13.5 Force on a moving charge in a magnetic field: examples and  (Page 3/4)

Thermonuclear fusion (like that occurring in the Sun) is a hope for a future clean energy source. One of the most promising devices is the tokamak , which uses magnetic fields to contain (or trap) and direct the reactive charged particles. (See [link] .) Less exotic, but more immediately practical, amplifiers in microwave ovens use a magnetic field to contain oscillating electrons. These oscillating electrons generate the microwaves sent into the oven.

Mass spectrometers have a variety of designs, and many use magnetic fields to measure mass. The curvature of a charged particle’s path in the field is related to its mass and is measured to obtain mass information. (See More Applications of Magnetism .) Historically, such techniques were employed in the first direct observations of electron charge and mass. Today, mass spectrometers (sometimes coupled with gas chromatographs) are used to determine the make-up and sequencing of large biological molecules.

Section summary

• Magnetic force can supply centripetal force and cause a charged particle to move in a circular path of radius
  $r=\frac{\text{mv}}{\text{qB}},$
  where $v$ is the component of the velocity perpendicular to $B$ for a charged particle with mass $m$ and charge $q$ .

Conceptual questions

What are the signs of the charges on the particles in [link] ?

Which of the particles in [link] has the greatest velocity, assuming they have identical charges and masses?

Problems&Exercises

If you need additional support for these problems, see More Applications of Magnetism .