Plotting Field vs. Distance

Consider now an insulating sphere with a uniform charge distribution placed inside a conducting spherical shell. Look at the graph of electric field versus distance. A positive field means the field points away from the center; a negative field means the field points toward the center.

What can you say about the net charge on each object?

  1. Both objects have a positive charge
  2. Both objects have a negative charge
  3. The insulating sphere is negative; the shell is positive
  4. The insulating sphere is positive; the shell is negative
  5. Not enough information is given to tell

Which object has the most net charge on it?

  1. The insulating sphere
  2. The conducting shell
  3. They have equal magnitude charges
  4. Not enough information is given to tell

The field in the space between the spheres is positive, so it points out from the center. The field here is solely determined by the charge on the insulating sphere, so that must be positive.

The field in the region outside the spheres is negative, so it points in toward the center. This field comes from the charge on the two objects combined, so the charge on the conducting shell must be negative and larger in magnitude than the positive charge on the insulating sphere.