# Homework Help: Radiation: simple, yet

1. Apr 9, 2006

### BooGTS

...im getting it wrong.

Calculate the heat flow through a 1.000m long copper bar with a cross sectional area of 2.00 cm2. One end at 2.0 degrees Celsius and the other end at 99.0 degrees Celsius. Rate of heat flow is Q/t. In this problem the coefficient of copper is 390.

P=(ka (Thot-Tcold))/L

so I input 390*(2/100)*97 and get 756.6=wrong

2. Apr 9, 2006

### lightgrav

You seem to be ignoring UNITS ...
some coefficient of copper is 390 ... WHATs?
cm? /cm? /m? J/K?

By the way, this is CONDUCTION , not radiation.
... and Conductivity (k) is not usually refered to as a coefficient.

3. Apr 9, 2006

### BooGTS

390 is thermal conductivity (J/s*m*deg C)

redoing with "UNITS" would be 390J/s*m*deg C*(2cm^2/100cm/m)*97degK

4. Apr 9, 2006

### lightgrav

so, which of the cm are you converting ... the height, or the width?

5. Apr 9, 2006

### BooGTS

so, which of the cm are you converting?

the area

6. Apr 9, 2006

### chroot

Staff Emeritus
Hint: To convert cm^2 into m^2, you don't just divide by 100. Think about this for a second: how many squared centimeters are in a squared meter?

- Warren

7. Apr 9, 2006

### BooGTS

^ yeah, that was it. I followed a similar example to a "T" too...