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Human power calculated from food instake is 100 Watts but Stefan Boltzmann power radiated from body temp is 1000 Watts? What am I doing wrong?
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On average, human consumes around 2000 kilocalories per day. This converts to roughly 2000000 calories / 86400 seconds or around 100 joules / second giving roughly 100 Watts.
But if you use human's body temperature of 310 kelvins degrees, Stefan Boltzmann gives Power radiated by a human, with surface area of roughly 2m^2, emissivity ~ 1, to be 1000 Watts.
$$P = e sigma A T^4$$
What am I doing wrong?
thermodynamics biophysics
edited 9 hours ago
DanielSank
16.5k44978
asked 11 hours ago
Azerty
83
add a comment |Â
up vote
1
down vote
favorite
On average, human consumes around 2000 kilocalories per day. This converts to roughly 2000000 calories / 86400 seconds or around 100 joules / second giving roughly 100 Watts.
But if you use human's body temperature of 310 kelvins degrees, Stefan Boltzmann gives Power radiated by a human, with surface area of roughly 2m^2, emissivity ~ 1, to be 1000 Watts.
$$P = e sigma A T^4$$
What am I doing wrong?
thermodynamics biophysics
edited 9 hours ago
DanielSank
16.5k44978
asked 11 hours ago
Azerty
83
It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
3
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
On average, human consumes around 2000 kilocalories per day. This converts to roughly 2000000 calories / 86400 seconds or around 100 joules / second giving roughly 100 Watts.
But if you use human's body temperature of 310 kelvins degrees, Stefan Boltzmann gives Power radiated by a human, with surface area of roughly 2m^2, emissivity ~ 1, to be 1000 Watts.
$$P = e sigma A T^4$$
What am I doing wrong?
thermodynamics biophysics
edited 9 hours ago
DanielSank
16.5k44978
asked 11 hours ago
Azerty
83
On average, human consumes around 2000 kilocalories per day. This converts to roughly 2000000 calories / 86400 seconds or around 100 joules / second giving roughly 100 Watts.
But if you use human's body temperature of 310 kelvins degrees, Stefan Boltzmann gives Power radiated by a human, with surface area of roughly 2m^2, emissivity ~ 1, to be 1000 Watts.
$$P = e sigma A T^4$$
What am I doing wrong?
thermodynamics biophysics
thermodynamics biophysics
edited 9 hours ago
DanielSank
16.5k44978
asked 11 hours ago
Azerty
83
edited 9 hours ago
DanielSank
16.5k44978
asked 11 hours ago
Azerty
83
edited 9 hours ago
DanielSank
16.5k44978
edited 9 hours ago
DanielSank
16.5k44978
edited 9 hours ago
DanielSank
16.5k44978
16.5k44978
asked 11 hours ago
Azerty
83
asked 11 hours ago
Azerty
83
asked 11 hours ago
Azerty
83
83
It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
3
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago
add a comment |Â
It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
3
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago
It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
3
3
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago
add a comment |Â
1 Answer
1
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oldest
votes
up vote
10
down vote
You are forgetting that you also absorb radiation from the environment. The formula you want is
$$P_net = epsilon Asigmaleft(T_skin^4 - T_env^4right)$$
You can find more info on Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
answered 10 hours ago
HiddenBabel
50619
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
10
down vote
You are forgetting that you also absorb radiation from the environment. The formula you want is
$$P_net = epsilon Asigmaleft(T_skin^4 - T_env^4right)$$
You can find more info on Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
answered 10 hours ago
HiddenBabel
50619
add a comment |Â
up vote
10
down vote
You are forgetting that you also absorb radiation from the environment. The formula you want is
$$P_net = epsilon Asigmaleft(T_skin^4 - T_env^4right)$$
You can find more info on Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
answered 10 hours ago
HiddenBabel
50619
add a comment |Â
up vote
10
down vote
up vote
10
down vote
You are forgetting that you also absorb radiation from the environment. The formula you want is
$$P_net = epsilon Asigmaleft(T_skin^4 - T_env^4right)$$
You can find more info on Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
answered 10 hours ago
HiddenBabel
50619
You are forgetting that you also absorb radiation from the environment. The formula you want is
$$P_net = epsilon Asigmaleft(T_skin^4 - T_env^4right)$$
You can find more info on Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bodrad.html
answered 10 hours ago
HiddenBabel
50619
answered 10 hours ago
HiddenBabel
50619
answered 10 hours ago
HiddenBabel
50619
answered 10 hours ago
HiddenBabel
50619
50619
add a comment |Â
add a comment |Â
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It is likely in your conversion to joules, you need to multiply by 4.18 in the conversion of calories to joules
– Triatticus
10 hours ago
3
This is exactly why I wear a sweater when it is cold outside.
– safesphere
10 hours ago
This means that you should be able to keep warm even when naked in extremely cold temperatures by working out hard. If you do 250 Watt on a bike, you'll burn energy at a rate of about 1000 Watt.
– Count Iblis
10 hours ago
@Triatticus. I think he did that. Multiply 2000000 by 4.18 and divide by 86400 you get 97 watts. No?
– Bob D
10 hours ago
Notes: the unit kelvin has no degree associated; and it's watt not Watt.
– Massimo Ortolano
7 hours ago