When balancing a chemical equation, you must
a) be able to count individual atoms on each side of the reaction arrows.
b) count only the polyatomic ions present in the reactants and the products.
c) count only complete molecules and ignore any atoms that are in their elemental (uncombined) forms.
d) make sure that there are always more atoms of reactant present than atoms of product, so that the reaction can proceed easily.
a
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In general, the time it takes from when in interstellar cloud fragment first begins collapsing until it gives birth to a main-sequence star is
A) shorter for less massive stars. B) longer for less massive stars. C) about the same for all stars. D) dependent on the rotation rate of the star.
A circumferential fin of rectangular cross section, 3.7-cm-OD and 0.3-cm-thick surrounds a 2.5- cm-diameter tube as shown below. The fin is constructed of mild steel. Air blowing over the fin produces a heat transfer coefficient of 28.4 W/(m2 K). If the temperatures of the base of the fin and the air are 260°C and 38°C, respectively, calculate the heat transfer rate from the fin.
GIVEN
FIND
ASSUMPTIONS
The system has reached steady state
The mild steel is 1% carbon steel
The thermal conductivity of the steel is uniform
Radial conduction only (temperature is uniform across the cross section of the fin)
The heat transfer from the end of the fin can be accounted for by increasing the length by half the
thickness and assuming the end is insulated
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity of 1% carbon steel (k) = 43 W/(m K) at 20°C
Two point charges, Q1 and Q2, are separated by a distance R. If the magnitudes of both charges are halved and their separation is also halved, what happens to the electrical force that each charge exerts on the other one?
A) It increases by a factor of 16. B) It increases by a factor of 8. C) It increases by a factor of 2. D) It increases by a factor of 4. E) It remains the same.
Significant Figures: In the sum A + B + C, A is accurate to 5 decimal places, B is accurate to 2 decimal places, and C is accurate to 3 decimal places. What is the correct number of decimal places in the sum?
A. 2 B. 3 C. 4 D. 5 E. 10