A metal rod is falling toward the surface of the Earth near the equator. As it falls, one end of the rod becomes positively charged due to the motional emf of the rod through the Earth's magnetic field. The rod is oriented so that:
a. the rod is horizontal with the positive end toward the east.
b. the rod is horizontal with the positive end toward the north.
c. the rod is vertical with the positive end higher.
d. the rod is horizontal with the positive end toward the west.
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
In science, the tendency of an experiment to produce a particular result is referred to as
A) a deviation B) bias C) an error D) an uncertainty
How many galaxies are there in the observable universe?
A) about as many as the number of stars we see in the sky with our naked eyes B) infinity C) roughly a thousand times more than the number of stars in our galaxy D) roughly (within a factor of 10 ) the same as the number of stars in our galaxy E) about as many as the number of grains of sand on all the beaches on Earth
How do we estimate the surface temperature of a star?
A) by measuring the Doppler shifts of absorption lines in its chromosphere B) by obtaining its spectrum and classifying the spectral type C) by estimating its parallax D) by estimating its luminosity