At what distance from a long straight wire carrying a current of 5.0 A is the magnitude of the magnetic field due to the wire equal to the strength of Earth's magnetic field of about 5.0 × 10-5 T? (?0 = 4? × 10-7 T ? m/A)
A) 3.0 cm B) 1.0 mm C) 1.0 cm D) 2.0 cm E) 4.0 cm
D
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Gravitational lensing of distant, faint irregular galaxies may be the key to:
A) mapping the dark matter. B) quasar energy production. C) understanding galactic rotation curves. D) understanding active galactic nuclei. E) determining galactic redshifts.
One of the most unexpected aspects of Mars' atmosphere is:
A) that it is 95% carbon dioxide. B) that its troposphere changes altitude. C) that it is cloudless. D) that it contains water vapor clouds. E) that it has such a low density.
In order to prevent frostbite to skiers on chair lifts, the weather report at most ski areas gives both an air temperature and the wind chill temperature. The air temperature is measured with a thermometer that is not affected by the wind. However, the rate of heat loss from the skier increases with wind velocity, and the wind-chill temperature is the temperature that would result in the same rate of heat loss in still air as occurs at the measured air temperature with the existing wind. Suppose that the inner temperature of a 3-mm-thick layer of skin with a thermal conductivity of 0.35 W/(m K) is 35°C and the ambient air temperature is –20°C. Under calm ambient conditions the heat transfer coefficient at the outer skin surface is about 20 W/(m2 K) but in a 40 mph wind it increases to
75 W/(m2 K). (a) If frostbite can occur when the skin temperature drops to about 10°C, would you advise the skier to wear a face mask? (b) What is the skin temperature drop due to wind?
GIVEN
• Skier’s skin exposed to cold air
• Skin thickness (L) = 3 mm = 0.003 m
• Inner surface temperature of skin (Tsi) = 35°C
• Thermal conductivity of skin (k) = 0.35 W/(m K)
• Ambient air temperature (T?) = –20°C
• Convective heat transfer coefficients
? Still air (hc0) = 20 W/(m2 K)
? 40 mph air (hc40) = 75 W/(m2 K)
• Frostbite occurs at an outer skin surface temperature (Tso) = 10°C FIND
(a) Will frostbite occur under still or 40 mph wind conditions? (b) Skin temperature drop due to wind chill.
ASSUMPTIONS
• Steady state conditions prevail
• One dimensional conduction occurs through the skin
• Radiative loss (or gain from sunshine) is negligible
SKETCH
A new generation of ground-based telescopes is currently being built that overcomes the limitations of the older large telescopes. Some of these new telescopes
a. use segmented mirrors. b. use mirrors that are very thin. c. use active optics to control the shape of the mirror. d. all of the above. e. none of the above