Intensity: The rate of energy flow per unit area of an electromagnetic wave has an average value of 0.695 W/m2. The wave is incident at right angles upon a rectangular area measuring 1.5 m by 2.0 m. How much total energy falls upon this rectangle each minute? (?0 = 8.85 × 10-12 C2/N ? m2, ?0 = 4? × 10-7 T ? m/A, c = 3.0 × 108 m/s)
A. 130 J
B. 160 J
C. 190 J
D. 220 J
E. 250 J
Answer: A
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A solid lead cylinder 0.6-m in diameter and 0.6-m long, initially at a uniform temperature of 121°C, is dropped into a 21°C liquid bath in which the heat transfer coefficient h cis 1135 W/(m2 K). Plot the temperature-time history of the center of this cylinder and compare it with the time histories of a 0.6 m-diameter, infinitely long lead cylinder and a lead slab 0.6 m-thick.
GIVEN
• A solid lead cylinder dropped into a liquid bath
• Cylinder diameter (D) = 0.6 m
• Cylinder (L) = 0.6 m
• Initial uniform temperature (To) = 121°C
• Liquid bath temperature (T?) = 21°C
• Heat transfer coefficient h c= 1135 W/(m2 K)
FIND
(a) Plot the temperature-time history of the cylinder center (b) Compare it with the time history of a 0.6 m diameter, infinitely long lead cylinder (c) Compare it with the time history of a lead slab 0.6 m thick
ASSUMPTIONS
• Two dimensional conduction within the cylinder
• Constant and uniform properties
• Constant liquid bath temperature
SKETCH
