A piece of chocolate melts in your mouth. This is an example of
a. heat conduction.
b. heat convection.
c. heat radiation.
d. thermal expansion.
e. none of the above.
a
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Cooled air is flowing through a long sheet metal air conditioning duct, 0.2 m high and 0.3 m wide. If the duct temperature is 10°C and passes through a crawl space under a house at 30°C, estimate
(a) The heat transfer rate to the cooled air per meter length of duct.
(b) The additional air conditioning load if the duct is 20-m-long.
(c) Discuss qualitatively the energy conservation if the duct were insulated with glass
wool.
GIVEN
• An air conditioning duct in a crawl space
• Duct height (H) = 0.2 m
• Duct width (w) = 0.3 m
• Duct temperature (Ts) = 10°C
• Ambient temperature (T?) = 30°C
FIND
(a) The heat transfer rate per meter length (qc/L) to the cooled air in the duct
(b) The additional air conditioning load (q20) if the duct length (L) = 20 m
(c) Discuss qualitatively the energy conservation if the duct were insulated with glass wool
ASSUMPTIONS
• Ambient air is still
• Duct temperature is constant and uniform
• Radiation is negligible
• Edge effects are negligible
• No condensation on the duct surface
SKETCH
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 20°C
Thermal expansion coefficient (?) = 0.00341 1/K
Thermal conductivity (k) = 0.0251 W/(m K)
Kinematic viscosity (?) = 15.7 × 10–6 m2/s
Prandtl number (Pr) = 0.71
Bert says that a charged particle in a vacuum can travel in a helix only if a uniform electric field and a uniform magnetic field are both present and both parallel to the axis of the helix. Stuart says that only a magnetic field with a component parallel to the axis of the helix is needed. Which one, if either, is correct, and why?
a. Bert, because the charged particle's velocity can have a vertical component only if an electric field in the vertical direction is present. b. Stuart, because a component of velocity in the vertical direction is not changed by a vertical component of a magnetic field. c. Bert, because a component of velocity in the vertical direction is changed by a vertical component of a magnetic field. d. Stuart, because an electric field in the vertical direction would cause the particle to come to a complete stop. e. Neither, because particles cannot move in helical paths in the presence of magnetic and electric fields.
A 25-L container holds hydrogen gas at a gauge pressure of 0.25 atm and a temperature of 0°C. What mass of hydrogen is in this container?
A) 1.4 g B) 2.8 g C) 4.2 g D) 5.6 g E) 6.3 g
When two strings of equal length on a guitar are played, two different pitches are heard. Comparing the waves on the lower pitch string to the higher, the waves on the higher pitch string have
A. a shorter wavelength. B. a larger period. C. a larger amplitude. D. a higher speed.