Heat Radiated by a Person - Edit In this problem you will consider the balance o

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Heat Radiated by a Person - Edit

In this problem you will consider the balance of thermal energy radiated and absorbed by a person.

Assume that the person is wearing only a skimpy bathing suit of negligible area. As a rough approximation, the area of a human body may be considered to be that of the sides of a cylinder of lengthL=2.0m and circumference , which is  A=1.6m2

For the Stefan-Boltzmann constant use=5.67×108W/m2/K4.

Part A

If the surface temperature of the skin is taken to be Tbody=30C, how much thermal power Prb does the body described in the introduction radiate? Take the emissivity to be e=0.6.

Express the power radiated into the room by the body numerically, rounded to the nearest 10 W.

460

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Part B

The basal metabolism of a human adult is the total rate of energy production when a person is not performing significant physical activity. It has a value around 125 W, most of which is lost by heat conduction to the surrounding air and especially to the exhaled air that was warmed while inside the lungs. Given this energy production rate, it would seem impossible for a human body to radiate 460 W as you calculated in the previous part.

Which of the following alternatives seems to best explain this conundrum?

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The human body contains significant amounts of water and organic compounds. These typically have many absorption bands in the infrared part of the spectrum where room temperature objects radiate thermal energy. Consequently, human skin has a high emissivity in this part of the spectrum regardless of the emissivity of the skin in the visible part of the spectrum.

Part C

Now calculate Pbr, the thermal power absorbed by the person from the thermal radiation field in the room, which is assumed to be at Troom=20C. If you do not understand the role played by the emissivities of room and person, be sure to open the hint on that topic.

Express the thermal power numerically, giving your answer to the nearest 10 W.

409.52

Please solve for part C only.. for Pbr=??

Heat Radiated by a Person - Edit

In this problem you will consider the balance of thermal energy radiated and absorbed by a person.

Assume that the person is wearing only a skimpy bathing suit of negligible area. As a rough approximation, the area of a human body may be considered to be that of the sides of a cylinder of lengthL=2.0m and circumference , which is  A=1.6m2

For the Stefan-Boltzmann constant use=5.67×108W/m2/K4.

Part A

If the surface temperature of the skin is taken to be Tbody=30C, how much thermal power Prb does the body described in the introduction radiate? Take the emissivity to be e=0.6.

Express the power radiated into the room by the body numerically, rounded to the nearest 10 W.

Prb =

460

W

SubmitHintsMy AnswersGive UpReview Part

Correct

Part B

The basal metabolism of a human adult is the total rate of energy production when a person is not performing significant physical activity. It has a value around 125 W, most of which is lost by heat conduction to the surrounding air and especially to the exhaled air that was warmed while inside the lungs. Given this energy production rate, it would seem impossible for a human body to radiate 460 W as you calculated in the previous part.

Which of the following alternatives seems to best explain this conundrum?

The human body is quite reflective in the infrared part of the spectrum (where it radiates) so e is in fact less than 0.1. The surrounding room is near the temperature of the body and radiates nearly the same power into the body.

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Correct

The human body contains significant amounts of water and organic compounds. These typically have many absorption bands in the infrared part of the spectrum where room temperature objects radiate thermal energy. Consequently, human skin has a high emissivity in this part of the spectrum regardless of the emissivity of the skin in the visible part of the spectrum.

Part C

Now calculate Pbr, the thermal power absorbed by the person from the thermal radiation field in the room, which is assumed to be at Troom=20C. If you do not understand the role played by the emissivities of room and person, be sure to open the hint on that topic.

Express the thermal power numerically, giving your answer to the nearest 10 W.

Pbr =

409.52

W

Please solve for part C only.. for Pbr=??

Explanation / Answer

The heat absorbed will be [(5.67*10-8)*(eAT4)] i.e. [(5.67*10-8)*{0.6*1.6*(293.15)4}] which equals to 402 Watts approximately. Rounding off the answer in 10 Watts it will become 400 Watts which will be the required answer.

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