1. The problem statement, all variables and given/known data This is Purcell 4.12... Given that conductivity in silicon at 500K is 0.3 sec-1, deduce the mean free time between collisions of an electron. 2. Relevant equations Electron mass is 9.1x10-28 grams (mass of a hole is exactly the same), the charge on an electron is 4.8x10-10, the density of electrons and holes is each 1015. 3. The attempt at a solution I solved Purcell's equation for conductivity to find mean free time instead: T=(Mass*σ)/(2Ne2)... and I get something like 5.92x10-25 seconds. Isn't this way too small? Or did I do something wrong?