The magnitude of the force created by the electric field is given by

F_E = Eq

F_E = EQ (using the values in this question)

The magnitude of the force created by the magnetic field is given by

F_M = Bqv

F_M = BQv (using the values in this question)

 

Q could be +ve or -ve.

If Q is +ve, then:

• • the electric field will exert a downward force on Q
  • the magnetic field will exert an upwards force on Q (from Fleming’s Left Hand Rule)

This could still create the motion indicated in the diagram, however F_M would need to be smaller than F_E.

F_M < F_E

BQv < EQ

v < E/Q

If Q is –ve, then:

• • the electric field will exert an upwards force on Q
  • the magnetic field will exert a downwards force on Q (from Fleming’s Left Hand Rule)

This could still create the motion indicated in the diagram, however F_M would need to be larger than F_E.

F_M > F_E

BQv > EQ

v > E/Q

Which is answer D