A 1.9-m2 Thermopane window is constructed by using two layers of glass 4.0 mm thick, separated by an air space of 5.0 mm. If the temperature difference is 25.0 °C from the inside of the house to the outside air, what is the rate of heat flow through this window? (Thermal conductivity for glass is 0.840 J/s/m/°C and for air 0.023 4 J/s/m/°C.)
a. 2 530 W
b. 426 W
c. 3 190 W
d. 210 W
e. 2 100
d
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Jet planes avoid bad weather by flying above it in the
a. stratosphere. b. troposphere. c. mesosphere. d. thermosphere.
A 2.5-cm-OD, 2-cm-ID copper pipe carries liquid oxygen to the storage site of a space shuttle at – 183°C and 0.04 m3/min. The ambient air is at 21°C and has a dew point of 10°C. How much insulation with a thermal conductivity of 0.02 W/(m K) is needed to prevent condensation on the exterior of the insulation if hc + hr = 17 W/(m2 K) on the outside?
GIVEN
nsulated copper pipe carrying liquid oxygen Inside diameter (Di) = 2 cm = 0.02 m Outside diameter (Do) = 2.5 cm = 0.025 m LOX temperature (Tox) = – 183°C LOX flow rate (mox) = 0.04 m3/min Thermal conductivity of insulation (ki) = 0.02 W/(m K) Exterior heat transfer coefficients (ho = hc + hr) = 17 W/(m2 K) Ambient air temperature (T ) = 21°C Ambient air dew point (Tdp) = 10°C
FIND
Thickness of insulation (L) needed to prevent condensation
ASSUMPTIONS
Steady-state conditions have been reached The thermal conductivity of the insulation does not vary appreciably with temperature Radial conduction only The thermal resistance between the inner surface of the pipe and the liquid oxygen is negligible, therefore Twi =
Tox
SKETCH
A stretched string is observed to vibrate in three equal segments when driven by a 480-Hz oscillator. What is the fundamental frequency of vibration for this string?
a. 480 Hz b. 320 Hz c. 160 Hz d. 640 Hz e. 240 Hz
Which of the following is not a common use of microwaves?
a. cooking c. radar b. medical imaging d. communications