A2L Item 129

Goal: Hone the concept of electrostatic potential

Source: 283-420 Change of PE

A uniform volume distribution of
charge has radius R and total charge Q. A point charge -q is released
from rest at point b, which is a distance 3R from the center of the
distribution. When the point charge reaches a, which of the following is
true regarding the potential energy, U?

  1. Ua = -Ub
  2. Ua = -2Ub/3
  3. Ua = -3Ub/2
  4. Ua = -9Ub/4
  5. Ua = Ub
  6. Ua = 2Ub/3
  7. Ua = 3Ub/2
  8. Ua = 9Ub/4
  9. None of the above
  10. Cannot be determined



(7) Many students use an inverse square dependence appropriate
for fields. Others will take the field at b and multiply by the
displacement. Still others will assert that the potential doubles
because they are using the distance to the surface of the sphere.