A bullet is fired into a wall and comes to rest. Considering only the bullet as the system we can say that
A. neither total kinetic energy nor momentum is conserved.
B. both total kinetic energy and momentum are conserved.
C. the momentum is conserved.
D. the total kinetic energy is conserved.
Answer: A
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Although Uranus and Neptune have no liquid metallic hydrogen in their interiors, they do have magnetic fields
Indicate whether the statement is true or false
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)
Are solar cell efficiencies of over 30% realizable under realistic use conditions (include cost in these conditions)? Explain
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Two identical silver spheres of mass m and radius r are placed a distance R (sphere 1) and 2R (sphere 2) from the Sun, respectively. The ratio of the pressure of solar radiation on sphere 2 to that on sphere 1 is
A. 0.25. B. 0.50. C. 1.0. D. 2.0. E. 4.0.