- #1
daleklama
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Homework Statement
The brick wall of a building has dimensions of 4m by 10m, it is 15cm thick with a coefficient of thermal conductivity of 0.8 (Wm^-1C^-1).
(a) How much heat flows through the wall in a 12-hour period when the average inside temperature is 20 C and the average outside temperature is 5 C?
(b) Will a 2kW heater operating in the room compensate for this?
(c) A layer of insulating material, thickness 5cm, k = 0.04 (Wm^-1C^-1) is added to the inside of the wall. Calculate the rate of heat loss after the addition of this layer.
Homework Equations
The Law of Heat Conduction:
H = (dQ/dt) = - k A (dT/dx)
Another equation I found is the following, but I'm not sure if it's relevant here:
H = A(T2 – T1)/(summation Ri), where the summation is taken from i = 1 to i = n,
and Ri = (Li/ki).
The Attempt at a Solution
I've attemped (a) using the Law of Heat Conduction, but I think my answer seems very wrong!
I said the area A = 4m x 10m = 40m^2
k = 0.8
dx = 15cm = 0.15m.
Using equation above:
(dQ/dt) = - (0.8) (40) (15/0.15)
(dQ/dt) = -3200
12 hours is 720 mins or 43200 seconds.
dQ = -3200 (43200)
dQ = - 138240000 W, or 138240 kW.
Fairly sure it's wrong, and even if it was right, I'm not sure how to do part (b). Do I just compare my answer to (a) with the 2kw given in (b)?
And c is beyond me at the moment, sadly! :(
Any help would be greatly appreciated, thanks.