# Homework Help: Heat flow by conduction

1. Oct 28, 2012

### prettykitty

Two 0.525 m rods, one lead and the other copper, are connected between metal plates held at 2.00°C and 106°C. The rods have a square cross section of 1.50 cm on a side. How much heat flows through the two rods in 1.0 s? Assume no heat is exchanged between the rods and the surroundings.

Relevant equations
Q=kA(ΔT/L)t
k for Pb is 34.3
k for Cu is 395

My attempt
I set up the equation for the rods in the exact same manner.
Q=(34.3)(1.77e-4)(104/.0150)(1)
Q=(395)(1.77e-4)(104/.0150)(1)

I know what is incorrect about these equations is the area I am using. I took the square cross section to mean diameter, which evidently it is not. The answers given in the text are:
Q=(34.3)(.0151)^2(104/.0150)(1)
Q=(395)(.0151)^2(104/.0150)(1)

How should I interpret square cross section in the future?
To finish this problem I am aware that Qtotal=QPb+QCu
Thanks!

2. Oct 28, 2012

### CWatters

A cross section is the surface you get when an object is cut (or intersected by a plane). If you cut a round rod at 90 degrees to the axis the cross section is circular. If you cut it at say 30 degrees to the axis the cross section would be an elipse. If in doubt assume it's cut at 90 degrees.

So when it says the rods have a "square cross section", that means the cut ends look like a square. When it says "1.50 cm on a side" that means the cross section has sides (so can't be round).

If it had said "the rods have a triangular cross section of 1.50 cm on a side" then the cross section would be an equilateral triangle of area 1.255 cm2.

3. Oct 28, 2012

### CWatters

Are you sure?

They seem to have L = 0.0150m = 1.5cm but the problem says L = 0.525m.