# Griffiths 7.49

1. Homework Statement
An atomic electron (charge q) circles about the nucleus (charge Q) in an orbit of radius r; the centripetal acceleration is provided, of course, by the Coulomb attraction of opposite charges. Now a small magnetic field dB is slowly turned on, perpendicular to the plane of the orbit. Show that the increase in kinetic energy, dT, imparted by the induced electric field is just right to sustain circular motion at the same radius r.

2. Homework Equations

3. The Attempt at a Solution
The induced emf (force per unit charge per loop) will be \pi r^2 dB. The Lorentz force will be qvdB.

I don't understand what they want me to do. The Lorentz force will be radially outward or inward. I do not see what could counteract it.