The magnetic force on a moving charge is given by:
where q is the charge, v is the speed of the charge, B is the magnetic field intensity and is the angle between the directions of B and v.
In our problem, the charge is an electron (), the velocity of the charge is , the magnetic force is and the angle between the direction of v and B is , so and we can ignore the sine in the formula. Therefore, if we rearrange the equation and we put these numbers in, we can find the intensity of the magnetic field: