Springs and Materials (including thermal and electrical properties)

Question 1

Hooke’s Law

Answer 1

For an object which is undergoing elastic deformation by a force, the subsequent extension is directly proportional to the force.

Question 2

Elastic Deformation

Answer 2

Object returns to originial size and shape when load is removed.

Question 3

Plastic deformation

Answer 3

Object does not return to originial size and shape when load is removed.

Question 4

Limit of Proportionality

Answer 4

Point beyond which the material no longer obeys Hooke’s Law

Question 5

Elastic limit

Answer 5

The point [technically a value of stress] beyond which an object will not return to its original size and shape.

Question 6

Yield Point

Answer 6

At this point the molecules/particles in the material realign to lower the stress. The strain rapidly increases.

Question 7

Ultimate Tensile Stress

Answer 7

The maximum stress a material can attain.

Question 8

Breaking stress

Answer 8

The stress (and strain) that causes a material to break.

Question 9

Strength

Answer 9

Determined by the stress at which a material fractures (i.e. ultimate tensile stress or breaking stress)

Question 10

Stiffness

Answer 10

Determined by the Young’s Modulus - more stiff means smaller strain for larger stress.

Question 11

Toughness

Answer 11

The energy required per unit surface area created during fracture (for a tough material lots of work must be done before it fractures).

Question 12

Hardness

Answer 12

Resistance to penetration/scratching

Question 13

Ductility

Answer 13

A ductile material undergoes large amounts of plastic deformation (in one dimension) before fracture, allowing it to be drawn into wires.

Question 14

Malleability

Answer 14

A malleable material undergoes large amounts of plastic deformation (in two dimensions) before fracture, allowing it to be beaten/drawn into sheets.

Question 15

Brittle

Answer 15

A brittle material undergoes little plastic deformation before fracture (opposite of ductile). Can also be said that it requires little work to be done before fracture (opposite of tough).

Question 16

Dislocations

Answer 16

Defects/flaws in the lines of a metal structure. These can cause movements in the arrangement of a metal or, if there are enough dislocations, they can actually inhibit the movement of the structure (see work hardening). Dislocations are often observed as extra half planes of atoms.

Question 17

Grain

Answer 17

Microcrystals of a metal. The arrangement of each grain differs in some way from neighbouring grain so one can observe the grain boundaries.

Question 18

Work Hardening

Answer 18

Metal is beaten/bent repeatedly, creating more and more dislocations. These dislocations inhibit the planes of atoms from moving. This makes the metal harder, stiffer and more brittle.

Question 19

Alloying

Answer 19

Another element is added to the metal structure. These atoms disrupt the regular arrangement of the metal atoms and stop dislocations from moving. This makes the alloy stiffer and more brittle.

Question 20

Vacancies

Answer 20

Gaps in the metal structure.

Question 21

Creep

Answer 21

Under stress and thermal agitation, a metal or polymer will grow in the direction of the stress over time, increasing the strain for the same tensile force. In a metal this is because the planes of atoms slide over each other, often starting at dislocations. In a polymer this is because the long chains can slowly unwind.

Question 22

Stretching polymers - the three sections.

Answer 22

1. Stretching Van der Waals Forces (harder) - elastic
2. Algining molecules through bond rotation (easier) - once enough stress to overcome the VdW’s, plastic deformation occurs as chains are unwound - significant increase in strain here.
3. Stretching/breaking covalent bonds (much harder)

Question 23

Two things that increase the stiffness of polymers

Answer 23

1. Large side chains - make it harder to rotate bonds

2. Cross links between/within chains - harder for the chains to slide over each other or to unwind.

Question 24

Specific Heat Capacity

Answer 24

The energy required to raise the temperature of 1 kg of a material by 1 K

Question 25

Specific Latent Heat of Fusion/Vaporisation

Answer 25

The energy required per unit mass of a substance to change it from solid to liquid/liquid to gas without a change in temperature.