Question

Constants A picture window has dimensions of 1.40 m×2.50 m and is made of glass 5.80 mm thick. On a winter day, the outside temperature is -15.0 ?C , while the inside temperature is a comfortable 20.5 ?C . a)At what rate is heat being lost through the window by conduction? b)At what rate would heat be lost through the window if you covered it with a 0.750 mm-thick layer of paper (thermal conductivity 0.0500 W/m?K)?

Answer 1

a)
we know, thermal conductivity og window glass, k_glass = 0.96 W/(m k )

Rate of heat being lost, dQ/dt = k*A*dT/L

= 0.96*1.4*2.5*(20.5 - (-15))/(5.8*10^-3)

= 2.06*10^4 W   <<<<<<<<------------Answer

b)

when paper layer is used,

the heat flux through the glass must be equal to the heat flux through the paper at steady state.

k_glass*(T_interior - T_interface)/t_glass = k_paper*(T_interfcae - T_out)/t_paper

0.96*(20.5 - T)/0.0058 = 0.05*(T - (-15))/0.00075

==> T = 10.3 degrees celsius

so, the rate of heat lost, dQ/dt = k_glass*A*(T_interior - T_interfcae)/t_glass

= 0.96*(20.5 - 10.3)*1.4*2.5/(5.8*10^-3)

= 5.90*10^3 W <<<<<<<<--------------------------Answer