In solar ponds constructed in israel, the sun’s energy is concentrated near the bottom of a salty pond. with the proper layering of salt in the water, convection, which would make the water temperature uniform, is prevented. below is a description of how it works: the pond is filled with a dense salt solution, which naturally separates into gradient levels: a weaker salt mix exists near the water's surface, and the higher—and thus denser —saline concentrations settle to the bottom. the topmost layer is, of course, relatively fresh water. as the sun beats down on the brine reservoir, its thermal energy passes through the "lighter" surface levels and heats the heavy saline mass below. the result is that the dense salt solution—held at the bottom of the pool and thermally protected by the surface layer—can reach temperatures between 158 and 176°f. the water at the surface of the pond, instead, stays at a warmth that's "normal" for the desert region, ranging from 68 to 90°f. the hot salt water is pumped through a heat exchanger, which is surrounded by a vessel filled with a substance similar to freon. this, in turn, is connected to a turbine that's specially designed to be driven by a much lower-temperature propellant than that used in a conventional steam turbine. since the medium changes from liquid to gas at a relatively low heat, the sun warmed water instantly flashes the fluid into a pressurized vapor which drives the turbine and its 150-kilowatt ac generator. what is the maximum efficiency with which useful mechanical work can be extracted from the pond?

19. explain why a magnet from your refrigerator could not be used to lift something as heavy as a car. (chapter 7 – pages 202-203) 20. can a magnet ever have a single pole? explain your answer. (chapter 7 – page 208) 21. what happens to the wavelength of a wave if the frequency is increased? (chapter 9 – pages 280-281) 22. an ocean wave has a wavelength of 10 m and a frequency of 4.0 hz. what is the velocity of the wave? show the appropriate equation from your book and show your work with units. (chapter 9 – page 282) 23. as you sit outside, the sound of a siren becomes lower in pitch. is the emergency vehicle moving away from or towards you? explain how you know. (chapter 10 – page 315-316) 24. why are two astronauts in space unable to hear one another? (chapter 10 – page 307) 25. explain in at least 3 sentences how electromagnetic waves form. (chapter 11 – page 338-339) 26. using the chart on page 345 in your textbook, what are the three types of electromagnetic wave with wavelengths shorter than those of visible light. give an example of each. (chapter 11 – pages 345-351) 27. explain why a leaf usually appears to be green, but a rose typically appears red. (chapter 12 – page 373) 28. what is the difference in light that is refracted compared to light that is reflected? think in terms of speed of light as well as what happens to light waves when they interact with a medium. (chapter 12 – pages 369-370) 29. how are concave and convex lenses different? (chapter 13 – page 408-410) 30. what type of mirror is used in rearview mirrors in cars? why is it important that these mirrors have the warning, “objects in mirror are closer than they appear.”? (chapter 13 – page 405)

In the circuit shown below, with each bulb holding a resistance of 60 ohms, how many amps of current are flowing? express your number as a decimal.

An insulated 40 ft^3 rigid tank contains air at 50 psia and 120°f. a valve connected to the tank is now opened, and air is allowed to escape until the pressure inside drops to 25 psia. the air temperature during this process is kept constant by an electric resistance heater placed in the tank. determine the electrical work done during this process.