# physics--heat

posted by on .

a piece of glass is 6mm thick and the coefficient of thermal conductivity of this glass is K=0.75Wm^-1K^-1.
if we add a 2nd identical piece of glass separated by an airgap of 5cm, the total thermal conduction will be reduced.
what is the ratio of heat transfer by conduction for the double pane to the single case.

K of air =0.023W m^-1K^-1

• physics--heat - ,

Add the thermal resistances of the two glass slabs and the air gap. The conductance is the reciprocal of the resistance. Are you sure the air gap is 6 cm and not 6 mm? With a gap that large, you are likely to get convection currents in the gap.

Thermal Resistance of one slab of glass:
R1 = W1/K1
Thermal Resistance of two slabs of glass with gap = Wair:
R2 = 2W1/K1+ Wair/Kair

where Wair is the air gap and kair is the termal conductivity of glass.
What you want is the ratio R1/R2

• physics--heat - ,

what is W1 and K1?
so is Wair=thermal conductivity of air gap
and Kair=thermal conductivity of glass?

• physics--heat - ,

W1 is the width of the glass slabs. They say it is 0.6 cm.
K1 is the thermal conductivity of the glass. Yes to your other questions.

Both W's should be in meters since the units of K contain m^-1, but the answer to your question will be a dimensionless ratio anyway