CDS (I) 2024 General Knowledge · Question 56 of 120

Correct — C, Let it be kept in a bowl of water (at room temperature) and operating a ceiling fan (or table fan) at full speed. Cooling is a race to move heat away from the rice, and this option is the only one that uses two mechanisms at once. Standing the hot bowl in water puts it in contact with a substance of very high specific heat capacity and far better thermal conductivity than air, so heat leaves the bowl into the surrounding water quickly. Running the fan at full speed then keeps carrying that heat away from the water surface and speeds up evaporation from it, so the water itself does not simply warm up and stall the process. Air alone, water alone, or a slow fan in a cooled room each supply only part of that.

• (a)Let it be kept on the table in a room where there is no fan, no air conditioner — The slowest option of the four. Still air is a poor conductor, and once a warm stagnant layer builds up around the bowl the only heat transfer left is weak natural convection. This is the baseline the other three improve on.
• (b)Let it be kept in a room with AC set at a temperature around 23 °C and a ceiling fan (or table fan) operating at slow speed — Better than still air, but still cooling through air alone. Air has a low specific heat capacity and low conductivity, the temperature difference is modest, and a slow fan moves far less air past the bowl than a fast one.
• (d)Let it be kept in a bowl of water at room temperature only — Uses the right medium but leaves out the transport. Water carries heat away from the bowl efficiently at first, but with no air movement over it the water warms, the temperature difference falls, and cooling slows down. Adding the fast fan is exactly what keeps it going.

Heat leaves a hot body by conduction, convection, radiation and evaporation, and how fast it goes depends on the temperature difference, the contact medium and how quickly the heated medium is replaced. Water beats air on two counts that matter here: its specific heat capacity is about four times that of air by mass and vastly greater by volume, so a given amount of heat raises its temperature far less, and its thermal conductivity is roughly twenty times higher, so heat crosses the boundary faster. Forced convection — a fan — matters because it strips away the warmed boundary layer that otherwise insulates the surface, and it also raises the rate of evaporation, which removes the latent heat of vaporisation from what is left behind.

Read the four options as a two-by-two grid: water bath or no water bath, crossed with fast air movement or not. The best cell is the one that has both, which is option (c). The air-conditioner in option (b) is a deliberate distraction — it lowers the room temperature by only a few degrees relative to a bowl of freshly cooked rice near 100 °C, so it barely changes the temperature difference that drives the cooling, while the slow fan wastes the one mechanism that would have helped. This is also why a hot pan is cooled under a running tap rather than left on a counter, and why blowing across hot food works better than waiting.

• Water has a much higher specific heat capacity and about twenty times the thermal conductivity of air, so it removes heat from a hot vessel far faster.
• A fan cools by forced convection — it removes the warm boundary layer of air and speeds up evaporation from the exposed surface.
• Evaporation carries away the latent heat of vaporisation, which is why a wetted surface cools below the surrounding air temperature.
• The rate of heat loss rises with the temperature difference between the body and its surroundings, which is why an air conditioner set near room temperature adds little here.

• Assuming an air conditioner must beat a bowl of water; the water's heat capacity and conductivity matter far more than a few degrees of room temperature.
• Forgetting that a water bath stalls without air movement above it, which is why the fan speed is part of the answer.
• Treating a fan as a cooling device in itself — it moves heat away faster but adds no cooling of its own.

Asked as an applied everyday-physics item with a short scenario — CDS increasingly frames heat-transfer questions this way rather than as definitions.

• practice — not a real PYQ

  Water cools a hot vessel faster than air mainly because water has

  • (a)a lower boiling point
  • (b)a higher specific heat capacity and much higher thermal conductivity
  • (c)a lower density
  • (d)a higher surface tension

  Answer(b) a higher specific heat capacity and much higher thermal conductivity — it absorbs more heat per degree and conducts it away far faster.

• practice — not a real PYQ

  An earthen pot keeps water cool in summer chiefly because

  • (a)clay is a good conductor of heat
  • (b)water seeping through the pores evaporates and takes away latent heat
  • (c)the pot reflects sunlight
  • (d)the pot is airtight

  Answer(b) water seeping through the pores evaporates and takes away latent heat — the same evaporative mechanism that helps in this question.