complex flow with mixing and eddies- no pattern
Occurs at high velocities or when there is a large change in velocity
Equation of continuity
Mass flowing in = mass flowing out
A1v1 = A2v2
What condition must the fluid fulfil for the equation of continuity to be correct?
Must be incompressible (constant fluid density)
What is the term for Av in a fluid?
Flow rate
Bernoulli’s equation (pressure in a fluid)
Pressure + kinetic energy + potential energy is constant at every point in a fluid
P + 0.5pv^2 + pgh = constant
(Where p = density of fluid)
Streamline
A line whose tangent- at every point - is in the direction of fluid velocity
What happens in places where streamlines converge?
Velocity increases
In a pipe, how do particles in the middle, and particles at the sides differ? What can we conclude from this?
Particles in contact with the pipe have lower velocity
The particles in the centre have the highest velocity, and they drag the other particles along
Velocity is dependant on distance from the wall of the pipe
η (liquids)
Viscosity
Stress over rate of strain = ?
Viscosity
Equation for η
η = (F/A) / (v/L)
Which requires more force to move: a low or high viscosity fluid?
High viscosity
Units for η viscosity
N s m^-2
Fluid flows from ____ to ____ pressure
High to low
What increases volume flow rate?
A higher difference in pressure between two points
What resists laminar flow?
Friction within the fluid
and friction between the fluid and container walls
What increases resistance to flow? (4)
Friction increase
Viscosity increase
Tube length increase
Radius decrease
What does a Reynolds number of <2000 suggest?
Flow is likely laminar
Which Reynolds number suggests turbulent flow?
> 3000
Free diffusion
Slow movement of a substance from area of high concentration to area of low concentration
What concept is xrms = √2Dt referring to?
Diffusion- average distance travelled by molecules
Ficks law (diffusion)
Free diffusion is proportional to the change of concentration with distance
What does osmosis cause at the membrane?
A pressure difference
Osmotic pressure
Minimum pressure that needs to be applied to prevent osmosis
