Module 2: Solids & Fluids Flashcards by Juliette Ward

Describe molecule movement in solids (3)

Molecules oscillate about fixed centres

Vibration is small

Fixed centres form a repeated spacial pattern (long-range order)

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

Describe the movement of gas molecules (4)

Great kinetic energy

Widely separated

Experience small attractive forces

Move in straight lines until they collide

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Stress = ?

F/A

How well did you know this?

Not at all

Perfectly

Units of stress

N m^-2 or Pascal Pa

How well did you know this?

Not at all

Perfectly

Describe stress

The effect of a force’s uniform distribution over an object

How well did you know this?

Not at all

Perfectly

Describe strain

The relative response of a material to stress

How well did you know this?

Not at all

Perfectly

Tensile strain =

ΔL/L(subscript o)

How well did you know this?

Not at all

Perfectly

Units of strain

No units

How well did you know this?

Not at all

Perfectly

Young’s modulus

Stress over strain

How well did you know this?

Not at all

Perfectly

Units of Young’s modulus

Nm^-2 / Pa

How well did you know this?

Not at all

Perfectly

What does and does not determine Young’s modulus?

Depends on material of object

Does not depend on size, shape or volume

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Shear strain

Deformation by slippage across a plane parallel to imposed stress

How well did you know this?

Not at all

Perfectly

Symbol for shear modulus

S (or G)

How well did you know this?

Not at all

Perfectly

Symbol for young’s modulus

How well did you know this?

Not at all

Perfectly

Symbol for bulk modulus

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Why is bulk modulus negative?

The volume shrinks

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Describe the stress-strain curve for a soft tissue

Strain increase dramatically per increase in stress

J shaped curve

How well did you know this?

Not at all

Perfectly

ρ = ?

ρ = mv

Density

How well did you know this?

Not at all

Perfectly

Units for density

kg cm^-3

How well did you know this?

Not at all

Perfectly

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