A solar cell has a light-gathering area of 10 cm2 and produces 0.2 A at 0.8 V (DC) when illuminated with S = 1000 W/m2 sunlight. What is the efficiency of the solar cell?
A. 16%
B. 7%
C. 23%
D. 4%
E. 32%
Answer: A
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A heater is rated at 1200 watts for a 120-volt source.The resistance in ohms of the heater is
a. 0.1 b. 10. c. 144,000 d. none of the above.
The heat transfer coefficient on the inside of a copper tube (1.9-cm-ID and 2.3-cm-OD) is 500 W/(m2 K) and 120 W/(m2 K) on the outside, but a deposit with a fouling factor of 0.009 (m2 K)/W (based on the tube outside diameter) has built up over time. Estimate the percent increase in the overall heat transfer coefficient if the deposit were removed.
GIVEN
• Heat transfer through a copper tube
• Heat transfer coefficients
? Inside h i= 500 W/(m2 K)
? Outside h o= 120 W/(m2 K)
• Tube diameters
? Inside (Di) = 1.9 cm = 0.019 m
? Outside (Do) = 2.3 cm = 0.023 m
• Fouling factor (Rd) = 0.009 (m2 K)/W
FIND
• Per cent increase in the overall heat transfer coefficient if the deposit were removed ASSUMPTIONS
• Constant thermal conductivity properties
• The copper is pure
SKETCH
PROPERTIES AND CONSTANTS
the thermal conductivity of copper (k) = 392 W/(m K) at 127°C
What is the magnitude of the horizontal displacement of the rock?
a. 3.53 m b. 13.3 m c. 222 m d. 26.1 m
Why is a complete image formed even if half of a lens or mirror is blocked?
What will be an ideal response?