Magnetic Field of a Long Wire: The magnetic field due to the current in a long, straight wire is 8.0 ?T at a distance of 4.0 cm from the center of the wire. What is the current in the wire? (?0 = 4? × 10-7 T ? m/A)
A. 0.20 A
B. 0.40 A
C. 0.80 A
D. 3.2 A
E. 1.6 A
Answer: E
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Constant Acceleration Kinematics: A cart starts from rest and accelerates uniformly at 4.0 m/s2 for 5.0 s. It next maintains the velocity it has reached for 10 s. Then it slows down at a steady rate of 2.0 m/s2 for 4.0 s. What is the final speed of the car?
A. 20 m/s B. 16 m/s C. 12 m/s D. 10 m/s
A small space heater is constructed of 1.25 cm, 18-gauge brass tubes that are 60 cm. The tubes are arranged in equilateral, staggered triangles on 3.6 cm centers, four rows of 15 tubes each. A fan blows 0.95 m3/s of atmospheric pressure air at 21°C uniformly over the tubes (see sketch). Estimate: (a) heat transfer rate (b) exit temperature of the air (c) rate of steam condensation, assuming that saturated steam at 115 kPa(abs) inside the tubes as the heat source. State your assumptions. Work parts a, b, and c of this problem by two methods. First use the LMTD, which requires a trial-and-error or graphical solution then use the effectiveness methods.
GIVEN
A small heater made of 4 rows of 15 tubes each as shown above
Tubes: 1.25 cm, 18 gauge brass
Tube length (L) = 60 cm
Distance between tube centers (2 ST) = 3.6 cm
Air flow rate (V) = 0.95 m3/s
Air inlet temperature (Ta,in) = 21°C
Saturated steam inside the tubes at pressure (ps) = 115 kPa (abs)
FIND
Using both the LMTD method and e method find:
(a) The heat transfer rate (q)
(b) Air exit temperature (Ta,out)
(c) Rate of steam condensation mc
PROPERTIES AND CONSTANTS
From Appendix 2, Table 42, 18 gauge tubes have a wall thickness (t) = 0.049 in.=0.124 cm
From Appendix 2, Table 10, the thermal conductivity of brass (kb) = 111 W/(m K)
From Appendix 2, Table 13, the temperature of saturated steam at 115 kPa Ts = 103°C and the heat of evaporation (hfg) = 2249 J/kg.
From Appendix 2, Table 28, for air at an estimated mean temperature of 40°C
Specific heat (cpa) = 1014 J/(kg K)
At the steam temperature of 103°C, Prs = 0.71.
In ductile cast iron, the carbon is in the form of _________rather than as iron carbide.
Fill in the blank(s) with the appropriate word(s).
Refer to the race described above. According to the spectators watching in the stands, what happens?
A) The light beam wins the race, but barely—it is going 1 percent of the speed of light faster than Ben. B) The light beam instantly finishes the race, before Ben even has a chance to start. C) The light beam wins the race by a large margin because it is going faster than Ben by the full speed of light. D) Ben beats the light beam to the finish line, becoming a hero.