What is the rate of heat conduction through a 30 m thickness of rhyolite lava?

[roam]

Homework Statement

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 celsius and its interior side is at 3100 degrees celsius. The thermal conductivity of rhyolite =0.84 Wm^-1 degrees celsius^-1.

Answer must be 75.6 W/m^2

Homework Equations

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

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

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!

 

[ideasrule]

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

 

[roam]

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

Yes, but I need a number for area A!

 

[roam]

Oh, never mind I get it. THANKS!