Question

9(d). A box has a volume of 0.28 m³ and is 70% filled with iron fillings at 25°C. Calculate: (i) the total mass of the iron fillings; (ii) the energy required to melt 10% of the iron fillings. [Density of iron = 8.00×10³ kgm⁻³, specific latent heat of fusion of iron = 1.38×10⁵ Jkg⁻¹, specific heat capacity of iron = 460 Jkg⁻¹K⁻¹, melting point of iron = 1500°C]

Accepted Answer

Volume of iron fillings = 0.7 × 0.28 = 0.196 m³.
(i) Mass = density × volume = 8000 × 0.196 = 1568 kg.
(ii) Mass of 10% of fillings = 0.1 × 1568 = 156.8 kg.
Heat required = heat to raise temperature from 25°C to 1500°C plus heat to melt at 1500°C:
Q = mCΔθ + mL_f = 156.8×460×(1500−25) + 156.8×1.38×10⁵ ≈ 1.06×10⁸ + 0.22×10⁸ ≈ 1.28×10⁸ J.

WAEC Physics 2015 Theory — Question 14

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