14. The "Hall potential difference" of a Hall device depends on (A) the local magnetic field； (B) the current； (C) the sign and concentration of charge carriers； (D) All； (E) None.

12. Which is not correct for the dielectric constant ( x ) of a substance? (A) It is a measure of the response of its charges to an external electric field； (B) The capacitance of a capacitor can be enhan ced by a factor of n when filling with this substance； (C) k = 0 for an ideal conductor； (D) All； (E) None.