# Thermal conductivity

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1. Sep 27, 2014

### Tonyuguccioni

• Warning! All queries in the Homework section must use the Posting Template provided when a new thread is started.
The information I am given is : a door has two steel layers both are .47 mm thick, the door itself is 725 mm by 1800mm. The question asks, how thick of a layer of wood (oak) would have to be put in the door to limit the heat loss to 740kJ per hour? Temp inside is 18C and outside is -20C

All work is done with only three significant digits.

I have been working with the equation ;
H=area(temp1-temp2)/((thickness1/k1)+(thickness2/k2))
Where; k is the thermal conductivity of the material
X is the unknown

I have run threw the equation several times but every time I either can't isolate the variable or end up with a wrong answer.

0.205kJ/sec= ((1.30m^2)(38C))/((9.40*10^-5m/43W/mC)+(Xm/0.17W/mC))

Is this the right equation and/or have I made a simple error with conversions ?

Please don't post full answer I want to find it but tips or suggestions would be awesome !!!

2. Sep 27, 2014

### Orodruin

Staff Emeritus
3. Sep 29, 2014

### Tonyuguccioni

thanks but it is still escaping me i feel like i am just one step away from getting it.... very frustrating but it i will prevail !! :P

4. Sep 29, 2014

### Orodruin

Staff Emeritus
Did you try simply solving for X in your equation? (Note that it is usually easier not to make mistakes if you solve the algebraic expression first and insert numbers when this is done)

5. Sep 29, 2014

### Felix Larrivee

whats the next step? and what is the equation you started with was it H= -kA (dt/dx)