In the wave model, light is ________.
A. a mix of electric and magnetic energy
B. a packet of energy moving through space
C. a compression wave moving through the air
D. a ripple in space-time
E. the absence of dark
Answer: A
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The addition of insulation to a cylindrical surface, such as a wire, may increase the rate of heat dissipation to the surroundings (a) For a No. 10 wire (0.26 cm in diameter), what is the thickness of rubber insulation [k = 0.16 W/(m K)] that maximizes the rate of heat loss if the heat transfer coefficient is 10 W/(m2 K)? (b) If the current-carrying capacity of this wire is considered to be limited by the insulation temperature, what per cent increase in capacity is realized by addition of the insulation? State your assumptions.
GIVEN
• An insulated cylindrical wire
• Diameter of wire (Dw) = 0.26 cm = 0.0026 m
• Thermal conductivity of rubber (k) = 0.16 W/(m K)
• Heat transfer coefficient ( h c) = 10 W/(m2 K)
FIND
(a) Thickness of insulation (Li) to maximize heat loss (b) Per cent increase in current carrying capacity
ASSUMPTIONS
• The system is in steady state
• The thermal conductivity of the rubber does not vary with temperature
SKETCH
A single slit is illuminated at normal incidence with a parallel beam of light having a wavelength of 411 nm. The entire central band of the diffraction pattern is observed at ±90°
The illumination is now replaced by a nonrelativistic beam of electrons, each having a kinetic energy of 980 eV. When this beam hits the slit at normal incidence, at what angle will the first minimum of the electron diffraction pattern occur? (h = 6.626 × 10-34 J • s, mel = 9.11 × 10-31 kg, 1 eV = 1.60 × 10-19 J) A) 0.095 mrad B) 0.071 mrad C) 0.046 mrad D) 0.12 mrad E) 0.14 mrad
Calculate the heat transfer coefficient for film boiling of water on a 1.3 cm horizontal tube if the tube temperature is 550°C and the system is placed under pressure of 50 kPa.
GIVEN
? Film boiling of water on a horizontal tube
? Tube outside diameter (D) = 1.3 cm = 0.013 m
? Tube temperature (Tt) = 550°C
? Pressure = 50 kPa = 50,000 N/m2
FIND
? The heat transfer coefficient (hc)
ASSUMPTIONS
? Steady state
? Tube temperature is uniform and constant
? The viscosity, specific heat, and thermal conductivity of the vapor can be approximated by those
of steam at 1 atm pressure
? Radiation across the vapor film is negligible
SKETCH
PROPERTIES AND CONSTANTS
Extrapolating from Appendix 2, Table 35, for steam at 1 atm and 81.5°C
A hot air balloon has a volume of 2.00 × 103 m3 when fully inflated, and the air inside the balloon is always at atmospheric pressure of 1.01 × 105 Pa because of the large opening used to fill the balloon and heat the air inside it
What is the mass of hot air inside the balloon if its temperature is 120°C? The universal gas constant is 8.314 J/mol ? K. (Assume a molecular weight of 28.8 g/mol for air.) A) 1780 kg B) 5850 kg C) 203 kg D) 62.0 kg