Walker, Halliday & Resnick — Magnetic Fields Due to Currents: Problem 16

16 An electron is shot into one end of a solenoid. As it enters the uniform magnetic field within the solenoid, its speed is 500 m/s and its velocity vector makes an angle of 30° with the central axis of the solenoid. The solenoid carries 4.0 A and has 8000 turns along its length. How many revolutions does the electron make along its helical path within the solenoid by the time it emerges from the solenoid’s opposite end? (In a real solenoid, where the field is not uniform at the two ends, the number of revolutions would be slightly less than the answer here.)