Air at 40ºC passes through an 80 cm by 60 cm rectangular duct 80 m long at 1 m/s. Estimate (a) the outlet temperature of the air if the duct wall temperature is 15ºC and (b) the hydraulic and thermal entrance lengths.
What will be an ideal response?
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A refrigerator
A) produces cold. B) causes internal energy to disappear. C) transfers internal energy from inside to outside. D) transfers heat into cold. E) none of the above
When a 20.8-? resistor is connected across a 15.6-V battery, a current of 500 mA flows. What is the internal resistance of the battery?
a. 20.8 ? b. 10.4 ? c. 31.2 ? d. 0.732 ? e. 1.25 ?
The excited state of a certain atom is 3.2 eV ± 0.21 eV. (h = 6.626 × 10-34 J ? s, 1eV = 1.60 × 10-19 J)
(a) What is the average lifetime of this state, in femtoseconds? (b) If the excited energy were doubled (to 6.4 eV ± 0.21 eV), how would the lifetime be affected?
At a certain instant in time, the electric field of an electromagnetic wave in free space points in the -z direction, and the magnetic field points in the +y direction. In what direction is this wave traveling?
A) +x direction B) -x direction C) +y direction D) -z direction E) +z direction