Assuming that no energy is lost in the form of heat, after the ball compresses the spring shown here, what is the maximum potential energy it may once again attain?
a. 0 J.
b. 5 J.
c. 2.5 J.
d. Significantly less than 5 J because of energy lost as heat because of friction.
e. Significantly less than 2.5 J because of energy lost as heat because of friction.
b
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Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except
A) a brown dwarf. B) a white dwarf. C) a neutron star. D) the core of a high-mass main-sequence star.
An electronic device is to be cooled in air at 20°C by an array of equally spaced vertical rectangular fins as shown in the sketch below. The fins are made of aluminum and their average temperature, Ts, is 100°C.
Estimate (a) The optimum spacing, s
(b) The number of fins
(c) The rate of heat transfer from one fin
(d) The total rate of heat dissipation
(e) Is the assumption of a uniform fin temperature justified?
GIVEN
? Electronic device with vertical aluminum fins in air
? Air temperature (T?) = 20°C
? Average fin temperature (Ts) = 100°C
FIND
(a) The optimum spacing (s)
(b) The number of fins
(c) The rate of heat transfer from one fin
(d) The total rate of heat dissipation
(e) Is the assumption of a uniform fin temperature justified?
ASSUMPTIONS
? Steady state
? Uniform fin temperature
? The air is still
? Heat transfer from the top and bottom of the fins is negligible
? The heat transfer coefficient on the wall area between the fins is approximately the same as on the fins
PROPERTIES AND CONSTANTS
From Appendix 2, Table 12
For aluminum: Thermal conductivity (kal) = 239 W/(m K) at 100°C
From Appendix 2, Table 28, for dry air at the mean temperature of 60°C
Refer to the accompanying narrative. What variable or variables were manipulated by the scientist?
a. DDT only b. temperature only c. weight differences only d. both DDT and weight differences e. both temperature and weight differences
Which of the following statements is true of an eclipsing binary system?
A) When one star moves in front of the other, it blocks some of the light. B) When one star moves in front of the other, the total brightness of the point of light increases. C) A light curve of an eclipsing binary system is a graph of temperature versus mass. D) When one star is in front of the other, the system looks like a dual point of light.