A 4.0-?F capacitor is charged to 6.0-V. It is then connected in series with a 3.0-M? resistor and connected to a 12-V battery. How long after being connected to the battery will the voltage across the capacitor be 9.0 V?
A)
5.5 s
B)
7.2 s
C)
8.3 s
D)
12 s
E)
17 s
C
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Discuss advantages and disadvantages of two methods for solving one-dimensional steady conduction problems.
What will be an ideal response?
A spaceship of mass 106 kg is to be accelerated to 0.75c. How much energy does this require?
a. 13.61 × 10^23 J b. 5.95 × 10^23 J c. 5.06 × 10^22 J d. 4.61 × 10^22 J e. 27.00 × 10^23 J
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
Light of wavelength 6.9 × 10^?7 m has an energy of: (h = 6.63 × 10^?34 J?s, c = 3.00 × 10^8 m/s)
a. 2.9E–19 J b. 9.6E–20 J c. 3.1E–19 J d. 4.3E–20 J e. 3.2E–20 J