# Homework Help: Rate of energy transfer by conduction through the window?

1. Jan 10, 2006

### cmilho10

A thermal window with an area of 6.00 m2 is constructed of two layers of glass, each 4.00 mm thick and separated from each other by an air space of 3.00 mm. If the inside is at 20.0°C and the outside is at -38.0°C, what is the rate of energy transfer by conduction through the window?

i am using the equation:

H=kA[(Thot-Tcold)/L]

where:
k=Thermal Conductivity constant of glass (0.8 W/m*C)
A=6.0 m^2
change in temp=58 C
and i have tried the lengths at 0.011 m, 0.008 m, and 0.003 m

2. Jan 10, 2006

### lightgrav

First, the change in Temperature is zero ... they're asking about a
static situation. There is a Temperature difference (gradient).

Have you done simple thermal conduction problems, with only one layer? If so, you know that you're computing the flow of Energy through the layer.

Now, what happens to the Energy that flows through the first layer?
How much of it later flows through the second layer?
How much of it eventually flows through the third layer?

You have the same H in each layer:
$$k_I A \frac{(T_1 - T_2)}{L_I} = k_M A \frac{(T_2 - T_3)}{L_M} = k_O A \frac{(T_3 - T_4)}{L_O}$$
this looks a lot neater with Tempaerature diferences across each layer:
$$k_I A \frac{\delta T_I }{L_I} = k_M A \frac{ \delta T_M}{L_M} = k_O A \frac{ \delta T_O}{L_O}$$ , where $$\delta T_I + \delta T_M + \delta T_O = 58 C$$
Now "just" solve these simultaneously.
It might help to make a new variable "R" = L / kA for each layer ...