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From Models to Materials · Paper 2 — Free Chemistry HL Practice Questions

1ChallengeSAQ-LPhysical properties of materials6 marksPaper 2~9 min

Data

Property — Aluminium — Steel Coefficient of linear thermal expansion, α\alpha — 23×10−6 ∘C−123 \times 10^{-6}\,^\circ\text{C}^{-1} — 12×10−6 ∘C−112 \times 10^{-6}\,^\circ\text{C}^{-1} Young's modulus, EE — 69 GPa69\,\text{GPa} — 200 GPa200\,\text{GPa}
A transmission line uses aluminium cables reinforced with a steel core (ACSR — Aluminium Conductor Steel Reinforced). During operation, electrical resistance heats the cable to temperatures up to 90 ∘C90\,^\circ\text{C} on hot days. The cable is installed at 20 ∘C20\,^\circ\text{C} with zero initial tension. The steel core occupies 15%15\% of the cross-sectional area; aluminium occupies 85%85\%.
(a)
Calculate the free thermal strain, ε=α ΔT\varepsilon = \alpha\,\Delta T, for each material when the cable heats from 20 ∘C20\,^\circ\text{C} to 90 ∘C90\,^\circ\text{C}. [2 marks]
(b)
Explain why bonding the two materials together creates internal stress in the cable when the temperature changes. [2 marks]
(c)
Determine which material is under tension and which is under compression at 90 ∘C90\,^\circ\text{C}. [1 mark]
(d)
Evaluate the mechanical design of the ACSR cable, referring to the role of each material at both low and high operating temperatures. [2] (e) Suggest one modification to the cable design that would reduce the internal stress at high temperatures. [1 mark]
diagram

Solutions

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