Thursday, 14 April 2016

thermodynamics - Humans have an average energy budget of $100$ Watts, but the power radiated from the body is $1000$ Watts?


On average a 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 Kelvin, the Stefan-Boltzmann law $$P = e \sigma A T^4$$ says the power radiated by a human with a surface area of $2 \, \text{m}^2$ and emissitivity $1$ is 1000 watts.



What's wrong here?




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