An observer on Earth's geographic north pole would find
a. Polaris directly overhead.
b. Polaris 40° above the northern horizon.
c. that the celestial equator coincides with the horizon.
d. celestial equator passing directly overhead.
e. that the ecliptic coincides with the horizon.
a
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Bottles of perfume sometimes have thick glass (n = 1.5) walls which give the impression the volume is larger than it really is. Assume a cylindrical bottle with an inner radius of 1.0 cm and an outer radius of 2.0 cm. What percentage of the apparent volume is the real volume?
A. 95% B. 60% C. 80% D. 64% E. 75%
A solar thermal central receiver generates heat by focusing sunlight with a field of mirrors on a bank of tubes through which a coolant flows. Solar energy absorbed by the tubes is transferred to the coolant which can then deliver useful heat to a load. Consider a receiver fabricated from multiple horizontal tubes in parallel. Each tube is 1-cm-ID and 1-m-long. The coolant is molten salt which enters the tubes at 370°C. Under start-up conditions, the salt flow is 10 gm/s in each tube and the net solar flux absorbed by the tubes is 104 W/m2. The tube wall material will tolerate temperatures up to 600°C. Will the tubes survive start-up? What is the salt outlet temperature?
GIVEN
• Molten salt flowing through a horizontal tube that is absorbing solar energy
• Tube inside diameter (D) = 1 cm = 0.01 m
• Tube length (L) = 1 m
• Entering salt temperature (Tb,in) = 370°C
• Start-up mass flow rate (m) = 10 gm/s = 0.01 kg/s
• Net solar energy absorbed by the tube (qs) = 104 W/m2
• Maximum tube wall temperature (Ts) = 600°C
FIND
(a) Salt outlet temperature (Tb,out)
(b) Will the tubes survive start-up?
SKETCH
PROPERTIES AND CONSTANTS
for molten salt at 370°C
Specific heat (c) = 1629 J/(kg K)
Capella is the same temperature as our Sun, but about ten times the Sun's diameter; it must be about ________ times as luminous as our star
Fill in the blank(s) with correct word
Which of the following general statements about light and Earth's atmosphere is not true?
A) X-rays from the Sun are absorbed in the thermosphere. B) Visible light from the Sun is absorbed in the exosphere. C) Ultraviolet from the Sun is absorbed in the stratosphere. D) Infrared light emitted by Earth itself is absorbed in the troposphere.