Question

Air has a thermal conductivity of 3 x 10-4 J cm-1 s-1 K-1. Consider a window with double panes of glass where the temperature of the inside pane is 25 oC and the temperature of the outside pane is -5 oC. The size of the window is 40 cm x 60 cm, and the panes are 8 mm apart. What is the heat flow in watts through the window? You can ignore convection.

Answer 1

rate of heat flow is

∆Q/∆t = -K×A×∆T/x

where ∆Q/∆t is the rate of heat flow;

K is the thermal conductivity factor;

A is the surface area;

∆T is the change in temperature and

x is the thickness of the material

(∆T/x is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).

∆Q/∆t = -K×A×∆T/x

∆Q/∆t = -3 x 10^-4 J cm^-1 s^-1 K^-1 x 40 cm x 60 cm x (298 - 268 )K/0.8cm

∆Q/∆t = - 27J s^-1

watt = joule / second

rate of heat flow through the window is 27 watts