# Homework Help: Lava conductive cooling

1. Mar 24, 2010

### roam

1. The problem statement, all variables and given/known data

A lava beds cool very slowly, remaining warm for years. Calculate the rate of heat conduction per square metre through a 30 m thickness of rhyolite, if the surface is 400 degrees celcius and its interior side is at 3100 degrees celcius. The thermal conductivity of rhyolite =0.84 Wm^-1 degrees celcius^-1.

2. Relevant equations

$$P=kA \left| \frac{dT}{dx}\right|$$

(The rate is P, k is thermal conductivity, dT/dx is temprature gradient)

3. The attempt at a solution

dT/dx = 3100-400 = 2700

$$0.84 \times 30 \times(2700) = 68040$$

Can anyone show me the problem? ...because the right answer is 75.6!

2. Mar 24, 2010

### ideasrule

Why are you multiplying by 30 instead of dividing? dT/dx is 2700/30.

3. Mar 24, 2010

### roam

Yes, but I need a number for area A!

4. Mar 24, 2010

### roam

Oh, never mind I get it. THANKS!