Question

Two particles with charges +5e and -6e are initially very far apart (effectively an infinite distance apart). They are then fixed at positions that are 4.08 x 10^-13 m apart. What is EPE_final - EPE_initial, which is the change in the electric potential energy?

Answer 1

r_i = initial distance between charges =

r_f = final distance between charges = 4.08 x 10^-13 m

Q₁ = 5e = 5 (1.6 x 10^-19) = 8 x 10^-19 C

Q₂ = - 6 e = - 6 (1.6 x 10^-19) = - 9.6 x 10^-19 C

U_i = initial electric potential energy = k Q₁ Q₂/r_i = (9 x 10⁹) (8 x 10^-19) (- 9.6 x 10^-19)/() = 0 J

U_f = final electric potential energy = k Q₁ Q₂/r_f = (9 x 10⁹) (8 x 10^-19) (- 9.6 x 10^-19)/(4.08 x 10^-13) = - 1.7 x 10^-14 J

change in electric potential energy is given as

U_f - U_i = (- 1.7 x 10^-14 ) - (0) = - 1.7 x 10^-14 J