In which of these situations is convection most likely the main form of heat transfer? a. warm air from a heater on the first floor of a house moves to the upper floors.
b. the sides of a metal pan become hot when the pan is placed on a stove burner.
c. a person gets a sunburn from lying on the beach too long.
d. an ice cube melts when a person holds it in his hand.

Athin electrical heater is inserted between a long circular rod and a concentric tube with inner and outer radii of 20 and 40 mm. the rod (a) has a thermal conductivity of ka = 0.15 w/(m*k), while the tube (b) has a thermal conductivity of kb = 0.8 w/(m*k) and its outer surface is subjected to convection with a fluid of temperature t= -15°c and heat transfer coefficient 60 w/(m2 *k). the thermal contact resistance between the cylinder surfaces and the heater is negligible. (a) determine the electrical power per unit length of the cylinders (w/m) that is required to maintain the outer surface of cylinder b at 15°c. (b) what is the temperature at the center of cylinder a?