A supernova that occurred 30,000 years ago in a galaxy 100,000 light years away will be observed on Earth:
A) 30,000 years from now.
B) 70,000 years ago.
C) 70,000 years from now.
D) 100,000 years ago.
E) 100,000 years from now.
C
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A thin metallic spherical shell of radius 50 cm has a total charge of 4.0 ?C placed on it. At the center of the shell is placed a point charge of 2.00 ?C. What is the electric field at a distance of 90 cm from the center of the spherical shell?
a. 
b. 
c. 
d. 
e. 
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 seen from Mars orbiters, ______________ and ______________ are cut, possibly by sudden floods or drainage that occur on a longer timescale
Fill in the blank(s) with correct word
A cable is 100 m long, has a cross-sectional area of 1.0 mm2, and is made of a material having a Young's modulus of 1.0 × 1011 N/m2. If a 1000-N force is applied to stretch the cable, how far does it stretch?
A) 0.0010 m B) 0.010 m C) 0.10 m D) 1.0 m E) 10 m