The crackling noise produced by aluminum roofing sheets on a house during a hot sunny day is a result of

Analyze the question: Why do aluminum roofing sheets make a crackling noise on a hot, sunny day?

Concept: Most solids expand when heated (thermal expansion). The amount of expansion is given by:

ΔL = L × α × ΔT

where L is the original length, α is the linear expansivity of the material, and ΔT is the temperature rise.

Aluminum has a fairly high linear expansivity (about 2.3 × 10⁻⁵ K⁻¹), so it expands noticeably when heated.

Step-by-step solution:

1. During a hot sunny day, the roofing sheets absorb a lot of heat from the Sun, and their temperature rises.
2. The sheets expand as they heat up.
3. The sheets are held in place by nails or screws and are overlapped with other sheets, so they cannot expand freely. As they push against these fixings and each other, they slip suddenly, which produces a crackling or popping sound.
4. Contraction would produce a similar noise, but it happens when the sheets cool, typically in the evening or at night, not during the hot sunny period.

Checking the options:

• A. Thermal equilibrium means no net heat flow, so it does not produce noise.
• B. Conduction is just heat transfer through the material and does not by itself cause sound.
• C. Contraction occurs on cooling, not on a hot sunny day.
• D. Expansion of the sheets is the cause of the crackling.

**Answer: D, expansion of the sheets.

Tip: In the WASSCE, whenever you see noises (cracking, ticking) linked to heating, such as roofing sheets, railway tracks, or cracking glass, think thermal expansion. Hot means expansion, and cold means contraction.