Which of the following choices best corresponds to what is required by the second law of thermodynamics for any process taking place in an isolated system?
a. entropy decreases
b. entropy remains constant
c. entropy increases
d. entropy equals work done on the system
c
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Which best describes the relationship between two systems in thermal equilibrium?
a. No net energy is exchanged. c. Masses are equal. b. Volumes are equal. d. The velocity is zero.
Consider a design for a nuclear reactor using natural-convection heating of liquid bismuth as shown. The reactor is to be constructed of parallel vertical plates 1.8 m tall and 1.2 m wide, in which heat is generated uniformly. Estimate the maximum possible heat dissipation rate from each plate if the average surface temperature of the plate is not to exceed 870°C and the lowest allowable bismuth temperature is 315°C.
GIVEN
• Vertical plates with uniform heat generation in bismuth
• Plate height (L) = 1.8 m
• Plate width (w) = 1.2 m
• Maximum average surface temperature (Ts) = 870°C
• Minimum bismuth temperature (T?) = 315°C
FIND
• Maximum possible heat dissipation rate (q) from each plate
ASSUMPTIONS
• Steady state
• Free convection only
• Edge effects are negligible
SKETCH
PROPERTIES AND CONSTANTS
for bismuth at the mean temperature of (592.5°C)
Thermal conductivity (k) = 15.58 W/(m K)
Kinematic viscosity (?) = 0.991 × 10–7 m2/s
Prandtl number (Pr) = 0.010 Also Density at 538°C (?538) = 9739 kg/m3 Density at 649°C (?649) = 9611 kg/m3
To find the thermal expansion coefficient (?)
All of the following are true. Which of the following is Not a consequence of precession of the Earth's spin axis?
a) the path that the earth orbits in a slightly elliptical b)which star is called the North Star changes in time c) the instant of the Vernal equinox occurs 20 minutes earlier each year
Acceleration due to gravity on the Moon is less than on Earth, and the Moon is smaller than Earth. This means that compared to an Earth satellite, a satellite in close orbit about the Moon would travel
A) slower. B) faster. C) the same. D) need more information