Physics
A scientific research team records temperature data of a freshwater lake in the Himalayas during winter. Three thermometers A, B, and C are used, where A is calibrated in the Celsius scale, B in the Fahrenheit scale, and C in the Kelvin scale.
On a particular night, when the surface water of the lake freezes, the recorded temperatures are :
A ⟶ 0 °C, B ⟶ 32 °F, C ⟶ 273 K
Next morning, the air temperature increases slightly, but the ice on the surface of the lake does not melt immediately.
(a) What happens to the fish present in the lake when the surface of the lake freezes?
(b) If the temperature of water at the bottom of the lake is measured, what would be the readings on thermometers A, B, and C?
(c) Why does the ice formed on the surface of the lake not sink?
(d) Why does the ice on the lake take time to melt even after the air temperature rises above 0 °C?
(e) Why is the Kelvin scale preferred in scientific calculations related to heat transfer?
Heat Energy
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Answer
(a) When the surface of the lake freezes, the fish remain safe in the water below because only the upper layer freezes, while the water beneath stays liquid (around 4 °C).
The ice layer acts as an insulating cover, preventing further heat loss, allowing aquatic life to survive.
(b) The water at the bottom of the lake remains at about 4°C because water has maximum density at 4°C.
Thermometer A = 4°C
To convert into Fahrenheit:
F = C + 32
F = × 4 + 32
F = 7.2 + 32 = 39.2°F
Thermometer B = 39.2°F
To convert into Kelvin:
K = °C + 273
K = 4 + 273 = 277 K
Thermometer C = 277 K
(c) Ice does not sink because it is less dense than liquid water. When water freezes, its structure expands, decreasing its density.
(d) Even when the air temperature rises above 0°C, the ice does not melt immediately because it must absorb latent heat of fusion.
This heat is required to change ice into water without raising its temperature. Hence, melting takes place gradually and the ice takes time to melt.
(e) The Kelvin scale is preferred in scientific calculations because it is an absolute scale of temperature starting from absolute zero.
It has no negative values and is directly related to the kinetic energy of molecules. Therefore, it is more convenient and accurate for calculations in heat transfer and thermodynamics.
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