Radioactive Decay: The half-life of radon-222 is 3.83 d. If a sample of radon initially contains 6.00 × 108 radon atoms, how many of them are left after 10.0 d?
A. 9.82 × 107
B. 7.67 × 107
C. 8.34 × 108
D. 7.29 × 108
E. 8.56 × 108
Answer: A
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A 0.15-m-square circuit board is to be cooled in a vertical position, as shown in the sketch. The board is insulated on one side while on the other, 100 closely spaced square chips are mounted, each of which dissipated 0.06 W of heat. The board is exposed to air at 25°C and the maximum allowable chip temperature is 60°C. Investigate the following cooling options
(a) Natural convection
(b) Air cooling with upward flow at a velocity of 0.5 m/s
(c) Air cooling with downward flow at the velocity of 0.5 m/s
GIVEN
? Square vertical circuit board insulated on one side, chips on the other side
? Length of each side (L) = 0.15 m
? Heat dissipation per chip (q) = 0.06 W
? Number of chips (N) = 100
? Ambient air temperature (T?) = 25°C
? Maximum allowable chip temperature (Ts) = 60°C
FIND
Investigate the following cooling options
(a) Natural convection
(b) Forced air cooling with an upward air velocity (U?) = 0.5 m/s
(c) Forced air cooling with a downward air velocity (U?) = 0.5 m/s
ASSUMPTIONS
? Steady state
? Uniform surface temperature
? Radiative heat transfer is negligible
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 2, Table 28, for dry air at the mean temperature of 42.5°C
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