A policeman in a stationary car measures the speed of approaching cars by means of an ultrasonic device that emits a sound with a frequency of 41.2 kHz
A car is approaching him at a speed of 33.0 m/s. The wave is reflected by the car and interferes with the emitted sound producing beats. What is the frequency of the beats? The speed of sound in air is 330 m/s.
A)
9.2 kHz
B)
4.1 kHz
C)
4.6 kHz
D)
1.2 kHz
E)
8.2 kHz
A
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The corona can reach temperatures of up to ____
a. 500,000 K b. 1,000,000 K c. 10,000 K d. 5800 K e. 2,000,000 K
If you could convert the entire mass of the Sun (mass 2 x 1033g) into pure energy, how much energy would be produced (in ergs) ? The speed of light c = 3 x 1010cm/s.
A. 2 x 1033ergs B. 1.8 x 1054ergs C. 6 x 10133ergs D. 1 x 1051ergs E. 6 x 1043ergs
Mercury at an inlet bulk temperature of 90°C flows through a 1.2-cm-ID tube at a flow rate of 4535 kg/h. This tube is part of a nuclear reactor in which heat can be generated uniformly at any desired rate by adjusting the neutron flux level. Determine the length of tube required to raise the bulk temperature of the mercury to 230°C without generating any mercury vapor, and determine the corresponding heat flux. The boiling point of mercury is 355°C.
GIVEN
• Mercury flow in a tube
• Inlet bulk temperature (Tb,in) = 90°C
• Inside tube diameter (D) = 1.2 cm = 0.012 m
• Flow rate (m ) = 4535 kg/h = 1.26 kg/s
• Outlet bulk temperature (Tb,out) = 230°C
• Boiling point of mercury = 355°C
FIND
(a) The length of tube (L) required to obtain Tb,out without generating mercury vapor
(b) The corresponding heat flux (q/A)
ASSUMPTIONS
• Steady state
• Fully developed flow
SKETCH
PROPERTIES AND CONSTANTS
for mercury at the average bulk temperature of 160°C
Density (?) = 13,240 kg/m3
Thermal conductivity (k) 11.66 W/(m K)
Absolute viscosity (?) = 11.16 × 10–4 (Ns)/m2
Prandtl number (Pr) = 0.0130
Specific heat (c) = 140.6 J/(kg K)
At what rate is thermal energy generated in the 20-? resistor when ? = 20 V?
a.
6.5 W
b.
1.6 W
c.
15 W
d.
26 W
e.
5.7 W