AP Physics C 2017

(B)

(C)

(D)

(E)

Answers

1 . A —The uniform electric field is downward from the positive plate to the negative plate. The top

surface has a   negative    electric    flux    of  –Ex^2 because   the E-field is  passing into    the closed  Gaussian

surface.    The bottom  surface of  the cube    will    have    a   positive    electric    flux    of  +Ex^2 as    the E-field is

passing out of  the closed  Gaussian    surface.    The four    sides   of  the cube    have    no  flux    because the E-field

does    not pass    through the Gaussian    surface.    Thus,   the net flux    for the entire  surface is  zero.   A   more

elegant solution    method  is  to  use Gauss’s law:        .   All of  the charge  is  spread

across  the capacitor   plates. Since   there   is  no  charge  enclosed    in  the Gaussian    surface,    the net flux

must    be  zero.

2 . A —In the original location x , the two electrostatic forces on +q add up, as shown in the figure on
the left. As the charge +q is moved toward +Q and –Q, the electrostatic forces increase in strength,
and their direction shifts as shown in the figure on the right. Thus, the net force increases in strength
but continues to point directly downward due to the symmetry of the charge arrangement.

3 . B —Using conservation of energy, the change in electrostatic potential energy plus the initial kinetic
energy equals the final kinetic energy: