Materials Flashcards

Question

How does pressure in a liquid change with depth and why?

Answer 1

The deeper you go, the greater the pressure you would experience. This is due to the force applied by the liquid (weight of the liquid) above, as the deeper you go then the greater the volume of liquid above you which exerts a force

Answer 2

Weight = mass of fluid x g = volume of fluid x density x g = area of fluid x depth x density x g so pressure beneath the column at depth h = force / area = weight / area = ρ x g x h (units Nm^-2)

Answer 3

Depth and density. If the density or depth is doubled then the pressure will double

Answer 4

``` P = Patm + ρgh (Patm= atmospheric pressure ) ```

Answer 5

The pressure at a given depth in a static liquid is a result of the weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid

Answer 6

Pressure in a liquid acts in all directions, and liquid pushes on all surfaces that are in contact with it, but because pressure increases with depth, the pressure on the bottom of the object is greater than that on the top. The difference in pressure creates a buoyancy force which pushes upwards on the block, which is upthrust

Answer 7

Considering an object of area A in a fluid of density ρ. Then F1 = P1 x A = h1 x ρ x g x A and F2 = P2 x A = h2 x ρ x g x A Upthrust = F2 - F1 = (h2 - h1) x A x ρ x g = V x ρ x g (= weight) The upthrust on an object is equal to the weight of the fluid displaced. This general rule is often called the Archimedes' principle and works for an object of any shape

Answer 8

When the upthrust caused by the pressure of the water on the object balances the weight of the object. An object will sink if the upthrust is less as there will be a resultant downwards force on the object

Answer 9

Initially the flow is laminar and there is no mixing between the fluid layers, but the flow becomes turbulent, where there is mixing between the layers and eddies and vortices are created, causing the smoke to become less structured and will disappear

Answer 10

At low speeds the flow is laminar (smooth, though may involve vortices on a large scale) but as the speed increases, the flow will change to turbulent flow.

Answer 11

Laminar flow often remains attached to a body and allows a pressure difference between faster and slower regions of flow (faster streamlines = lower pressure) which give rise to a lift force. If the flow becomes turbulent or separated from the body then the pressure difference disappears and so does the lift force

Answer 12

Turbulence is caused when flow has to make sharp changes around a body, which causes it to become separated from the body and will cause large swirls of flow. By altering the shape and making a body more streamlined the turbulence can be reduced and so air resistance will also be reduced

Answer 13

Turbulent flow has more kinetic energy, and, because of the law of conservation of energy, this kinetic energy has to come from the object and the object then loses speed as energy is lost/transferred to turbulence

Answer 14

At low speeds, the fluid nearest the wall of the pipe is dragged back by friction against the wall and so it moves the slowest. This layer then drags on the next fluid layer and so on, meaning that the fastest layer is that which is furthest away from the wall (i.e. the centre), and because there is no movement between the layers there is no 'diffusion' of fluid velocity. Laminar flow is often also called Poiseulle Flow

Answer 15

If the volume rate is increased then flow may become turbulent and so the mixing of fluid layers causes fluid layers from near the wall to mix with layers from the centre so the fluid moves with a uniform velocity

Answer 16

ΔP = 8ηLQ / πr^4 Pressure required increases with the length of the pipe but decreases enormously with pipe radius; pressure increases as the fluid becomes more viscous

Answer 17

An expression for viscous drag force exerted on spherical objects when the movement relative to the fluid is laminar (so for small spheres or ones at very slow speeds) Fd = 6πηrv Drag force is directly proportional to the speed and radius to the sphere

Answer 18

Weight, upthrust and viscous drag

Answer 19

Weight = Fg (mg) Upthrust = Fu (Vρg) Viscous drag = Fd (6πηrv) Resultant force = Fg - Fd - Fu = mg - 6πηrv - Vρg When object reaches terminal velocity, the resultant force is 0 so Vterminal = mg - Vρg / 6πηr

Answer 20

An increase in temperature causes a decrease in viscosity

Answer 21

An increase in temperature causes an increase in viscosity

Answer 22

mg = Vρg = 4/3πr^3 x ρs x g Vt = 4/3πr^3 x ρs x g - 4/3πr^3 x ρ x g / 6πηr = 2r^2g(ρs - ρ) / 9η

Answer 23

A material made up of more than 1 substance, such as carbon fibre

Answer 24

How a material behaves when subject to forces. When it is subject to external forces then internal forces are set up in the material which oppose the internal forces

Answer 25

When 2 forces extend a material, it is under tension. External forces pull against the attractive forces acting between atoms/molecules within the material

Answer 26

When 2 forces squeeze a material, the compressive force are opposed by repulsive forces between the atoms/molecules

Answer 27

One part of the material is under tension and another part is compressed

Answer 28

If 2 forces are applied to a body which cause it to twist then they are shear forces

Answer 29

This is the region in which, if the tension is removed, the spring will return to its original length

Answer 30

Above the elastic limit, the spring's behaviour is plastic, and it deforms permanently, so when the tension is removed the extension will not return to 0.

Answer 31

Proportional limit, elastic limit (elastic deformation up until this point), failure

Answer 32

Force is proportional to extension, if the extension is elastic then if forces are removed then the material returns to its original length, the deformation is reversible and elastic energy stored in the material may be recovered as it is released

Answer 33

F ∝ Δx, or F = -k Δx

Answer 34

k is the constant of proportionality and represents the stiffness of the material. The higher the value of k the steeper the force extension curve and the stiffer the material. For a spring, k is the spring constant, and the negative sign indicates that the force exerted by the spring is in the opposite direction to the extension (spring tries to push back up against the load)

Answer 35

A ductile material will often absorb lots of energy before it fails, and much more than a brittle material of equal strength and stiffness. On a stress strain graph the area under the curve of a ductile material is much greater as it requires more energy to break

Answer 36

F ∝ Δx Work done = area under graph = 1/2 x base x height = 1/2 Fx F=kx so energy stored = 1/2kx x x = 1/2 kx^2 k is positive because the force on the spring is being considered, not the force exerted by the spring. x is in m, k is in N/m so energy is stored in J

Answer 37

Have one curve for loading and one curve for unloading - hysteresis. There is a greater area under the loading curve than under the unloading curve, which shows there is a difference in energy taken in vs energy released when load is released; some energy is stored in the material (and it gets hot). Area under the graph would be the amount of energy that is transferred to KE

Answer 38

- G-clamp to clamp the wire - Ruler to measure extension - Tape to allow the extension to be measured easily - Masses to provide tension - Micrometer to measure diameter of wire and calculate area * Long length of wire to ensure a measurable extension

Answer 39

They are quantities based on force and extension that take into account the material dimensions which allows the mechanical properties of different materials to ve compared without having to consider their particular dimensions

Answer 40

Tension / cross sectional area (σ = F/A) | Units are the same as pressure - Nm^-2 or Pa

Answer 41

Extension / original length (ε = Δx / x or ε = Δx / L) | Strain accounts for the fact that samples strain in proportion to their lengths so it has no units

Answer 42

A measure of the stiffness of an elastic material and a stiff material has a high Young modulus. Young modulus = stress / strain = σ / ε = E Gradient of a stress against strain graph

Answer 43

E = σ / ε = F/A ÷ Δx/L | = F x L / Δx x A

Answer 44

There are more atoms in the longer sample and each one moves apart from the next when extended. A longer sample then gives a greater total extension due to the larger number of atoms

Answer 45

They are the standard way of comparing the mechanical properties of different materials under tension. If a material obeys Hooke's law then the stress strain is a scaled version of a tension extension graph, but, beyond the elastic limit, the stress strain graph gives a much clearer picture of the material's properties as they yield.

Answer 46

``` Limit of proportionality (where Hooke's law is n longer obeyed and where the graph is no longer linear) Elastic limit (where the material starts behaving plastically) Yield point (material shows large increase in strain for small increase in stress) Ultimate tensile strength (point at which plastic deformation becomes unstable and a neck forms in the sample, which then increases the stress in this region. Maximum value of stress the material can endure before it breaks) Fracture stress (the stress where the fracture occurs) ```

Answer 47

Relatively high stiffness and strength, can be ductile and malleable and have high thermal and electrical conductivities. Properties can be improved by alloying

Answer 48

Classified as thermoplastics or thermosets. Thermoplastics become soft when heated and are generally flexible and relatively soft. Thermosets do not soften when heated by decompose and are rigid and hard. Polymers have low electrical and thermal conductivity and properties are temperature dependent

Answer 49

Elastomers are polymers which the property of elasticity. They are soft and deformable and their force extension graphs show a non-linear hysteresis curve which shows some energy is stored in the material increasing its temperature

Answer 50

Crystalline, but glass is an example of a non-crystalline form of ceramic. Brittle, relatively stiff. stronger in compression than tension, hard, chemically inert and bad conductors of electricity and heat

Answer 51

Materials composed of 2+ materials bonded together, e.g. steel and reinforced concrete. Composites made of fibres all aligned in the same direction have different properties to those with fibres in different directions. The main advantage of composites is that they combine the good properties of each material while avoiding some of the less desirable