Some mass spectrometers use an electric field to give the incoming ions a certain known...

Some mass spectrometers use an electric field to give the incoming ions a certain known kinetic energy as they enter the magnetic field region. (a) If the ions have charge q and kinetic energy KE, what is the value of r in Figure 20.27 in terms of m, q, B, and KE? (b) How does the final kinetic energy of an ion (just before it reaches the ion detector) compare with the initial kinetic energy (just as the ion enters the region where the magnetic field is nonzero)?