The circuit below contains three 100-watt light bulbs. The emf ? = 110 V. Which light bulb(s) is(are) the brightest?
A. A
B. B
C. C
D. B and C
E. All three are equally bright.
Answer: A
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Consider the electrical space heater, where the electric resistance heating coil is enclosed in a straight horizontal polished metallic cylinder that has an outer diameter of 1 cm and an emissivity of 0.9. If its surface temperature is at 110°C and the room air temperature is at 20°C, calculate the total heat loss per unit length of the cylinder due to both convection and radiation. For the purpose of this calculation, assume that the reflector is so designed that it “collects” all of the radiant heat from the back half of the horizontal heater and that it reflects 95% of the incident radiation.
GIVEN
• An electric resistance heating coil enclosed in horizontal metallic cylinder.
• Emissivity (?) = 0.9
• Diameter of cylinder (D) = 1 cm= 0.01 m
• Surface temperature (Ts) = 110°C= 383 K
• Ambient air temperature (T?) = 20°C= 293 K
• Reflects 95% of incident radiation.
FIND
Total heat loss per unit length of cylinder due to convection and radiation.
ASSUMPTIONS
• Air is still
• Surface temperature is uniform and constant
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 65°C
Thermal expansion coefficient (?) = 0.00296 1/K
Thermal conductivity (k) = 0.02825 W/(m K)
Kinematic viscosity (?) = 19.92 × 10–6 m2/s
Prandtl number (Pr) = 0.71
A sound wave radiates from a source in all directions. If the power of the sound source is 200 watts, then the intensity of the sound wave 100 m from the source is
A. 2.2 milliWatts/m2. B. 2.0 milliWatts/m2. C. 1.8 milliWatts/m2. D. 1.6 milliWatts/m2. E. 1.2 milliWatts/m2.
A(n) ______ layout presents multiple topics, each in its own square or rectangle.
A. card B. geometric C. grid D. gallery
The change in entropy of 20.0 moles of ideal monatomic constant volume gas as it changes from an initial temperature 300 K to its final temperature is 200 J/K. What is the final temperature of the gas?
A) 669 K B) 562 K C) 427 K D) 187 K E) 345 K