I need help with this homework assignment

[Jenn_Lane2001]

The average human body has a surface tempreture of 30 degrees Celsius, a skin surface of 1.20m and an emissivity of 1.0.

A. If the surrrounding tempreture is 20 degress Celsuis, the body's net rate of heat loss due to radiation is ________ Watts.

B. How many food calories must be consumed per day to maintain that body tempreture if only radiative heat lossess are considered?________.

Thanks for the help.

[Tom Mattson]

Hi Jenn, and welcome to PF.

We are happy to help you with your homework, but we need to see that you have attempted it first. Show us how you started and where you got stuck, and we will assist you through the rough spots. I think you will find that this problem is a straightforward "plug-and-chug" when you locate the formula in your book.

Thanks,

[Jenn_Lane2001]

Well I had a problem finding a forumla for this but I came up with .. P = sigma*A*e[T^4-T0^4]..I was give the tempretures and Area and emissivity. I am not sure if I was able to just plug it in or if im using the right equation. It might be simple but since I wasnt given a book to solve this I am a little stuck..This was extra credit,to see if we can solve with research. Thank you for the help in advance.

[Jenn_Lane2001]

Segement of 12m-long steel rails are laid end to end when the tempreture is -12 degrees Celsius. A space is left between rails segments to allow for thermal expansion.
A.If they are to just touch when the tempreture is 33 degrees Celsisu the spacing is..____________.
My answer is 5.94*10^-3

I used deltaL = [(11*10^-6)(12)][45]=.00594

Thanks for the help everyone.

[Tom Mattson]

Originally posted by Jenn_Lane2001
[B]Well I had a problem finding a forumla for this but I came up with .. P = sigma*A*e[T^4-T0^4]..I was give the tempretures and Area and emissivity. I am not sure if I was able to just plug it in or if im using the right equation.


Well, if the symbols take on their usual meaning (check this!), then...

P=power
sigma=Stefan-Bolzmann constant
A=area
e=emissivity
T,T0=temps

then you have the right equation.

[Tom Mattson]

Originally posted by Jenn_Lane2001
I used deltaL = [(11*10^-6)(12)][45]=.00594



Since I do not know where all those numbers came from, I cannot say.