Here, mobility is defined as |e| \tau / m, but e was already defined to be positive, so I feel like taking the absolute value is redundant. Is the mobility normally defined for any charge q, which could be positive or negative?
Furthermore, in the equation for \mathbf{v}, the absolute value of \mu is used. But if the absolute value of the charge is already taken in the definition of \mu, this absolute value doesn’t do anything, right?
