# Homework Help: Help! Heat Loss

1. Oct 18, 2005

### jdog6

Help! Heat Loss!!!

A cup of coffee (with a lid) is enclosed in an insulated cup 0.5cm thick in the shape of a cube 13.1cm on a side. The thermal conductivity of the cup is 0.0002 cal/s*cm*DegreeC. The temperature of the coffee is 87C and the temperature of the surroundings is 14C. Find the heat loss due to conduction. Answer in units of J/s.

I've tried P=kA(Th-Tc)/L ------> Energy transfer by conduction but somehow it's wrong?

I see no other way!

I know that:
k = 0.0002cal/s*cm*DegC which = 0.0008372J/s*cm*DegC
L=0.5cm
A=13.1cm*13.1cm (because cube)
A=171.61 cm^2
Th=87C
Tc=14C

ANY IDEAS?!

Last edited: Oct 18, 2005
2. Oct 18, 2005

### lightgrav

You're actually using kA(Th - Tc)/L , right?

Are you leaking Heat thru all 6 sides?

Did you convert from cal to J?

Are you using 13.1 cm as inside measure, or outside?
How do you know it's wrong?

3. Oct 18, 2005

### jdog6

I know its wrong because my teacher said so

Yes im using that equation.

I think I'm leaking on all sides

I did convert from cal to J.

And I'm using 13.1cm as an outside measure I believe.

Last edited: Oct 18, 2005
4. Oct 18, 2005

### lightgrav

Did you multiply the area of each side by 6 sides?

5. Oct 18, 2005

### jdog6

Why x6 we are looking for overvall heat loss from this thermos like apparatus.

6. Oct 18, 2005

### lightgrav

you think Heat only leaks out the top?
How many sides does a cube have?

7. Oct 18, 2005

### jdog6

Ok i understand but how do i fit it into the equation? k(6A)(Th-Tc)/L ?

Last edited: Oct 18, 2005
8. Oct 18, 2005

### lightgrav

yea, or A = 6 s^2

9. Oct 18, 2005

### jdog6

Correct! Thank you very much!