F = Magnetic Force
B = Magnetic Field
V = Velocity
*The vectors from the photo you get doing the left-hand rule.
The magnetic force is always perpendicular to the magnetic field.
And as told in the statement, the electron is moving perpendicular to a magnetic field, that is, the Velocity forms an 90 degree angle / Right angle with the magnetic field.
The formula to find the Magnetic Force is:

Where "q" is the Charge and the sin theta is the angle formed by the Velocity and Magnetic Field, in this case it's 90°. Sin 90° = 1.

Newton (N) = C x m/s x T = (C x m x T)/s
It gets larger since the heat is expanding it outward