The change in entropy of a mass m of a solid substance which has a latent heat of fusion LF and melts at a temperature T is
A. LFT/m
B. mLFln(T)
C. mLFT
D. mLF/T
E. LF/mT
Answer: D
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Estimation: A reasonable estimate for the mass of a typical female college student is
A. 20 kg. B. 50 kg. C. 150 kg. D. 200 kg.
In a ball bearing production facility, steel balls that are each of 15 mm in diameter are annealed by first heating them to 8700C and then slowly cooling in air to 1250C. If the cooling air stream temperature is 600C, and it has a convective heat transfer coefficient of 35 W/(m2 K), determine the time required for the cooling.
GIVEN
• Steel balls of diameter D=15 mm=0.015 mm
• Initial temperature of balls before cooling (To) =870°C
• Cooling air stream temperature (T?) = 60°C
• Convective heat transfer coefficient ( ch ) = 35 W/(m2 K)
FIND
• The time required for cooling ball bearing to 1250C.
ASSUMPTIONS
• Constant thermal conductivity
• End effects are negligible
SKETCH
PROPERTIES AND CONSTANTS
For steel ball:
Thermal conductivity (k) = 50 W/(m K) Specific heat (c) = 460 J/(kg K) Density (?) = 7865 kg/m3
This painting represents the Sun and planets (and two dwarf planets) of our solar system. What is not to scale in this painting?
A) The sizes of the planets are not correctly scaled compared to each other or the Sun. B) The distances between the planets are not shown to scale. C) The Sun is too big compare to the planets. D) Everything is correctly scaled, but the planets are shown in the wrong order from the Sun. E) Neither distances nor sizes are correctly scaled.
A +4.0-?C charge is placed on the x axis at x = +3.0 m, and a ?2.0-?C charge is located on the y axis at y = ?1.0 m. Point A is on the y axis at y = +4.0 m. Determine the electric potential at point A (relative to zero at the origin).
A. 6.0 kV B. 8.4 kV C. 9.6 kV D. 4.8 kV E. 3.6 kV