Physical properties

Question 1

Define a physical property (simple).

Answer 1

A measurable characteristic of a material that does not involve a chemical reaction, e.g., density, hardness, conductivity.

Question 2

Density—definition and formula.

Answer 2

Mass per unit volume. ρ = m / V.

Question 3

Density—typical values: aluminium vs steel vs acrylic (approx).

Answer 3

Aluminium ≈ 2.7 g/cm³; Steel ≈ 7.8 g/cm³; Acrylic ≈ 1.2 g/cm³ (approximate).

Question 4

Young’s modulus (stiffness)—definition.

Answer 4

Ratio of stress to strain in the elastic region; measures how much a material resists elastic stretch (units: GPa).

Question 5

Strength vs stiffness—difference.

Answer 5

Strength = load before failure/yield; Stiffness = resistance to elastic deformation (slope of stress–strain).

Question 6

Tensile strength—what is it?

Answer 6

Maximum stress a material can withstand in tension before fracture.

Question 7

Compressive strength—what is it?

Answer 7

Maximum stress in compression before failure (important for brittle materials like ceramics).

Question 8

Shear strength—simple definition.

Answer 8

Resistance to sliding failure along a plane within the material.

Question 9

Hardness—definition and common tests.

Answer 9

Resistance to indentation/scratch. Tests: Vickers, Brinell, Rockwell.

Question 10

Toughness—definition.

Answer 10

Ability to absorb energy before fracturing; area under the stress–strain curve (impact resistance).

Question 11

Brittleness—definition.

Answer 11

Tendency to fracture with little plastic deformation (e.g., glass, some ceramics).

Question 12

Ductility—definition and indicator.

Answer 12

Ability to be drawn into wire/plastically deform in tension; measured as % elongation at break.

Question 13

Malleability—definition.

Answer 13

Ability to be deformed in compression (e.g., rolled/pressed) without cracking.

Question 14

Elasticity vs plasticity.

Answer 14

Elasticity: returns to original shape after unloading. Plasticity: permanent deformation remains after unloading.

Question 15

Fatigue—what is it?

Answer 15

Failure under repeated cyclic loading at stresses lower than the static strength.

Question 16

Creep—definition.

Answer 16

Time‑dependent permanent deformation under constant load, significant at high temperature.

Question 17

Wear resistance—meaning.

Answer 17

Ability to resist material loss due to friction/abrasion/erosion.

Question 18

Thermal conductivity—definition and unit.

Answer 18

Rate of heat flow through a material (W/m·K). High = metal pans; low = insulators.

Question 19

Thermal expansion—simple equation.

Answer 19

ΔL = α·L·ΔT (change in length = coefficient × original length × temperature change).

Question 20

Coefficient of thermal expansion—units.

Answer 20

Per degree (e.g., /°C or 1/K).

Question 21

Specific heat capacity—definition.

Answer 21

Energy needed to raise 1 kg by 1 °C; affects heating/cooling rates.

Question 22

Thermal shock resistance—what is it?

Answer 22

Ability to withstand rapid temperature change without cracking (important for glass/ceramics).

Question 23

Electrical conductivity vs resistivity.

Answer 23

Conductivity: ease of current flow (S/m). Resistivity: opposition to flow (Ω·m); they are inverses.

Question 24

Dielectric strength—definition.

Answer 24

Maximum electric field an insulator can withstand without breakdown.

Question 25

Magnetic behaviour—simple categories.

Answer 25

Ferromagnetic (strongly attracted, e.g., steels), paramagnetic (weakly), diamagnetic (weakly repelled).

Question 26

Corrosion resistance—meaning.

Answer 26

Ability to resist chemical attack/oxidation (e.g., stainless steel, aluminium oxide layer).

Question 27

Water absorption/permeability—relevance.

Answer 27

Important for polymers/wood; affects swelling, dimensional stability and finish adhesion.

Question 28

Optical properties—two examples.

Answer 28

Transparency/opacity and refractive index; relevant for acrylic and glass components.

Question 29

Acoustic properties—simple note.

Answer 29

Sound absorption vs reflection; dense stiff materials reflect, porous/soft absorb.

Question 30

Test: how is hardness commonly measured in school/college?

Answer 30

Vickers or Rockwell indentation tests; compare indent size or depth to a scale.

Question 31

Test: what does a tensile test give you?

Answer 31

Stress–strain curve → yield strength, ultimate tensile strength, Young’s modulus, % elongation.

Question 32

Test: Izod/Charpy—what property?

Answer 32

Impact toughness (energy absorbed by a notched specimen).

Question 33

Simple calc: A 0.5 kg aluminium block has volume 1.85×10⁻⁴ m³. Density?

Answer 33

ρ = m/V = 0.5 / 1.85e−4 ≈ 2700 kg/m³ (≈ 2.7 g/cm³).

Question 34

Simple calc: A 1 m steel bar (α≈12×10⁻⁶/°C) heats by 50 °C. ΔL?

Answer 34

ΔL = α·L·ΔT = 12e−6×1×50 ≈ 0.0006 m = 0.6 mm.

Question 35

Material choice: a heatsink needs high thermal conductivity—metal or polymer?

Answer 35

Metal (e.g., aluminium) due to high thermal conductivity.

Question 36

Material choice: an electrical insulator near heat—pick a property set.

Answer 36

Low electrical conductivity + high thermal stability (e.g., alumina ceramic).

Question 37

Material choice: a flexible hinge on a prototype—key properties?

Answer 37

High ductility and toughness; sufficient fatigue resistance (e.g., polypropylene).

Question 38

TIMWOOD link: which waste can poor toughness cause?

Answer 38

Defects (scrap) due to brittle fracture; Overprocessing if rework is needed.

Question 39

Finish selection: why is corrosion resistance important?

Answer 39

Reduces maintenance and extends life; may allow lighter coatings or none at all.

Question 40

Laser cutting acrylic: which two properties matter most?

Answer 40

Thermal behaviour (melting/vapourisation characteristics) and optical absorption at laser wavelength.

Question 41

Lathe tool selection: which two properties are critical?

Answer 41

Hardness and hot hardness (retain hardness at temperature), plus wear resistance.

Question 42

Stiffness‑to‑weight—what is a good metric?

Answer 42

Specific modulus = Young's modulus / density (useful for lightweight design).

Question 43

Strength‑to‑weight—what is a good metric?

Answer 43

Specific strength = tensile strength / density.