The energy stored in a compressed spring
A. Is larger than if the spring is stretched the same distance
B. Is not in this chapter
C. Is constant
D. Is proportional to the distance compressed
Ans: B. Is not in this chapter
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The electronic controls of a medical imaging device are housed in a compartment such that it dissipates heat to the ambient from a flat vertical plate surface, 15 cm wide and 20 cm high, to which fins are attached so as to enhance the heat transfer and improve the cooling of the electric systems. If 10 fins made of aluminum with a rectangular cross section are attached that extend over the full vertical height of the plate surface, where each fin is 0.15 cm thick with equal spacing of 1.5 cm along the width of the plate, determine the optimum fin height and the rate of heat transfer to the surrounding air. The room air temperature is at 20°C and the base temperature of the plate surface to which the fins are attached should not exceed 70°C.
GIVEN
? An electronic device with vertical aluminum fins in air
? Air temperature (T?) = 20°C
? Vertical Height of fin (H) = 0.2 m
? Width of surface (w) = 0.15 m
? Fin thickness(t) = 0.0015 m
? Maximum base temperatures: Tb= 70°C
? Fin spacing (s)= 1.5 cm=0.015 m
FIND
(a) Optimum fin height perpendicular to surface.
(b) Rate of heat transfer to surrounding air.
ASSUMPTIONS
? The fins are at a uniform temperature equal to the base temperature
PROPERTIES AND CONSTANTS
From Appendix 2, Table 28, for dry air at the mean temperatures for each of the base temperatures
From Appendix 2, Table 12, the thermal conductivity of aluminum in the range of around 70°C
(ka) = 240 (W/(m K))
When a 100-kg astronaut ejects 0.1 kg of gas at a speed of 50 m/s from her propulsion pistol, her recoil speed is
A) 50 m/s. B) 5.0 m/s. C) 0.5 m/s. D) 0.05 m/s.
A certain ac signal at 1000 Hz is applied across an inductor and a 100-? resistor. If the power factor of the circuit is 0.400, what is the impedance of this circuit?
A) 150 ? B) 200 ? C) 250 ? D) 100 ? E) 300 ?
Which statement regarding the mix of state revenues is FALSE?
a. The 50 states combined had over $1.6 trillion annual revenue in 2011.
b. Over 70 percent of all stategenerated (or "own source") revenues come from taxation.
c. Most revenue from direct charges for public services is in the form of tuition for higher education.
d. With few exceptions, states generate nearly all of their tax revenues from the general sales tax.