Which of the following statements about spectral types of stars is not generally true?
A) The spectral type of a star can be determined by identifying lines in its spectrum.
B) The spectral type of a star can be used to determine its surface temperature.
C) The spectral type of a star can be used to determine its color.
D) The spectral type of a star can be used to determine its distance.
D
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A small gray sphere having an emissivity of 0.5 and a surface temperature of 5370C is located in a blackbody enclosure having a temperature of 37°C. Calculate for this system: (a) the net rate of heat transfer by radiation per unit of surface area of the sphere, (b) the radiative thermal conductance in W/K if the surface area of the sphere is 95 cm2 , (c) the thermal resistance for radiation between the sphere and its surroundings, (d) the ratio of thermal resistance for radiation to thermal resistance for convection if the convective heat transfer coefficient between the sphere and its surroundings is 11 W/(m2 K), (e) the total rate of heat transfer from the sphere to the surroundings, and (f) the combined heat transfer coefficient for the sphere.
GIVEN
• Small gray sphere in a blackbody enclosure
• Sphere emissivity (?s) = 0.5
• Sphere surface temperature (T1) = 3370C= 810 K
• Enclosure temperature (T2) = 37°C = 310 K
• The surface area of the sphere (A) = 95 cm2=9.5*10-3 m2
• The convective transfer coefficient ( ch ) = 11 W/(m2 K)
FIND
(a) Rate of heat transfer by radiation per unit surface area (b) Radiative thermal conductance (Kr) in W/K (c) Thermal resistance for radiation (Rr) (d) Ratio of the radiative and conductive resistance (e) Total rate of heat transfer (qT) to the surroundings (f) Combined heat transfer coefficient ( cr h )
ASSUMPTIONS
• Steady state prevails
• The temperature of the fluid in the enclosure is equal to the enclosure temperature
SKETCH
A ping-pong ball covered with a conducting graphite coating has a mass of 2.5 × 10^-3 kg and a charge of 4.0 µC. What electric field directed upward will exactly balance the weight of the ball? (g = 9.8 m/s2)
a. 6.1 × 10^3 N/C c. 5.1 × 10^6 N/C b. 2.0 × 10^-7 N/C d. 1.2 × 10^4 N/C
In the context of life in the universe, which of the following discoveries of astronomy suggests that there has been plenty of time for life to have appeared and evolved elsewhere?
A) the universe is very large B) the universe is very old C) the laws of physics are the same everywhere D) the building blocks of life are common
A car of mass m1 traveling at velocity v passes a car of mass m2 parked at the side of the road. The momentum of the system of two cars is
A.
B.
C.
D.
E.