# Energy Transfer Question

1. Nov 15, 2005

### blue88

The average thermal conductivity of the walls (including windows) and roof of a house in Figure P11.32 is 4.8 10-4 kW/m°C, and their average thickness is 20.0 cm. The house is heated with natural gas, with a heat of combustion (energy given off per cubic meter of gas burned) of 9300 kcal/m3. How many cubic meters of gas must be burned each day to maintain an inside temperature of 28.0°C if the outside temperature is 0.0°C? Disregard radiation and loss by heat through the ground.

The house's dimensions are 10.0m x 8.0m x 5.0m

I tried this problem, and I keep getting the answer completely wrong... if anyone could show me how to do this one, that would be awesome.

Thanks.

2. Nov 15, 2005

### Physics Monkey

What have you tried so far? We can only help you if you tell us what you've done up to this point.

3. Nov 15, 2005

### blue88

I did P=(4.8E-4)(.2)(28)

P= 2.688W

Q=Pnet(t)
Q=(2.688)(86400)
Q= 232243.2 J of heat

but i don't think that I am even starting it right.

4. Nov 15, 2005

### Physics Monkey

Why did you multiply the thermal conductivity by length when computing the emitted power? How do you use the thermal conductivity to calculate the heat flux through a surface of area A and thickness L with a temperature difference $$\Delta T$$?