When two objects at different temperatures are placed in thermal contact, heat flows
A. only if both objects are at the same temperature
B. from the higher temperature object to the lower temperature object
C. from the lower temperature object to the higher temperature object
B. from the higher temperature object to the lower temperature object
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In a metal manufacturing plant and its heat treatment process, a long, hexagonal copper extrusion (or rod) comes out of heat-treatment furnace at 400°C and is then quenched by immersing it in a 50°C air stream flowing perpendicular to its axis at 10 m/s as depicted in the figure. The surface of the copper has an emissivity of 0.9 due to oxidation in this process. The rod is 3 cm across opposing flat sides, and it has a cross-sectional area of 7.79 cm2, and a perimeter of 10.4 cm. Determine the time required for the center of the copper to cool to 100°C.
GIVEN
• A long hexagonal copper extrusion in an air stream flowing perpendicular to its axis
• Initial temperature (To) = 400°C
• Air temperature (T?) = 50°C
• Air velocity (V?) = 10 m/s
• Surface emissivity (?) = 0.9
• Distance across the flats (D) = 3 cm = 0.03 m
• Cross sectional area of the extrusion (Ac) = 7.79 cm2 = 7.79 × 10–4 m2
• Perimeter of the extrusion (P) = 10.4 cm = 0.104 m
FIND
• The time (t) required for the center of the copper to cool to 100°C
ASSUMPTIONS
• Variations of the copper properties with temperature are negligible
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity (ka) = 0.0339 W/(m K) Kinematic viscosity (?) = 29.6 × 10–6 m2/s Prandtl number (Pr) = 0.71 For Appendix 2, Table 12, for copper
Thermal Conductivity (k) = 386 W/(m K) at 250°C Density (?) = 8933 kg/m3 at 20°C Specific heat (c) = 383 J/(kg K) at 20°C
As a body cools, its ________ decreases.
A. brightness B. total emitted energy C. internal energy D. All of these choices are correct.
At what rate is thermal energy generated in the 5-? resistor when ? = 24 V?
a.
13 W
b.
3.2 W
c.
23 W
d.
39 W
e.
51 W
Two stars of masses M and 6M are separated by a distance D. Determine the distance (measured from M) to a point at which the net gravitational force on a third mass would be zero
a. 0.41 D b. 0.33 D c. 0.37 D d. 0.29 D e. 0.14 D