Two identical amounts of coffee at are poured into two mugs and left in still room air at
PHYSICS · Single correct · CBSE · Physics · Thermal Properties of Matter · Thermal Properties of Matter
Problem
Two identical amounts of coffee at 90∘C are poured into two mugs and left in still room air at 20∘C. Mug X is wide and shallow, giving the coffee twice the exposed surface area of Mug Y, which is tall and narrow. Which prediction about the cooling of the coffee is most accurate?
(a) Coffee in Mug Y cools faster because its greater depth increases internal convection currents that carry thermal energy to the surface more efficiently.
(b) Both mugs cool at essentially the same rate because the identical masses and temperatures mean Newton’s law of cooling gives the same overall heat loss.
(c) Coffee in Mug X cools faster because the larger exposed surface area allows a greater rate of energy loss by convection, radiation, and evaporation.
(d) Coffee in Mug X actually cools more slowly because the larger area spreads the same heat loss over a wider surface, reducing the heat flux from any given spot.