Figure 20.7b Curving path of a positively charged particle moving in a magnetic field directed out
of the page.Now our proton is moving straight down. The force exerted on it by the magnetic field, using the right-
hand rule, is now directed to the left. So the proton will begin to bend leftward. You probably see where
this is going—a charged particle, traveling perpendicular to a uniform magnetic field, will follow a
circular path.
We can figure out the radius of this path with some basic math. The force of the magnetic field is
causing the particle to go in a circle, so this force must cause centripetal acceleration. That is, qvB = mv 2
/r .
We didn’t include the “sin θ ” term because the particle is always traveling perpendicular to the
magnetic field. We can now solve for the radius of the particle’s path:
The real-world application of this particle-in-a-circle trick is called a mass spectrometer. A mass
spectrometer is a device used to determine the mass of a particle.
A mass spectrometer, in simplified form, is drawn in Figure 20.8 .
Figure 20.8 Basic mass spectrometer.A charged particle enters a uniform electric field (shown at the left in Figure 20.8 ). It is accelerated by