# Finding free electron concentration to check authors' calculations for electron drift velocity

I am currently studying Practical Electronics for Inventors, Fourth Edition, by Scherz and Monk. Chapter 2.5.1 How the Shape of a Conductor Affects Resistance presents the following image:

I wanted to check the authors' calculations for drift velocity, since there has been some issues[2][3] related to their electron drift velocity calculations in prior sections of the textbook.

My understanding is that the drift velocity of electrons is calculated as

$$\dfrac{\text{current density}}{\text{free electron concentration} \times \text{electron charge}}$$

But, as far as I can tell, we are not provided with the free electron concentration in this case, so how does one calculate the drift velocity (to verify that the authors' calculations are correct)?

I would greatly appreciate it if people would please take the time to clarify this.

• omnicalculator.com/physics/…
– G36
Commented Jan 29, 2020 at 18:48
• @G36 Doesn't this assume that you already know the free electron concentration (in order to calculate the drift velocity)? Also, I'm not sure how we take the values we have here and apply it to their "number density", since it is set to a variable multiple of $10^{28} \text{carriers}/m^3$ as default? Commented Jan 29, 2020 at 19:17
• Yes, this is what we normally do we, use a number between 8.4.....8.5 as a ballpark number. In fact, in EE we never does such calculations. So why even bother about it?
– G36
Commented Jan 29, 2020 at 19:25
• @G36 Ahh, that actually produces a value that is approximately what the authors got: omnicalculator.com/physics/… !!! So it seems that you are correct about the 8.4-8.5 value. This is what I was looking for. Thanks! Do you want to post an answer? I will then accept it, so that we can close this question. Commented Jan 29, 2020 at 19:49
• Also, you should check this onedrive.live.com/…
– G36
Commented Jan 29, 2020 at 20:22