In fission bombs, a certain "critical mass" is needed because
A) the fissionable material will not reach a sufficiently high temperature unless this much material is present.
B) if the mass is too small, too many neutrons pass through the uranium and out the sides without hitting a nucleus.
C) too small an amount of uranium will not hold together throughout the reaction, and thus the uranium falls apart and the reaction goes out before it is completed.
D) it is impossible to fission a uranium nucleus unless it is near a large number of other uranium nuclei.
E) for smaller masses the pressure is so low that neutrons move too slowly through the uranium to sustain a chain reaction.
B
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The emissivity of an ideal reflector has which of the following values?
a. 0 c. 100 b. 1 d. infinity
Two ideal Carnot thermodynamic engines have the same efficiency. One operates between 5.0 × 102 K and 3.0 × 102 K, and the other between 4.0 × 102 K and some lower temperature. What is the lower temperature?
A) 200 K B) 220 K C) 240 K D) 260 K E) 280 K