Heat flow through aluminium rod

[em3]

Homework Statement

An Aluminium rod has a diameter of 40mm. and is 750mm long. If the temperature difference between the ends of the rod is 100C .

Take the thermal conductivity of Aluminium to be 235 Wm-1K-1

Homework Equations

k * A * ΔT / L

The Attempt at a Solution

235 * 0.097 m * 100 / 0.75

The answer is supposed to be 39.4 Js^-1 but I can't get it right? So I don't know if I'm doing something wrong, and I've tried so many times. I don't know what I'm doing wrong.
Thanks

 

[gneill]

Hi em3,

Welcome to Physics Forums!

235 * 0.095 m * 100 / 0.75

How did you arrive at 0.095 m for the cross sectional area?

 

[em3]

40mm. and is 750mm into m, 0.04m and 0.75m. Then did A=2πrL+2πr2

2 * ∏ * 0.02 * 0.75 + 2∏0.02²≈ 0.097
Sorry, made a typo in first post

 

[gneill]

40mm. and is 750mm into m, 0.04m and 0.75m. Then did A=2πrL+2πr2

You want the cross sectional area, not the total surface area.

 

[em3]

How do I find that? The question only gives me the diameter and length, no inner or outer diameter.

 

[gneill]

How do I find that? The question only gives me the diameter and length, no inner or outer diameter.

Assume that a rod is a solid cylinder.

 

[em3]

Oh ok thanks, I got it. Thanks a lot!