Dark matter is detectable in the coronae of ____
a. elliptical galaxies only
b. spiral galaxies only
c. irregular galaxies only
d. spiral and elliptical galaxies only
e. all galaxies
e
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Except for the Local Group, galaxies seem to be isolated and randomly in space
Indicate whether the statement is true or false
Which represents v vs. t for a ball which is thrown straight up, reaches the top, and falls back down (but before it hits the ground)?
A. 
B. 
C. 
D. 
A 60.0-Hz ac generator with a peak voltage of 110 V drives a series RC circuit with R = 10.0 ? and C = 300 ?F. The power factor, cos ?, is
a. ?1.00. b. ?0.749. c. +0.749. d. +0.834. e. +1.00.
One method of storing solar energy for use during cloudy days, or at night, is to store it in the form of sensible heat in a rock bed. Suppose such a rock bed has been heated to 70°C and it is desired to heat a stream of air by blowing it through the bed. If the air inlet temperature is 10°C and the mass velocity of the air in the bed is 0.5 kg/(s m2), how long must the bed be in order for the initial outlet air temperature to be 65°C? Assume that the rocks are spherical, 2 cm in diameter, and that the bed void fraction is 0.5. (Hint: The surface area of the rocks per unit volume of the bed is (6/Dp)(1 –(?).)
GIVEN
Packed bed of rocks with air blowing through it
Initial temperature of rocks (Tr) = 70°C
Inlet air temperature (Ta,in) = 10°C
Mass velocity of air (m/A) = 0.5 kg/(s m2)
Outlet air temperature (Ta,out) = 65°C
Surface area per unit volume (As/V) = (6/Dp)(1 –(?) )
FIND
Length of bed required (L)
ASSUMPTIONS
Rocks are spherical with diameter (D) = 2 cm = 0.02 m
Void fraction of the bed (?) = 0.5
Rock temperature remains practically constant
SKETCH
