1. The problem statement, all variables and given/known data A charged oil drop is allowed to fall through the electric field created by the plates as shown: + - l l l l l l In order to give the oil drop a straight trajectory, a magnetic field should be established with field lines pointing: A. to the right B. to the left C. out of the page D. into the page 2. Relevant equations --------- 3. The attempt at a solution The back of my book says that "the electric field between the plates will push a positive charge to the right, so the magnetic field must push the drop to the left to counter the force." What I don't understand is how am I to know if the oil drop has a positive or negative charge since it is not stated in the question?