Suppose we use a baseball to represent Earth. On this scale, the other terrestrial worlds (Mercury, Venus, the Moon, and Mars) would range in size approximately from that of ________
A) a golf ball to a baseball B) a dust speck to a basketball
C) a golf ball to a beach ball D) a dust speck to a golf ball
A
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An ideal diode in an ac circuit
A. doubles the current. B. halves the current. C. permits the current to flow in only one direction. D. permits the current to flow in one direction and double the current to flow in the other. E. permits the current to flow in one direction and half the current to flow in the other.
A long, heated, cylindrical steel rod is removed from a heat treatment furnace and has to be cooled to complete the process. If the surface temperature of the rod is at 150°C and the cooling fluid temperature is maintained at 25°C, what is the minimum diameter of the rod to produce turbulent natural convection heat transfer if it is held in a horizontal position in (a) air and (b) water? Also, determine the initial heat transfer rate by natural convection from a 10-cm diameter rod in each of the two fluids.
GIVEN
• Long heated cylindrical steel rod, cooled by natural convection.
• Surface temperature of rod (Ts=1500C
• Fluid temperature (T?) = 250C
FIND
(a) Minimum diameter of rod to produce turbulent natural convection in (i) Air (ii) water
(b) Initial heat transfer rate by natural convection from 10 cm diameter rod in each fluids.
ASSUMPTIONS
• Radiation is negligible
SKETCH
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 87.5°C
Thermal expansion coefficient (?) = 0.00277 1/K
Thermal conductivity (k) = 0.0298 W/(m K)
Kinematic viscosity (?) = 22.55 × 10–6 m2/s
Prandtl number (Pr) = 0.71
for water at the mean temperature of 87.5°C
Thermal expansion coefficient (?) = 0.000678 1/K
Thermal conductivity (k) = 0.6765 W/(m K)
Kinematic viscosity (?) = 0.33 × 10–6 m2/s
Prandtl number (Pr) = 67.8
Must we see both spectral lines splitting to find a spectroscopic binary?
What will be an ideal response?
What is the net power radiated by a little animal with a surface area of 0.075 m2 if his emissivity is 0.75, his skin temperature is 315 K, and he is in a room with a temperature of 290 K? (? = 5.67 × 10-8 W/m2 ? K4 )
A) 8.8 W B) 6.0 W C) 8.0 W D) 15 W E) 18 W