A pre-electronic thermostat employs a coiled
A) wire of low conductivity.
B) bimetallic strip.
C) tube of mercury.
Answer: B
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Energy Levels: The wavelength of the emitted photon if an electron in the hydrogen atom makes a transition from the n = 2 state to the ground state is closest to which of the following values? (c = 3.0 × 108 m/s, h = 6.626 × 10-34 J ? s, 1 eV = 1.60 × 10-19 J)
A. 122 nm B. 203 nm C. 243 nm D. 389 nm E. 411 nm
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
The operation of a cloud chamber relies on
A) magnetization. B) evaporation. C) acceleration. D) polarization. E) condensation.
It is harder to map the structure of the Milky Way in visible wavelengths than with radio and infrared waves
Indicate whether the statement is true or false