Problem 3.406

In a betatron the magnetic induction on an equilibrium orbit with radius \(r=20 \mathrm{~cm}\) varies during a time interval \(\Delta t\) \(=1.0 \mathrm{~ms}\) at practically constant rate from zero to \(B=0.40 \mathrm{~T} .\) Find the energy acquired by the electron per revolution.