Thermodymanics heat loss question. Please help

  • Thread starter name_ask17
  • Start date
  • #1
name_ask17
146
0
Question: How much heat is lost in one hour through a 15 cm x 3.7m x 6.1m concrete floor if the inside temperature is 22 degrees c and the ground temperature is 13 degrees c?

some info:
thermal conductivity of concrete=1.1
change in temperature=8

Relative Equations:
kAchangeT/L

Attempt at problem:
1.1(.15x3.7)(22-13)/6.1
this is not giving me the right answer. the answer should be 5.363E6J
can someone explain to me what I am doing wrong.
thanks
 

Answers and Replies

  • #2
JaredJames
2,817
22
The change in temperature is 8 degrees celsius?

k = thermal conductivity
A = area
T = change in temerature
L = thickness of the surface

Is it more likely the concrete floor is 0.15mx3.7m with a thickness 6.1m or is it 3.7mx6.1m with a thickness of 0.15m?

You've got the values in the wrong place.
 
  • #3
name_ask17
146
0
yes i tries rearranging the numbers several different ways but i still can't get the correct answer
 
  • #4
JaredJames
2,817
22
yes i tries rearranging the numbers several different ways but i still can't get the correct answer

22-13 isn't 8 for a start...

The equation only gives you the heat flow per second. You now need to convert it into heat flow per hour.
 
  • #5
facenian
436
25
I suppose the answer should be
1.1(6.1x3.7)(22-13)/0.15*3600 but this is equal to 4,767E6
 
  • #6
name_ask17
146
0
yes but even with that, io am getting the answer to br 3242.65 (still not correct)
 
  • #7
JaredJames
2,817
22
I suppose the answer should be
1.1(6.1x3.7)(22-13)/0.15*3600 but this is equal to 4,7676

I've worked it out and I get the exact answer specified by the OP.
 
  • #8
JaredJames
2,817
22
yes but even with that, io am getting the answer to br 3242.65 (still not correct)

The value you get should is per second, multiply by 3600 to get the value per hour.

3242.65 is incorrect.
 
  • #9
name_ask17
146
0
but the answer should be 5.363 x 10^6 J
 
  • #10
JaredJames
2,817
22
k = 1.1
A = (3.7 x 6.1)
T = 9
L = 0.15

kAT/L = 1.1(3.7x6.1)9 / 0.15 = 223.443 / 0.15 = 1489.62

1489.62 x 3600 = ?
 
Last edited:
  • #11
name_ask17
146
0
am i using the correct equation? could that be the problem?
 
  • #12
name_ask17
146
0
Oh! thank you so much!
 
  • #13
JaredJames
2,817
22
am i using the correct equation? could that be the problem?

I've given the actual answer above.

I don't know what facenian did.

You need: Q/t = kAT/L

Where Q/t = heat loss per unit time.

When you solve the second part you get heat loss (Q) per second (t).

So you simply convert this to an hour.
 
  • #14
name_ask17
146
0
Thank you(:
 
  • #15
facenian
436
25
I've worked it out and I get the exact answer specified by the OP.

you are right I used 22-13=8!
 

Suggested for: Thermodymanics heat loss question. Please help

  • Last Post
Replies
1
Views
275
  • Last Post
Replies
2
Views
258
Replies
16
Views
527
Replies
3
Views
427
  • Last Post
2
Replies
35
Views
767
  • Last Post
Replies
6
Views
469
Replies
4
Views
105
Replies
2
Views
314
Replies
10
Views
677
Replies
10
Views
303
Top