WAEC Physics 2012 Theory — Question 9

Model answer

The cathode ray consists of a stream of negatively charged electrons. When this beam passes through the magnetic field (directed from N to S, i.e. into the region between the poles), the force on the moving charge is given by F = qv×B (the motor effect). Using Fleming's left-hand rule (with the current direction taken as opposite to electron flow, since electrons are negative), the ray will be deflected in a particular direction determined by the directions of the electron velocity and the magnetic field — in this arrangement (electron beam moving horizontally, field directed from S to N i.e. upward between the poles as drawn), the ray will bend downward, since the resulting force on the negative charges points downward according to the left-hand rule applied with reversed (conventional) current direction.