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FUNDAMENTALS OF THERMAL RADIATION TEST 1
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Question 1 of 15
1. Question
1 pointsConsider a 20cmdiameter spherical ball at 800 K suspended in air as shown in Figure . Assuming the ball closely approximates a blackbody, determine the total blackbody emissive power (ANSWER UPTO ONE DECIMAL)
Assumptions The ball behaves as a blackbody
Correct
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined.
Analysis : The total blackbody emissive power is determined from the Stefan–Boltzmann law to be
Eb=σT^4=(5.67 x 10^8 W/m²· K^4 )(800 K)^4=23.2 x 10³ W/m²=23.2 kW/m²
That is, the ball emits 23.2 kJ of energy in the form of electromagnetic radiation per second per m2 of the surface area of the ball.
Incorrect
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined.
Analysis : The total blackbody emissive power is determined from the Stefan–Boltzmann law to be
Eb=σT^4=(5.67 x 10^8 W/m²· K^4 )(800 K)^4=23.2 x 10³ W/m²=23.2 kW/m²
That is, the ball emits 23.2 kJ of energy in the form of electromagnetic radiation per second per m2 of the surface area of the ball.

Question 2 of 15
2. Question
1 pointsConsider a 20cmdiameter spherical ball at 800 K suspended in air as shown in Figure . Assuming the ball closely approximates a blackbody, determine the total amount of radiation emitted by the ball in 5 min.
Assumptions The ball behaves as a blackbody
Correct
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined
ANALYSIS:The total amount of radiation energy emitted from the entire ball in 5 min is determined by multiplying the blackbody emissive power obtained above by the total surface area of the ball and the given time interval:
As=πD²=π(0.2m)²=0.1257 m²
Δt=(5 min)(60S/1min)=300s
Qrad=EbAsΔt=(23.2 kW/m²)(0.1257 m² )(300 s)(1/KJ/100 W.S=876 KJ
That is, the ball loses 876 kJ of its internal energy in the form of electromagnetic waves to the surroundings in 5 min, which is enough energy to raise the temperature of 1 kg of water by 50°C. Note that the surface temperature of the ball cannot remain constant at 800 K unless there is an equal amount of energy flow to the surface from the surroundings or from the interior regions of the ball through some mechanisms such as chemical or nuclear reactions
Incorrect
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined
ANALYSIS:The total amount of radiation energy emitted from the entire ball in 5 min is determined by multiplying the blackbody emissive power obtained above by the total surface area of the ball and the given time interval:
As=πD²=π(0.2m)²=0.1257 m²
Δt=(5 min)(60S/1min)=300s
Qrad=EbAsΔt=(23.2 kW/m²)(0.1257 m² )(300 s)(1/KJ/100 W.S=876 KJ
That is, the ball loses 876 kJ of its internal energy in the form of electromagnetic waves to the surroundings in 5 min, which is enough energy to raise the temperature of 1 kg of water by 50°C. Note that the surface temperature of the ball cannot remain constant at 800 K unless there is an equal amount of energy flow to the surface from the surroundings or from the interior regions of the ball through some mechanisms such as chemical or nuclear reactions

Question 3 of 15
3. Question
1 pointsConsider a 20cmdiameter spherical ball at 800 K suspended in air as shown in Figure . Assuming the ball closely approximates a blackbody, determine the spectral blackbody emissive power at a wavelength of 3 μm.
Assumptions The ball behaves as a blackbody.
Correct
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined.
ANALYSIS: The spectral blackbody emissive power at a wavelength of 3 μm is determined from Planck’s distribution law to be
Incorrect
SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 minutes, and the spectral blackbody emissive power at 3 mm are to be determined.
ANALYSIS: The spectral blackbody emissive power at a wavelength of 3 μm is determined from Planck’s distribution law to be

Question 4 of 15
4. Question
1 pointsA radio station is broadcasting radio waves at a wavelength of 200 m. Determine the frequency of these waves
Correct
1.5 x 10^6 Hz
Incorrect
1.5 x 10^6 Hz

Question 5 of 15
5. Question
1 pointsA 3mmthick glass window transmits 90 percent of the radiation between λ= 0.3 and 3.0 μm and is essentially opaque for radiation at other wavelengths. Determine the rate of radiation transmitted through a 2m x 2m glass window from blackbody sources at 5800 K
Correct
218,400 kW
Incorrect
218,400 kW

Question 6 of 15
6. Question
1 pointsA 3mmthick glass window transmits 90 percent of the radiation between λ= 0.3 and 3.0 μm and is essentially opaque for radiation at other wavelengths. Determine the rate of radiation transmitted through a 2m x 2m glass window from blackbody sources at 1000 K (answer in one decimal)
Correct
55.8 kW
Incorrect
55.8 kW

Question 7 of 15
7. Question
1 pointsThe emissivity of a surface coated with aluminum oxide can be approximated to be 0.2 for radiation at wavelengths less than 5 μm and 0.9 for radiation at wavelengths greater than 5 μm. Determine the average emissivity of this surface at 5800 K(answer upto three decimal)
Correct
0.203
Incorrect
0.203

Question 8 of 15
8. Question
1 pointsThe emissivity of a surface coated with aluminum oxide can be approximated to be 0.2 for radiation at wavelengths less than 5 μm and 0.9 for radiation at wavelengths greater than 5 μm. Determine the average emissivity of this surface at 300 K (answer upto two decimal)
Correct
0.89
Incorrect
0.89

Question 9 of 15
9. Question
1 pointsThe variation of the spectral transmissivity of a 0.6cmthick glass window is as given in Figure . Determine the average transmissivity of this window for solar radiation (T ≈ 5800 K) ( answer upto three decimal)
Correct
0.848
Incorrect
0.848

Question 10 of 15
10. Question
1 pointsThe variation of the spectral transmissivity of a 0.6cmthick glass window is as given in Figure . Determine the radiation coming from surfaces at room temperature (T ≈300 K) ( answer upto five decimal)
Correct
0.00015
Incorrect
0.00015

Question 11 of 15
11. Question
1 pointsThe variation of the spectral transmissivity of a 0.6cmthick glass window is as given in Figure determine the amount of solar radiation transmitted through the window for incident solar radiation of 650 W/m²(answer upto one decimal)
Correct
551.1 W/m²
Incorrect
551.1 W/m²

Question 12 of 15
12. Question
1 pointsSolar radiation is incident on the outer surface of a spaceship at a rate of 400 Btu/h · ft² . The surface has an absorptivity of ∝s =0.10 for solar radiation and an emissivity of ε=0.8 at room temperature. The outer surface radiates heat into space at 0 R. If there is no net heat transfer into the spaceship, determine the equilibrium temperature of the surface(answer upto one decimal)
Correct
413.3 R
Incorrect
413.3 R

Question 13 of 15
13. Question
1 pointsA manufacturing facility located at 32° N latitude has a glazing area of 60 m² facing west that consists of doublepane windows made of clear glass (SHGC =0.766). To reduce the solar heat gain in summer, a reflective film that will reduce the SHGC to 0.35 is considered. The cooling season consists of June, July, August, and September, and the heating season, October through April. The average daily solar heat fluxes incident on the west side at this latitude are 2.35, 3.03, 3.62, 4.00, 4.20, 4.24, 4.16, 3.93, 3.48, 2.94, 2.33, and 2.07 kWh/day · m² for January through December, respectively. Also, the unit costs of electricity and natural gas are $0.09/kWh and $0.45/therm, respectively. If the coefficient of performance of the cooling system is 3.2 and the efficiency of the furnace is 0.90, determine the net annual cost savings due to installing reflective coating on the windows.
Correct
$53
Incorrect
$53

Question 14 of 15
14. Question
1 pointsA manufacturing facility located at 32° N latitude has a glazing area of 60 m² facing west that consists of doublepane windows made of clear glass (SHGC =0.766). To reduce the solar heat gain in summer, a reflective film that will reduce the SHGC to 0.35 is considered. The cooling season consists of June, July, August, and September, and the heating season, October through April. The average daily solar heat fluxes incident on the west side at this latitude are 2.35, 3.03, 3.62, 4.00, 4.20, 4.24, 4.16, 3.93, 3.48, 2.94, 2.33, and 2.07 kWh/day · m2 for January through December, respectively. Also, the unit costs of electricity and natural gas are $0.09/kWh and $0.45/therm, respectively determine the simple payback period if the installation cost of reflective film is $20/m²
.Correct
23 years
Incorrect
23 years

Question 15 of 15
15. Question
1 pointsDetermine the rate of net heat gain (or loss) through a 9fthigh, 15ftwide, fixed 1/8 in. singleglass window with aluminum frames on the west wall at 3 PM solar time during a typical day in January at a location near 40° N latitude when the indoor and outdoor temperatures are 70°F and 45°F, respectively
Correct
16,840 Btu/h gain
Incorrect
16,840 Btu/h gain