Anything enclosed in a metal shell is shielded by the shell from external electric fields. In other words, the external fields do not penetrate into the shell. This is why one of the safest places to be in a lighting storm is inside a car.

Consider two charges, -Q and +8Q. Which one feels the largest force?

  1. The -Q charge
  2. The +8Q charge
  3. Neither, the forces are equal-and-opposite

As usual, Newton's Third Law tells us that the two charges feel equal-and-opposite charges.

A spherical conducting shell is now placed around the -Q charge so the -Q charge is at the center of a conducting shell. How does this change the force the -Q charge experiences?

  1. It doesn't
  2. The -Q charge now experiences no force
  3. The force is reduced, but is not zero
  4. The force is increased
  5. It depends on whether the shell is charged or not

The shell shields the -Q charge from the external field, so the -Q charge now feels no force.

Does the +8Q charge feel a force?

  1. No
  2. Yes, the -Q charge still exerts the same force on it
  3. Yes, but it's the conducting shell that exerts the force
  4. Yes, both the -Q charge and the conducting shell exert forces on the +8Q charge

If the -Q charge is shifted toward the +8Q charge, but is still inside the conducting shell, what happens to the force experienced by each charge?

  1. Nothing
  2. The -Q charge still feels no force; the +8Q charge feels more force
  3. The -Q charge feels a force; the force on the +8Q charge is unchanged
  4. The -Q charge feels a force; the force on the +8Q charge is increased
  5. The -Q charge feels a force; the force on the +8Q charge is decreased

The -Q charge now feels a force because the situation is no longer symmetric. What happens inside the shell can not affect the situation outside the shell, however, so the force on the +8Q charge is unchanged.