What will happen if a collection of positive and negative charges are passed at a high speed through a magnetic field which is perpendicular to the direction of motion of the charges ? (Assume that both kind of charges are NOT going to recombine)

The correct answer is B, Positive charges and negative charges will separate out. A magnetic field pushes a moving charge sideways, and the push on a negative charge is opposite to that on a positive one, so the two kinds bend away from each other. The force on a charge moving through a magnetic field is the Lorentz force, F = qvB sin θ. It is greatest when the motion is perpendicular to the field, and its direction, found by Fleming's left-hand rule, is at right angles to both the velocity and the field. Because the force is always sideways, it changes the direction of motion but not the speed, so the charges curve along circular paths. Reversing the sign of the charge reverses the force, which is how mass spectrometers and particle detectors sort charged particles. Option A is wrong because a magnetic force does no work and cannot stop a charge. Option C is wrong because both kinds of charge feel a force, not only the positive ones. Option D is wrong because a perpendicular field always deflects moving charges. Exam tip: magnetic force bends moving charges sideways, opposite charges opposite ways, without changing their speed.