Imagine that we place two point charges a certain distance apart. The electric field vector at a certain point on a line connecting the charges (but not between the charges) is zero. What can we conclude about the signs and magnitudes of the charges?
  1. the same sign and magnitude
  2. opposite signs but the same magnitude
  3. the same sign, but might have different magnitudes
  4. opposite signs, but must have different magnitudes
  5. completely unknown signs and magnitudes