If someone tells you they have a sample of niobium-90, can you tell how many neutrons are in the nucleus?
a. Yes, there are 41 neutrons.
b. Yes, there are 90 neutrons.
c. Yes, there are 49 neutrons.
d. No, more information must be given.
e. No, there is no such thing as "niobium.".
c
RATIONALE: You do need to look up the atomic number of niobium in the periodic table.
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LR Circuits: A 45-mH inductor is connected in series with a 60-? resistor through an ideal 15-V dc power supply and an open switch. What is the current 7.0 ms after closing the switch?
A. 250 mA B. 650 mA C. 550 mA D. 280 mA E. 850 mA
Which of these is a fictitious force?
a. inertial force b. gravitational force c. electric force d. magnetic force e. centripetal force
The density of a ___________ is greater than the density of a ___________
a. white dwarf, neutron star b. neutron star, black hole c. pulsar, neutron star d. pulsar, white dwarf e. white dwarf, black hole
A shell-and-tube heat exchanger is to be used to cool 25.2 kg/s of water from 38°C to 32°C. The exchanger has one shell-side pass and two tube side passes. The hot water flows through the tubes and the cooling water flows through the shell. The cooling water enters at 24°C and leaves at 32°C. The shell-side (outside) heat transfer coefficient is estimated to be 5678W/(m 2 K). Design specifications require that the pressure drop through the tubes be as close to 13.8 kPa as possible and that the tubes be 18 BWG copper tubing (1.24 mm wall thickness), and each pass is 4.9 m long. Assume that the pressure losses at the inlet and outlet are equal to one and one half of a velocity heat (?V2/gc), respectively. For these specifications, what tube diameter and how many tubes are needed?
GIVEN
A water-to-water shell-and-tube exchanger, hot water in tubes, cooling water in shell One shell and two tube passes Hot water flow rate m
h= 25.2 kg/s Water temperatures
? Hot: Th,in = 38°C Th,out = 32°C
? Cold: Tc,in = 24°C Tc,out = 32°C Shell-side transfer coefficient h o= 5678 W/(m2 K) Pressure drop (?p) = 13.8 kPa Tube wall thickness (t) = 1.24 mm = 0.00124 m Tube length per pass (Lp) = 4.9 m
FIND
The tube diameter (Do) and number of tubes (N)
ASSUMPTIONS
Pressure losses at inlet and outlet (?pii) = 1.5 (? V2/gc) Variation of thermal properties with temperature is negligible Fouling resistance is negligible Thermal resistance of the tube walls is negligible
SKETCH
PROPERTIES AND CONSTANTS
for water at 30°C Density (?) = 995.7 kg/m3
Specific heat (cp) = 4176 J/(kg K)
Thermal conductivity (k) = 0.615 W/(m K)
Kinematic viscosity (?) = 0.805 × 10–6 m2/s
Prandtl number (Pr) = 5.4