Which of the following is the main reason for the fact that hydroelectric power plants are far more efficient than coal-fired electric power plants?
A) Hydroelectric plants have far fewer moving parts.
B) Coal is only partially combustible, due to the presence of large amounts of impurities.
C) The law of conservation of energy tells us that heat engines, such as hydroelectric plants, are more efficient than other forms of energy conversion.
D) Since the hydroelectric plant is not a heat engine, it is not restricted by the second law of thermodynamics.
E) Actually, hydroelectric plants are not significantly more efficient than coal-fired plants.
D
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Does the cold streaming gas heat up or cool down when it enters the central cloud?
This frame from Figure 16.12 is from a supercomputer model of the formation of one of the first stars in the universe. Color indicates gas temperature: Blue and green represent cool gas (100—300 K), orange and red represent warm gas (500—3000 K), and yellow represents gas at intermediate temperatures (300—500 K). Use the information in the figure to answer the following questions.
How do x-ray measurements help us measure the amount of dark matter in galaxy clusters?
A) The gravity of dark matter causes intracluster gas to be heated to very high temperatures, and the higher the temperature, the stronger the x-ray emission we will detect from the gas. B) Dark matter absorbs x-rays, so we can map the dark matter distribution by looking for absorption lines in x-ray spectra. C) Collisions between dark matter particles emit x-rays, allowing us to map dark matter with x-ray images.
The x-rays that occur when a high energy electron beam is incident on a metal target will show what type of spectrum?
a. continuous b. line c. continuous spectrum superimposed with a line spectrum d. absorption
A small underwater pool light is 1.26 m below the surface of a swimming pool. What is the radius of the circle of light on the surface, from which light emerges from the water? (nwater = 1.333)
a. 0.77 m c. 1.13 m b. 0.57 m d. 1.43 m