(a) Calculate the magnetic flux density of the field required for protons to move in the circular path when their speed is 1.2 × 105 ms1.
∴ BQv = mv2/r
B = mv/rQ
B = 1.67 × 10−27 × 1.2 × 105 /(120 x 1 .6 x 10-19) = 1.04 × 10−5
B = 1.0 × 10−5 T
This magnetic field must act upwards, out of the plane of the page (Fleming's left-hand rule)
(b) Explain how the magnetic flux density required to maintain the circular path has to change as the kinetic energy of the protons increases.
As the kinetic energy of the protons increases the radius of the path they take would increase if the magnetic field strength remained steady. The magnetic flux density therefore has to increase because in a path of constant radius (using the same charged particles) magnetic flux density B ∝ v