Which of the following models best explains why our galaxy has spiral arms?
A) The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.
B) The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates.
C) The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation.
D) No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy.
A) The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.
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Calculate the net radiant-heat-transfer rate if the two surfaces in Problem 11.18 are black and are connected by a refractory surface with an area of 500 m2. A1 is at 555 K and A2 is at 278 K. What is the refractory surface temperature?
GIVEN
• Rectangular surfaces A1 and A2 connected by a refractory surface A3
• A1 is parallel to and centered 5 m above A2
• Dimensions of A1 = 1 m × 20 m
• A2 is 20 m2
• A3 is 500 m2
• Temperature of A1 (T1) = 555 K
• Temperature of A2 (T2) = 278 K
• A1 and A2 are black
FIND
(a) The net radiating heat transfer rate (q1)
(b) The refractory surface temperature (T3)
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
All heat engines:
a. move heat from colder regions to hotter regions. b. convert some of the heat into work. c. add work to the movement of heat. d. can potentially extract nearly limitless energy from the thermal energy of their environment. e. are too large and cumbersome to be used in automobiles.
Astronomers have determined the detailed composition of all confirmed exoplanets
Indicate whether the statement is true or false
In an isovolumetric process
a. the temperature remains constant. b. no heat is transferred between a system and its surroundings. c. the pressure remains constant. d. the volume remains constant. e. the internal energy is constant.