13. A conductor has a surface charge density σ C/m2. The force per unit area on the surface is (A)0；(B) σ2/2εo N/m2；(C) σ2/εo N/m2；(D)2σ2/εo N/m2；(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.