# Homework Help: Thermal Conductivity Problem.

1. Apr 18, 2010

### Hendon100

1. The problem statement, all variables and given/known data
This is my first post on these forums, this problem is part of a lab and its part my end of year result but we never did anything on it in class so i have'nt a clue how to do it. Help would be really appreciated

The handle of a freezer door 12cm thick is attached by two brass bolts 0.75cm in diameter that pass through the entire door and are secured on the inside by nuts. The interior of the freezer is maintained at -32 degrees celsius and the room temperature is 34 degrees celsius; the thermal conductivity is 109W.M-1.K-1. Find the heat lost per hour through the bolts.

2. Relevant equations

Like i said, i haven't a clue. I dont know why we even were given this question.

3. The attempt at a solution

2. Apr 18, 2010

### tiny-tim

Welcome to PF!

Hi Hendon100! Welcome to PF!

You don't need to know anything about heat loss …

just look at the units!

you're told the power loss of the material in watts per metre per ºK, and power times time = energy, so (remembering to use both the diameter and the length) the energy loss in this case is … ?

3. Apr 18, 2010

### Hendon100

Thanks for the help, I used this equation. I dont know if im right though.

Power
= (thermal conductivity)(C.S.A)(difference in temp)
=
Distance ​

Thermal conductivity= 109
C.S.A= (3.75x10-3)2(3.14)= 4.42x10-5m2
Difference in temp=34-(-32)=66
Distance= .12m

Power=
(109)(4.42x10-5)(66)
= 2.65 Watts per bolt
.12​

I then multiplied this by 2 because theres 2 bolts. So 5.3 Watts overall

And then i just used the equation Power= Energy x Time

5.3(3600)= 19080 joules lost per hour

:uhh::uhh: which seems a bit high for two little brass bolts but i could be wrong. Please reply to tell me if i did it right, and if not how do i do it. Thanks alot.

4. Apr 18, 2010

### tiny-tim

(you meant Energy = Power x Time )

Yes, it does look high, but the maths seems fine.

5. Apr 18, 2010

### Hendon100

alright, thank you very much