# Homework Help: Conductance of heat through concrete

1. Aug 20, 2014

### slaw155

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

In an electrically heated home, the temperature of the ground in contact with a concrete basement wall is 10.7 oC. The temperature at the inside surface of the wall is 21.4 oC. The wall is 0.17 m thick and has an area of 6.4 m2. Assume that one kilowatt hour of electrical energy costs \$0.10. How many hours are required for one dollar's worth of energy to be conducted through the wall?

2. Relevant equations

heat = (thermal conductivity constant x change in temp x area x time)/length

3. The attempt at a solution
heat energy to be conducted = 3.6 x10^6J x 10
(1.1 x (21.4-10.1) x 6.4t)/0.17 = 443 x time
so equating these gives 3.6 x 10^7 = 443t
however this t value is much too large, where have I gone wrong?

2. Aug 20, 2014

### Staff: Mentor

So, you calculated how many hours, exactly?

3. Aug 22, 2014

### rude man

right so far.

so what is the thermal conductivity constant?
where did the numbers "3.6e6J" and "1.1" come from? Is "10" the number of kwh? kw-h is not an SI unit.

use the thermal conductivity number and change total energy from kw-h to J.