Posted by noah on Wednesday, February 22, 2012 at 1:27am.
First you get to know the surface area of a sphere, that is A=4*pi*r^2
All the info listed down:
Emissivity,e=1 (blackbody has a good emissivity)
radius,r=0.30m
Temperature, T2=77K T1=4.2K
Latent heat of vaporization,L=2.1*10^4 J/kg
Time,t=1hour=3600sec
First, use the Steven-Boltzman Rule,
Power,P=Q/t=e*sigma*A*(T2^4-T1^4)
Thus, we get
P=1*(5.67x10^-8)*4*pi*(0.30)^2*(77^4-4.2^4)
P=2.254 J/s
Then as we found Power,and time=3600sec
Substitute into the formula, Q=Pt
Thus, we get
Q=2.254*3600 =8115.16Joule
Refer to the question the Latent heat is given, we calculate for the mass with the formula, Q=mL
m=Q/L =8115.16/(2.1*10^4) =0.386kg
hope i do get it right. :)
oo
thx dude