Materials

Question 1

Hooks law states

Answer 1

Extention is proportional to force

Question 2

In springs k is called the…

Answer 2

Spring constant

Question 3

A force extention graph starts to curve at…

Answer 3

The limit of proportionallity

Question 4

What is the elastic limit

Answer 4

The point at which the material is permanently deformed

Question 5

What is elastic deformation?

Answer 5

When the atoms in a material return to there equillibrium distance after the tension force pulling them apart is removed

Question 6

What is plastic deformation

Answer 6

When the tension force is removed the atoms dotn return to orignal positions

Question 7

How to investigate extention

Answer 7

• Spring on clamp w ruler
• adding masses
• can do in parralel and series to show relationship of k like with resistors

Question 8

Tension in opposite directions on materials are either

Answer 8

Compresive (squash)

Tensile (stretch)

Question 9

What is fracture stress

Answer 9

• stress pulls atoms apart
• at a point atoms completely sperate and material fractire
• the stress at this point is fracture stress

Question 10

Calculate elastic strain energy stored in a force extension graph.

Answer 10

Area
Workdone=1/2 Fx
This gives average force
-only works if obeying hookes law

Question 11

What dimensions of wire should be use when finding youngs modulus?

Answer 11

-very long and thin wire so more extemtion for the same force therefore reducing measurement uncertaintys

Question 12

Method for findign youngs modulus

Answer 12

• use micro meter to measure diameter at 3 points in wire and take average and assume circular
• clamp wire at one end and put pulley at the other so weights cna be hung
• start w smallest weight that straightens wire
• measure distance between fixed end and marker
• increase the weight in intervals recording the marker each time from above
• calculate stress and strain and plot so gradient is youngs modulus
• make sure not to go past elastic limit

Question 13

Brittle materials….. w example

Answer 13

break without plastic deformation 
Eg ceramics (glass ,pottery)

Question 14

Ductile material….w example

Answer 14

Can be drawn into a wire without losing strength

eg. Copper

Question 15

Strong materials…. W example

Answer 15

Can withstand high stresses without deforming or breaking

Eg. Steel

Question 16

Hard materials…

Answer 16

Very difficult to scratch

Eg. Diamond

Question 17

Stiff materials….. w example

Answer 17

High resistance to bending and stretching
High youngs modulus (measure of stiffness)
Eg. Helmet

Question 18

Tough materials…. W example

Answer 18

Absorb lots of energy before fracture

Eg.polyethene

Question 19

What is yield stress

Answer 19

Stress at which plastic takes place w a constant load

Question 20

Metal structure

Answer 20

• usually crstalline lattice ( atoms arranged in a regular repeating pattern)
• strong electrostatic attraction between positive ion lattice and free electrons making it stiff and tought
• the ions in lattice can move when force is applied so ductile
• when the stress is high enough to cause plastic deformation the planes slip over each other
• if dislocation then lower stress needed

-

Question 21

What is a dislocation?

Answer 21

Missing atoms in regular structure that allow atoms to slip with a lower stress

Question 22

What is alloying?

Answer 22

Second metal pins dislocations to increase stress needed for slipping
Making them harder and less ductile

Question 23

What does polycrystalline mean?

Answer 23

Made up of grains of crystalline structure facing different directions

Question 24

Structure of ceramics

Answer 24

• Can be crystalline or polycrystalline
• some are amorphous like glass
• All covalently or ionically bonded in a giant rigid structure
• rigid structure means bonds are directional meaning locked in place and cannot slip so hard and brittle
• strong bonds make them stiff
• Cracks spread through when they fracture as force applied to small area gives high stress

Question 25

What does amphorpous mean?

Answer 25

• randomly arranged (disordered) atoms so no slip plames or mobile disclocationa meaning rare plastic deformation before fracture
• rapid cooling traps ceramics in amorphous state

Question 26

How does speed of cooling affect ceramics

Answer 26

• rapid cooling traps in amorphous state
• slow controlled cooling can lead to crystalline structure
• in bertween is polycrystalline

Question 27

Explain crack propagation

Answer 27

• In ceramics cracks lead to fracture
• rigid structures so when force is concentrated at tip of crack neighbouring atoms are pulled apart and then the next two erc
• the crack propagates through until fracture
• very high stress as such small area

Question 28

What is a polymer? Example

Answer 28

• Long of chain of repeating monomers

- eg natural rubber or man made polyethene

Question 29

Explain polymer structure?

Answer 29

• covalently bonded so hard to seperate so relatively strong
• Chains are entangled together so can unwravel bu roatating about bonds when stretched making them flexible

Question 30

What can make a polymer stiffer?

Answer 30

• stronger cross links make bonds harder to rotate so stiffer
• adding cross links means can only stretc hebtween each pne so becomes stiffer

Question 31

How to carryout Rayleighs oil drop experiment?

Answer 31

-Drop oil onto water and assume it spreads to be 1 molecule thick
-measure surface aread and volume of oil dropper to find thickness
Size of atom=size of oil molecule divided by no of atoms in molecule so gives upper bound

Question 32

What is scanning tunneling microscope

Answer 32

• scanning tunnelling microscope detects bumps amd dips by keeping a constant current
• 2d images
• has high resoloution so can calculate spacing and size

Question 33

How do Atomic force microscopes work?

Answer 33

—needle detects contours on sample on an atomic scale

• keeps force constant by an actuator physically moving soecies
• records surface in 3d
• lower resolution