Posted by Lily on Monday, March 8, 2010 at 3:33am.
A rectangular piece of gold is 3.1 cm long and 3 cm wide. Resistivity of gold is 2.4*10^8. How deep is the piece when it displays a resistance of R = 1.65 μω for a current that flows along the depthdirection?
I used R=(p*l)/A and solved for l. I got the answer wrong. For the area I used 0.03*0.03 as the cross sectional area. What did I do wrong?

Physics  drwls, Monday, March 8, 2010 at 7:42am
The area is 0.03 * d, and you are supposed to solve for d, knowing the total resistance. Current flow is along the length, which you know to be 3.1 cm.
There is possibly another problem as well. The resistivity of gold is approximately 2.44 × 108 ohmm. The way you are using it, with gold block dimensions in cm, you have to change the value of p to 2.44 × 106 ohmcm 
Physics  Lily, Monday, March 8, 2010 at 2:25pm
Thank you.