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AS Physics PiA Spec > Designer materials > Flashcards

Designer materials Flashcards

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1
Q

Hooke’s Law

A

Extension is proportional to the force applied. F=ke, where k is the stiffness/spring constant. Works for metal wires and springs, NOT rubber.

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2
Q

Limits of Hooke’s Law

A

Only works up until the elastic limit - here, force (y) against extension (x) curves (was a straight, linear relationship).

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3
Q

Elastic limit

A

Beyond the elastic limit, Hooke’s law is no longer obeyed and the material will not return to its original shape.

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4
Q

Elastic deformation

A

Material returns to original shape after force is removed. Atoms have been moved small distances from their equilibrium position, and return after force is removed.

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5
Q

Plastic deformation

A

Material is permanently deformed. Atoms in material move relative to one another, and don’t return to original position.

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6
Q

Tensile/Compressive

A

Tensile: stretching forces, positive
Compressive: squashing forces, negative

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7
Q

Tensile stress, sigma=

A

F/A (force/area)

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8
Q

Tensile strain, funny e thing=

A

e/l (extension/original length). Change in length of material.

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9
Q

Breaking stress

A

A stress big enough to break the material

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10
Q

Shape of stress strain graph (general)

A
  • from origin
  • straight line for a bit
  • starts to bend
  • reaches peak, then falls back down
  • ends, as material breaks
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11
Q

Fracture stress

A

Stress on a stress strain graph where the material breaks

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12
Q

UTS

A

Ultimate tensile stress. The maximum amount of stress a material can withstand

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13
Q

Young modulus, E=

A

Stress/strain. Up until limit of proportionality (where stress strain graph starts to curve). Measured in N/m^2

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14
Q

Measuring young modulus

A

Clamp a wire to a bench, set up a marker, hang some weights off it. Measure original length of wire (from marker to clamped end). Add weights, recording extension. At end, measure diameter of wire with micrometer, so can calculate cross sectional area.

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15
Q

Structure of metals

A
  • crystalline/polycrysalline
  • Sea of free electrons surrounding positive ions
  • Metallic bond between ions and free electrons
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16
Q

Properties of metals

A
  • Stiff, as strong metallic bonds
  • Ductile, as ions in lattice can move when load applied
  • Good conductor, as free electrons as charge carriers
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17
Q

Structure of ceramics

A
  • Crystalline/polycrystalline/amorphous structure
  • ionic/covalent bonds
  • giant rigid structure
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18
Q

Properties of ceramics

A
  • Stiff, as strong ionic/covalent bonds

- Brittle, as rigid structure

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19
Q

Making ceramics

A

Melting materials, then cooling them

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20
Q

Structure of polymers

A
  • Molecular chain of a repeating unit (monomer)
  • Natural (e.g. rubber) and man-made
  • Covalent bonds
  • Chains often scrunched up inside the material
  • Sometimes have cross links between chains
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21
Q

Properties of polymers

A
  • Strong, as covalently bonded monomers
  • Flexible, as monomer chains can rotate around bonds and can unfold
  • Rigid if many cross links, as restricts rotation
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22
Q

Composites

A

2 different materials combined together
E.g. reinforced concrete (steel rods in concrete). Concrete is strong in compression but brittle in tension; steel is strong in tension

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23
Q

‘Seeing atoms’

A
  • Using scanning electron microscopes/atomic force microscope
  • Allows you to build up an image of the surface of a material; can’t see atoms underneath!! (need x-ray crystallography)
24
Q

Brittle

A

Opposite of tough. Brittle materials don’t undergo much plastic deformation and break soon after elastic limit. They crack easily and fracture suddenly. Brittle materials can be strong though!!!

25
Q

Ductile

A

Can easily be drawn into a wire, without losing strength

26
Q

Malleable

A

Can easily be hammered and pressed into shape (may lose strength though)

27
Q

Hard

A

Resistant to scratches, cuts, denting and abrasion. Measured in Pa

28
Q

Stiff

A

Resistant to bending/stretching.

29
Q

Tough

A

The energy required to create new surface area (J/m^2) or energy absorbed per unit volume (J/m^3). A tough material needs a large amount of energy to break it, undergoes a large amount of plastic deformation before breaking, resists crack propagation

30
Q

Shape of stress strain graphs for ductile materials

A
  • Linear until limit of proportionality
  • Curves through elastic limit
  • Reaches yield point - dip in graph
  • continues to curve
31
Q

Limit of proportionality

A

Stress strain graph starts to bend after this point (not obeying Hooke’s Law), but material will still return to original shape.

32
Q

Yield point

A

Where material suddenly starts to stretch without any more load being added. Large amount of plastic deformation takes place with constant/reduced load

33
Q

Things determining resistance

A
  • Length (longer a wire is, more difficult for a current to flow
  • Area (wider it is, easier for passing electrons)
  • Resistivity (a material/environmental - e.g. temperature etc - constant - how easy it is for a current to flow through the material)
34
Q

Resistivity

A

The resistance of a 1, length material with a 1m^2 cross-sectional area

35
Q

Resistivity, rho=

A

RA/l (where R is resistance, A is area and l is length)

36
Q

Resistance, R=

A

(rho)l/A

37
Q

Conductivity, sigma=

A

Gl/A (where G is conductance, l is length and A is area. In siemens per metre, Sm^-1)

38
Q

Conductance, G=

A

(sigma)A/l

39
Q

Charge carriers in metals

A

Are free electrons. As temperature increases, the positive ions vibrate more, slightly restricting the movement of the electrons

40
Q

Charge carriers in semiconductors

A

Are free electrons. As temperature increases, more electrons are freed (leaving behind holes - which can also conduct), so conductivity increases dramatically

41
Q

Crystalline

A

Regular layers of atoms or molecules, e.g. metals (polycrystalline) and salts

42
Q

Amorphous

A

No long range order, e.g. glass, some ceramics

43
Q

Polymers

A

Molecular chains, e.g. polythene, rubber. Can be amorphous or crystalline

44
Q

Ionic bonds

A

e.g. salts. Rigid, directional, stiff - little deformation before breaking - usually brittle

45
Q

Covalent bonds

A

e.g. ceramics, glass. Rigid, directional, stiff - little deformation before breaking - usually brittle

46
Q

Polymer chains

A

Very variable - if bonds can rotate, chains straighten out - giving very large strains. If cross-linked,, chains are rigid and inflexible

47
Q

Bonding in metals

A

Polycrystalline: many crystal grains surrounded by irregular grain boundaries. In each grain, regular lattice of positive ions, surrounded by a ‘sea’ of delocalised electrons. Bonds are stiff and strong, but also non-directional, so that layers can slip over one-another

48
Q

Elastic deformation of metals

A
  • Ordered lattice structure
  • Atoms arranged in layers
  • Allows atoms to move relative to each other
  • Restorative forces return atoms to original position
49
Q

Impurity atoms

A

Lodge within the lattice and obstruct the movement of layers or dislocation - material is stiffer, harder and more brittle

50
Q

Brittle materials cracking

A

High concentration of stress at the top of the crack; crack propagates through the material.

51
Q

Tough materials cracking

A

Plastic flow around the tip of the crack reduces stress concentration - crack tip is blunted

52
Q

Fibrous material cracking

A

Cracks are unlikely to propagate through successive fibres - the resin tends to fail first, blunting the tip of the crack

53
Q

N-type silicon

A

A small amount of impurity is added which donates an extra electron for each atom, e.g. an atom with 5 valence electrons. Provides extra conduction electrons

54
Q

P-type silicon

A

A small amount of impurity is added which has a deficiency of one electron per atom, e.g. an atom with 3 valence electrons. These provide electron holes, which behave like positive charge carriers

55
Q

Deformation of glass

A
  • Amorphous structure and directional bonding
  • Atoms aren’t free to move
  • Stress can’t be relieved by plastic flow
  • Stress is concentrated at crack tips, causing cracks to propagate through the structure (therefore brittle)

AS Physics PiA Spec (3 decks)

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