Chapter 5

Question 1

How did Rayleigh carry out his oil drop experiment?

Answer 1

1) measuring the diameter of the oil drop, then calculating the radius
2) Placing the oil drop on still water to observe it spreading
3) Measuring the diameter of the patch of oil after it spreads, and calculating the radius of that patch

Question 2

Equation for the volume of a sphere

Answer 2

4/3πr³

Question 3

Equation for the volume of an oil patch

Answer 3

πR²h

Question 4

What is the largest uncertainty in the oil drop experiment?

Answer 4

Comes from the difficulty in measuring the diameter of an oil drop more precisely than about +- 0.5mm

Question 5

What are the 2 types of microscopes to view atoms?

Answer 5

Atomic Force Microscope (AFM)

Scanning Tunnelling Microscope (SEM)

Question 6

Scanning tunnelling microscopes show [ ] structures

Answer 6

large scale

Question 7

How does an AFM work?

Answer 7

It moves a needle over a sample to detect the contours of the surface. It can detect changes on an atomic scale. A fine point is mounted on the arm and forces between the surface and the tip make the arm bend. A laser beam is reflected from the arm detects the bending. One way of using the AFM is to move the specimen to keep the force on the tip constant. The up and down movement corresponds to the surface profile.

Question 8

Metals are [ ]

Answer 8

Crystalline

Question 9

What does crystalline mean?

Answer 9

The individual particles are arranged in a regular pattern over distances many times the spacing between the particles.

Question 10

What are dislocations?

Answer 10

Mismatches in the regular rows of atoms - missing atoms in the otherwise orderly arrangement.

Question 11

What makes some pure metals ductile? How does this apply to energy?

Answer 11

The movement of dislocations.

This greatly reduces the energy needed to deform the metal.

Question 12

Why are ceramic metals brittle?

Answer 12

They have dislocations within their structure however they are not mobile

Question 13

Are metal alloys usually less or more ductile than pure metals? Why?

Answer 13

Less ductile
Metal alloys can be formed by the addition of other metallic elements that usually have different size atoms. These can pin down the dislocations in the metal structure, making slippages between the layers of atoms more difficult.

Question 14

Solids form when liquids [ ]

Answer 14

Cool

Question 15

The internal structure of a solid can be [ ] or [ ]

Answer 15

Crystalline or amorphous

Question 16

Rapid cooling is more likely to result in [ ]

Answer 16

Amorphous state, resembling the disordered arrangement in a liquid

Question 17

Slow controlled cooling results in a [ ]

Answer 17

a single pure Crystal

Question 18

If a material is not crystalline or amorphous, it is?

Answer 18

Polycrystalline

Question 19

Describe a Polycrystalline material

Answer 19

Consists a number of grains all orientated differently relative to one other but with an ordered, regular structure within each individual grain

Question 20

How are Polycrystalline solids formed?

Answer 20

As a liquid cools, crystals star to form at different points within it. Each crystal grows out into the remaining liquid until it runs into its neighbours. The result is a patchwork of tin crystals or grains. The interface where these grains meet is known as the grain boundary

Question 21

The strength of a material is affected by tiny [ ] and [ ] in the structure of the material

Answer 21

cracks and flaws

Question 22

Describe the stresses around cracks and how that propagates

Answer 22

The stress concentration around such cracks can be 100s or even 1000s of times the applied stress. This can lead to cracks working though a specimen until it fractures (Look at pg98)

Question 23

Describe toughness in terms of cracks

Answer 23

Toughness is a measure of the energy needed to extent cracks through a material

Question 24

What happens when stress is applied to a crack in a material?

Answer 24

The metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack

Question 25

What does microscopic mean?

Answer 25

Not visible to the unaided eye

Question 26

Ceramics undergo [ ] fracture

Answer 26

Brittle

Question 27

What are the three types of bond?

Answer 27

Ionic
Covalent
Metallic

Question 28

Which bonds are directional?

Answer 28

Bonds in ceramics and ionic compounds

Question 29

What does directional bonds mean?

Answer 29

This means that the atoms are locked in place and cannot slip, making the material hard and brittle

Question 30

Ceramics have [ ] structures

Answer 30

Rigid

Question 31

Describe ceramics, using bonds

Answer 31

Atoms share electrons with neighbouring atoms to form covalent bonds. These bonds are directional - they lock atoms in place, like scaffolding
The bonds are strong - stiff
The atoms cannot slip - hard and brittle

Question 32

Describe metals, using bonds

Answer 32

Non directional bonds
Atoms in metals are ionised
The free electrons move between the ions
The -ve charge of the electrons ‘glues’ the ions together, but the atoms can easily change places
The bonds are strong - metals are stiff
The ions can slip - metals are ductile and tough

Question 33

Describe elasticity in metals, using strain

Answer 33

Metals behave elastically for small strains, up to strains of 0.1% Up to this point the metal extends because the pacing between the +ve ions increases. When the tensile force is removed the metal returns to its original length

Question 34

Describe elasticity in polymers, using strain

Answer 34

They can extend elastically up to 1% strains. Polythene is very floppy because it is free to rotate about its bonds. The bonds are strong so they are difficult to break. this gives polythene the macroscopic properties of strength and flexibility

Question 35

What are polymers?

Answer 35

They are long-chained molecules. Long chains of monomers

Question 36

How do you produce a stiffer material from polymers

Answer 36

Adding cross-linkages, where polymer chains are tied together at regular intervals along the chains, produces a stiffer material

Question 37

What is vulcanisation?

Answer 37

Where natural rubber is heated with sulfur.

Question 38

Describe cracks in brittle materials

Answer 38

microscopic flaws (crack) will tend to elongate rather induce plastic deformation when a stress is applied.

Question 39

Describe cracks in plastic materials

Answer 39

cracks tend to emitdislocationsand become more blunt as the raft is stressed