• Support PF! Buy your school textbooks, materials and every day products Here!

Thermal radiation problem

  • Thread starter Gaith
  • Start date
  • #1
5
0

Homework Statement


Energy can be transmitted via radiation, the rate at which this happens is H=σAT4. A bar of iron is 0.5m long, 0.2m wide and 0.2m high. Iron has a density of 7900 kg/m3, a heat capacity of 400J/kgC, and a coefficent of linear expansion of 1.2x10-5

What is the rate at which iron radiates energy assuming it is initially at 500K and that the emissivity is 0.8?

Homework Equations


H=σAT4

The Attempt at a Solution


I started by trying to figure out the final Temperature so I could plug it in to the formula above and get the H. However Im stuck at which equation to use in order to find the final temperature.

Im not sure if I'm going about it the right way either. Help would be greatly appreciated. Thank you
 

Answers and Replies

  • #2
Simon Bridge
Science Advisor
Homework Helper
17,847
1,644
You don't need the final temperature, the formula does not call for it.
 
  • #3
5
0
You don't need the final temperature, the formula does not call for it.
Ok so you're telling me I can just plug in the numbers and get the answer like this

H=(5.67x10^-8)(0.8)(0.2x0.5)(500)^4

The answer is still incorrect...
 
  • #4
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,798
1,666
Ok so you're telling me I can just plug in the numbers and get the answer like this

H=(5.67x10^-8)(0.8)(0.2x0.5)(500)^4

The answer is still incorrect...
Think carefully about what area A represents in the formula for the thermal radiation. Does heat only radiate from one side of the iron bar?
 
  • #5
Simon Bridge
Science Advisor
Homework Helper
17,847
1,644
How did you work out the total surface area of the block of iron?
Ah, HM beat me to it.
 
  • #6
5
0
Think carefully about what area A represents in the formula for the thermal radiation. Does heat only radiate from one side of the iron bar?
Wow I cant believe how I didnt see that. Thank you for pointing out my stupid mistake!
 

Related Threads on Thermal radiation problem

  • Last Post
Replies
2
Views
469
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
2
Views
4K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
11
Views
619
  • Last Post
Replies
3
Views
1K
Replies
11
Views
1K
  • Last Post
Replies
2
Views
3K
  • Last Post
Replies
4
Views
922
Top