The filament in a light bulb has a diameter of 0.050 mm and an emissivity e = 1.0. The temperature of the filament is 3000°C. How long should the filament be in order to radiate 60 W of power?
The Stefan-Boltzmann constant is 5.67 × 10-8 W/(m2?K4).
A)
11 cm
B)
9.4 cm
C)
8.6 cm
D)
7.2 cm
E)
5.9 cm
E
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Two football players moving as shown, collide. Which vector triangle represents their total momentum before the collision?
The magnetic fields carried by jets coming from active galactic nuclei are generated by
A) the combined magnetic fields of the many stars near the galaxy's center. B) the central black hole. C) the movement of the interstellar matter of the galaxy. D) the movement of the stars in the galaxy. E) charged matter in the accretion disk of the central black hole.
Two lightning bolts are seen to strike two distant locations at the same time. Seen from a different location, the two lightning bolts
A) will also be seen at the same time. B) will not be seen at the same time. C) may or may not be seen at the same time.
How do we know the expansion of the universe is accelerating?
A) We're really not that sure the expansion is accelerating; it's all just a theory. B) The rotation curves of galaxies indicate the universe is speeding up. C) White dwarf supernova in distant galaxies are fainter than they would be if the expansion were not accelerating. D) Redshifts of distant galaxies are increasing with time, so when we repeat the measurements, the redshifts are larger. E) We see more of the universe every night, as the observable universe grows in size.