On a cold night the temperatures of the two surfaces of a glass windowpane in a
ID: 2238911 • Letter: O
Question
On a cold night the temperatures of the two surfaces of a glass windowpane in a house are 7.8Explanation / Answer
sol: as we know that A. (dQ/dt)/A = k1*dT/d = 0.8*0.2/0.00348 = 45.977 W/m^2 B. From the ref, dQ/dt = k2*A*dT ==> k2 = (dQ/dt)/(A*dT) We know (dQ/dt)/A from answer A. To solve for k2 we simply need to divide that by dT, which is 21.8-8 C on the inside and 7.8-0 C on the outside. Inside: k2 = 45.977/13.8 = 3.332 W/m^2-C Outside: k2 = 45.977/7.8 = 5.894 W/m^2-C Note that k2 is a property of the air, not the glass. Typical values for air are quoted in the ref as 10 to 100, so these answers are below the "typical" range. use this