A certain factory whistle can be heard up to a distance of 2.5 km. Assuming that the acoustic output of the whistle is uniform in all directions, how much acoustic power is emitted by the whistle? The threshold of human hearing is 1.0 × 10-12 W/m2
A) 0.079 mW B) 0.26 mW C) 0.026 mW D) 2.6 mW E) 0.79 mW
A
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The moons of Uranus have been named after ____
a. Roman gods b. Greek goddesses c. characters of Shakespearean plays d. characters of Tennessee Williams' plays e. the Romanoff family children's names
A container is filled with a mixture of helium and oxygen gases. A thermometer in the container indicates that the temperature is 22.°C. Which gas molecules have the greater average kinetic energy?
A) It is the same for both because the temperatures are the same. B) The oxygen molecules do because they are more massive. C) The helium molecules do because they are monatomic. D) The oxygen molecules do because they are diatomic. E) The helium molecules do because they are less massive.
A uniform 100-lb beam is held in a vertical position by a pin at its lower end and a cable at its upper end. A horizontal force (magnitude P) acts as shown in the figure. If P = 75 lb, what is the tension in the cable?
a.
54 lb
b.
69 lb
c.
47 lb
d.
61 lb
e.
75 lb
A homeowner wants to replace an electric hot-water heater. There are two models in the store. The inexpensive model costs $280 and has no insulation between the inner and outer walls. Due to natural convection, the space between the inner and outer walls has an effective conductivity of 3 times that of air. The more expensive model costs $310 and has fiberglass insulation in the gap between the walls. Both models are 3.0 m tall and have a cylindrical shape with an inner wall diameter of 0.60 m and a 5 cm gap. The surrounding air is at 25°C, and the convective heat transfer coefficient on the outside is 15 W/(m2 K). The hot water inside the tank results in an inside wall temperature of 60°C. If energy costs 6 cents per kilowatt-hour, estimate how long it will take to pay back the extra
investment in the more expensive hot-water heater. State your assumptions.
GIVEN
FIND
- The time it will take to pay back the extra investment in the more expensive hot-water heater
ASSUMPTIONS
- Since the diameter is large compared to the wall thickness, one-dimensional heat transfer is
assumed
- To simplify the analysis, we will assume there is no water drawn from the heater, therefore the
inside wall is always at 60°C
- Steady state conditions prevail
SKETCH
PROPERTIES AND CONSTANTS
fiberglass (ki) = 0.035 W/(m K) at 20°C
dry air (ka) = 0.0279 W/(m K) at 60°C