Find the magnitude of the force on an electron that is travelling with velocity 2 x 10^4 ms^(-1) in the x direction through a uniform magnetic field of strength 2T in the y direction.

This question tells us we only need to consider the magnitude of the force, and since the magnetic field and electron's velocity are perpendicular, we can simply use the equation

F = Bqv.

We have B = 2 T, q = 1.6 x 10^-19 C (the charge of an electron) and v = 2 x 10⁴ ms^-1.

Substituting these values into the equation gives 

F = 2 * (1.6  x 10^-19 ) * (2 x 10⁴) = 6.4 x 10^-15 N   (Remember your units!)