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## Homework Statement

An 8.0m by 12m house is built on a concrete slab 23cm thick. What is the heat-loss rate through the floor if the interior is at 20°C while the ground is at 10°C?

## Homework Equations

[itex]H=-k A \frac{\Delta T}{\Delta x}[/itex]

k

_{concrete}= 1.0 W / m K (approx)

## The Attempt at a Solution

[itex]H=\frac{-1.0 (8 * 12) (10-20)}{0.23} = 4.17[/itex] kW

My problem is that the book (Essential University Physics, Wolfson) has the answer as -4.17 kW, but that would imply that heat is being transferred from the ground to the house, which it clearly cannot be from the temperature of the house being larger than the ground's temperature.

I usually have problems with the sign of this value of H, how do I know what direction they are asking for by "through" an object - it could be either. Would either value be acceptable in an exam?

Thanks.