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In thermal physics, the rate at which a material cools is governed by its heat transfer coefficient, measured in watts per square meter per degree Celsius (W/m²·°C). This value quantifies how readily heat moves from a material to its surroundings.
Newton’s law of cooling tells us that a lower heat transfer coefficient means slower cooling. The coefficient represents the energy, in watts, required to raise or lower a material’s temperature by one degree over a square meter per second.
Wood is a fibrous material primarily composed of cellulose and lignin. Its heat transfer coefficient is 0.13 W/m²·°C, indicating a relatively fast loss of heat. A 1-kg slab of wood cooled from 104°F (50°C) to 68°F (20°C) in about 2h 20min.
Sand, made of silicon dioxide, has a coefficient of 0.06 W/m²·°C. It retains heat longer than wood, which explains why beach sand can stay warm hours after sunset. A 1-kg container cooled from 104°F to 68°F in roughly 5h 30min.
EPS, a synthetic plastic polymer used in packaging and insulation, boasts the lowest coefficient among the materials discussed—0.03 W/m²·°C. This makes it an excellent insulator. A 1-kg block cooled from 104°F to 68°F in about 11h 20min.
The air we breathe—78% nitrogen, 21% oxygen, 0.03% CO₂, and trace gases—has a heat transfer coefficient of 0.024 W/m²·°C. In a sealed room, air can hold heat for over 14h, keeping indoor temperatures comfortable after heating systems shut down. A 1-kg container of air cooled from 104°F to 68°F in 14h 15min.
