materials Flashcards

Question

how do you investigate the extensions of an object by stretching an object?(practical) MATERIALS

Answer 1

method: 1) set up experiment as shown in diagram - support object being tested at top (eg. by clamp) and measure its original length with ruler 2) add masses on at time to bottom of object 3) after each weight is added, measure new length of object, then CALCULATE EXTENSION: extension = new length - original length 4) plot graph of FORCE (weight, which = mass x g) against EXTENSION for results analysing the data: ⋅ where line of best fit is STRAIGHT, object obeys hooke's law + gradient = k (as F = kx, like y = mx) ⋅ if you've loaded object beyond its limit of proportionality, graph will start to curve

Answer 2

⋅ how much a material stretches for a particular applied force depends on its dimensions ⋅ so the spring constant (k) won't be constant for all samples of a particular material

Answer 3

if you want to compare one material to another, you need to compare their stresses and strains instead

Answer 4

stress causes strain

Answer 5

⋅ as said before, a material subjected to a pair of opposite forces might deform, i.e. change shape ⋅ if forces stretch a material, they are tensile forces ⋅ if forces squash materials, they are compressive forces

Answer 6

⋅ stress is defined as the tension (force applied) divided by the cross-sectional area of the material ⋅ the units for stress are: N m^-2 or Pascals, Pa

Answer 7

⋅ stress = force applied / cross-sectional area ⋅ σ = F/A

Answer 8

⋅ strain is defined as extension (i.e. change in length) divided by the original length of material ⋅ no, strain has no units as it is a ratio (of lengths)

Answer 9

⋅ strain = extension / original length ⋅ ε = x/L

Answer 10

⋅ it doesn't matter whether the forces producing the stress and strain are tensile or compressive, the same equations still apply ⋅ the only difference is that you tend to think of the tensile forces as positive and the compressive forces as negative (so compressive forces would cause negative extension)

Answer 11

the stress that is big enough to cause a material to break is called the fracture stress

Answer 12

as the tensile force applied to a material increases, the stress on the material increases

Answer 13

⋅ UTS on the graph shows the ultimate tensile strength of the material ⋅ point B shows the fracture stress (stress at which a material fractures): 1) as the stress increases, the atoms start to be pulled apart from one another, causing atomic planes to slip over one another 2) eventually the stress becomes so great that atomic planes can no longer slip over each other and you're pulling on the actual bonds between atoms, causing the atoms to separate completely 3) when the atoms separate completely, the material fractures (breaks)

Answer 14

the ultimate tensile strength (UTS) the maximum stress that a material can withstand before breaking (/fracturing)

Answer 15

elastic strain energy is stored in the material that is being stretched

Answer 16

when a material is stretched or compressed, work is done in deforming the material (bc energy is transferred to material)

Answer 17

1) on a graph of force against extension, the work done [/energy transferred to a material when it is stretched or compressed] is given by the area under the graph 2) before the elastic limit, all work done in stretching or compressing the material is stored as energy in the material 3) this stored energy is called elastic strain energy

Answer 18

⋅ the strain energy stored inside a material can be calculated quite easily if the material is obeying hooke’s law

Answer 19

the work done on an elastic material in stretching it is equal to the energy stored in the material as elastic strain energy

Answer 20

⋅ usually, work done = force x displacement ⋅ however, the force exerted on the material isn't constant - it rises from zero to a force F ⋅ to calculate the work done, you use the average force between zero and F - which is 0.5F ⋅ therefore, work done = 0.5 x force x displacement ⋅ therefore, work done = 0.5Fx

Answer 21

⋅ elastic strain energy = 0.5 x force x displacement ⋅ E = 0.5Fx (like work done = 0.5Fx)

Answer 22

⋅ E = 0.5Fx ⋅ this is the triangular area under a force-extension graph for a material (so the elastic energy stored by the material until the limit of proportionality)

Answer 23

⋅ bc hooke's law is being obeyed, the F in 'F = kx' can be replaced with 'E = 0.5Fx' (rearranged to give F first though befire plugging it in) ⋅ this gives E: E = 0.5k(x^2)

Answer 24

⋅ if a material is stretched beyond its elastic limit, some work is done in separating the atoms ⋅ this energy (work done) will NOT be stored as elastic strain energy by the material, and so isn't released when the force is removed

Answer 25

⋅ young modulus = stress / strain ⋅ E = σ/ε

Answer 26

⋅ the young modulus of a material is a measure of the stiffness of a material ⋅ the units for the young modulus are: N m^2 or pascals, Pa (young modulus has the same units as stress since strain has no units [bc strain is a ratio])

Answer 27

⋅ up to the point called the limit of proportionality, stress and strain are proportional to each other ⋅ so before this limit, for a particular material, stress/strain = constant ⋅ this constant is called the young modulus, E

Answer 28

when you apply a load in order to stretch a material, the material experiences stress and strain

Answer 29

engineers can use E to make sure their materials can withstand sufficient forces

Answer 30

1) set up apparatus as shown below: 2) find cross-sectional area of wire using micrometer ⋅ do this by measuring diameter of wire at three different points along wire + find average diameter ⋅ then use area of circle = πr^2 3) clamp wire to bench (as shown in diagram) so you can hang weights off one end of it ⋅ start with smallest weight necessary to straighten wire (DON'T include this weight in your final calculations) 4) measure distance between fixed end of wire + marker - this is your unstretched length then if you increase weight, wire stretches + marker moves 5) increase weight in steps (eg, in 1 N intervals), measuring extension each time ⋅ extension = marker's position - unstretched length ⋅ use balance to accurately find weight you add at each step 6) use your results from this experiment to calculate stress + strain on wire, + plot stress-strain graph 7) gradient of stress-strain graph gives young modulus, E

Answer 31

you should: ⋅ conduct the experiment standing up so you can get out of the way quickly if the weights fall ⋅ wear goggles to protect eyes if wire snaps

Answer 32

⋅ when setting up the equipment, the test wire used should be thin, and as long as possible ⋅ bc the longer and the thinner the wire, the more it extends for the same force, reducing the uncertainty in your measurements [for the extension, x]

Answer 33

⋅ the gradient of a stress-strain graph gives the young modulus (E) of a material ⋅ the area under the graph gives the elastic strain energy (or elastic strain energy stored) per unit volume of material (i.e. energy stored per 1 m^3 of wire)

Answer 34

⋅ brittle materials break suddenly without deforming plastically ⋅ if you apply a force to a brittle material, it won't deform plastically, but will suddenly snap when the force reaches a certain size ⋅ brittle materials can also be quite weak if they have cracks in them ⋅ you can break pieces off of a ceramic without the whole thing breaking ⋅ ceramics fracture/shatter via crack propagation

Answer 35

⋅ ductile materials can be drawn into wires without losing their strength ⋅ you can change the shape of ductile materials by drawing them into wires or other shapes - the important thing is that they keep their strength when deformed like this

Answer 36

⋅ copper is ductile, and with its high electrical conductivity, this means that it's ideal for electrical wires

Answer 37

⋅ strong materials can withstand high stresses without deforming or breaking

Answer 38

⋅ strength is a measure of how much a material can resist being deformed by a force without breaking ⋅ this can be resisting a pulling force (so has tensile strength) or a squeezing force (so has compressive strength)

Answer 39

steel beams used to create structures like bridges are very strong - they withstand the forces caused by lots of vehicles going over them without bending or breaking

Answer 40

⋅ bc of their structure, hard materials are very resistant to cutting, indentation (becoming dented) and abrasion (scratching) ⋅ (this is bc their structure makes it very difficult to dislodge atoms from the surface of hard material)

Answer 41

cutting tools need to be harder than the stuff they're cutting

Answer 42

cutting tools are often made from hardened steel

Answer 43

stiff materials have high resistance to bending and stretching

Answer 44

⋅ the stiffness of a material is measured by the young modulus (E) ⋅ so E of material ∝ stiffness of material ⋅ so the higher the E, the stiffer the material

Answer 45

the outer protective casing of safety helmets and safety boots need to be very stiff so that they keep their shape and don't crush onto your body when something impacts on them

Answer 46

tough materials are really difficult to break bc they can absorb a lot of energy before they break

Answer 47

⋅ toughness is a measure of how much energy a material can absorb before it breaks

Answer 48

some polymers, such as polythene (material that plastic bags are made from)

Answer 49

the hull of a kayak must be made from a tough material so it won't break on rocks (so it can absorb energy from the collision and not break)

Answer 50

⋅ the stress-strain graphs for ductile materials curve

Answer 51

⋅ yes, they do change ⋅ bc different solids have different properties, their stress-strain graphs look different too

Answer 52

Y - point Y is the yield stress ⋅ here the material suddenly starts to stretch without any extra load or with a reduced load E - point E is the elastic limit ⋅ at this point, the material starts to behave plastically ⋅ from point E onwards, the material would no longer return to its original shape once the stress is removed P - point P is the limit of proportionality ⋅ after point P, the graph is no longer a straight line but instead starts to bend ⋅ at this point, the material stops obeying hooke's law, but would still return to its original shape if the stress was removed (so behaves elastically up to point P) ⋅ before point P, the graph is a straight line through the origin ⋅ this shows that, before point P, the material is obeying hooke's law

Answer 53

⋅ brittle materials don't tend to behave plastically ⋅ they fracture before they reach the elastic limit

Answer 54

1) the atoms in a metal are usually arranged in a crystalline lattice, where metal atoms are arranged in a regular repeating pattern ⋅ the structure of metals can also be polycrystalline 2) metals are good conductors bc: ⋅ outer electrons of metal atoms don't need much energy to escape their atoms in this crystalline structure ⋅ electrons then form a 'sea' of free electrons, leaving behind a lattice of ions ⋅ these free electrons make metals good conductors of heat and electricity 3) metals are stiff bc: ⋅ electrostatic attraction between ion lattice + free electrons forms metallic bonds ⋅ metallic bonds are very strong 4) metals are tough bc: ⋅ the strongly bonded lattice structure of metals makes metals tough 5) metals are ductile bc: ⋅ ions within the lattice can move when you apply a force to the metal - making it ductile

Answer 55

1) when a force is applied to a metal, the interatomic spacing between ions increases 2) this increase in interatomic spacing is uniform during elastic deformation 3) once the stress is high enough to cause plastic deformation, planes of atoms within the metal slip over each other (+ form new bonds [holding planes in place]) 4) if there is a dislocation in the metal, the stress needed to cause slipping is lower than the stress needed to cause slipping in a perfect metal

Answer 56

⋅ alloying is where the atoms of a second metal can be placed inside dislocations of metal (impurities) to pin them down ⋅ this increases the stress needed to cause slipping ⋅ alloying makes metals harder and less ductile

Answer 57

⋅ where the atoms in the structure of a metal have a regular, repeating pattern ⋅ such as atoms all lining up in the same direction (/having the same orientation)

Answer 58

where the structure of the material has many regions (or grains) of crystalline structure - but the structure is not completely crystalline

Answer 59

1) ceramics like pottery, brick and glass are made by melting certain materials and then letting them cool 2) arrangements of the atoms in a ceramic can be crystalline or polycrystalline (as shown below) ⋅ the atoms in each grain of crystalline structure line up in the same direction 3) some ceramics like glass are amorphous ⋅ amorphous: there's no overall pattern; atoms are arranged at random ⋅ the quicker a molten ceramic is cooled, the more likely it is to be amorphous 4) ceramics rarely deform plastically before they fracture bc: ⋅ the random atomic bonding in [amorphous] ceramics means there are no slip planes in ceramic lattices ⋅ bc of random atomic bonding, ceramics also don't have mobile dislocations (dislocations that can move) - meaning that ceramics rarely deform plastically before they fracture 5) ceramics are stiff and brittle bc: ⋅ the atoms in ceramics are ionically or covalently bonded in a giant rigid structure ⋅ the strong [covalent] bonds between atoms make ceramics very stiff ⋅ while the rigid structure means that ceramics are very brittle 6) ceramics break via crack propagation bc: ⋅ ceramics being brittle means that cracks spread through them when they fracture ⋅cracks spread bc the applied force acts on a small area (the tip of the crack) so the stress is high

Answer 60

polymers are molecular chains made up of single repeating units (called monomers) joined together

Answer 61

1) polymers are very strong bc: ⋅ the monomers in a polymer chain are covalently bonded together, and so are very hard to separate ⋅ this makes even the thinnest polymer materials very strong 2) polymers are very flexible bc: ⋅ polymer chains are often entangled, and can be unravelled by rotating about their bonds when you pull them ⋅ unravelling polymers by rotating about their bonds is what makes polymer materials flexible ⋅ the more easily the monomers can rotate, the more the chains will untangle and the more flexible the polymer will be 3) polymers can be very rigid bc: ⋅ the strength and the number of bonds between the polymer chains also affect the polymer's flexibility ⋅ the stronger the cross-linking bonds and/or the more cross-linking bonds in the polymer material, the more rigid the material

Answer 62

the two types of polymers are natural polymers (eg, rubber) and man-made polymers (eg, polythene)

Answer 63

1) the radius of a drop of oil was measured and its volume calculated 2) the drop of oil was then released into a tub of distilled water 3) rayleigh assumed the oil drop would spread as much as it could, so the thickness of the oil patch would be the size of one molecule of oil 4) by knowing the volume of oil dropped and the radius of the oil patch, you could equate the equation for the volume of a sphere (approximately the shape of the oil drop) and the equation for the volume of a cylinder (approximately the shape of the oil patch) together to find h (the thickness of the oil patch), finding the size of one atom

Answer 64

⋅ x-ray crystallography ⋅ scanning tunnelling microscope (STM) ⋅ scanning electron microscope (SEM) ⋅ transmission electron microscope (TEM) ⋅ atomic force microscope (AFM)

Answer 65

⋅ in x-ray crystallography, x-rays are fired at a sample ⋅ the x-rays’ diffraction patterns are then used to investigate atomic spacing and the atomic structure of the sample

Answer 66

1) a scanning tunnelling microscope (STM) has a very fine tip to which voltage is applied to 2) electrons from the sample's surface tunnel from the surface to the tip and cause a current to flow 3) the tip is moved across the surface of the sample, and the height of the tip is adjusted to keep the current constant, meaning any small bumps or dips in the surface can be detected ⋅ an STM has such a fine resolution that individual atoms can be resolved and their size and spacing measured

Answer 67

⋅ SEMs and AFMs don't let you 'see' the material's surface directly, but they can be used to build an atom-by-atom image of the surface on a computer screen ⋅ by knowing the magnification of the image on the computer screen and the size of the 'blobs' representing each atom, the sizes of atoms can be calculated

Answer 68

⋅ modern techniques give the diameter of an atom as 0.1-0.5 nm, depending on the atom being measured ⋅ this is much more accurate than the values rayleigh estimated from his experiment

Answer 69

⋅ the biggest percentage uncertainty was given by trying to measure the very small radius of the oil drop precisely, while using equipment that has a very low resolution ⋅ the radius is cubed in the equation for the volume of a sphere, so the percentage uncertainty contributed by radius would end up being very large