202 Week 6: Moving Charges and Magnetic Force
Figure 65: The first photograph of a positron ever taken in a cloud chamber. Note the curvature
carefully. Which way is the particle travelling while slowing down? What direction does the magnetic
field in the chamber point?
to the charge and mass by:
r=mv^0
qB 0(413)
and can be determined directly from a photographed trajectory.
At the same time, the particle slows down because the same processthat causes supersaturated
gas molecules to precipitate out along its trajectory exerts a “drag” force on the particle. By looking
at the rate the particle’s trajectory curvaturechanges(and various other things), one can estimate
its momentum, the charge of the particle, and its mass. Using this and many other specialized
detectors, an enormous “zoo” of particles has been discovered and categorized and transformed into
a quantitative model for the nuclear force that has at least some predictive power, although it is not
yet a complete or perfect theory.
A simple cloud chamber is not too difficult to build – it requires a bowl, dry ice, alcohol, cotton,
black paint, a light source, and a few other things, but they are all fairly readily obtainable. It is
therefore a good candidate for an extra credit project, if your program has one.
Example 6.2.4: Region of Crossed Fields
Another extremely useful application of magnetic fields acting on individual charged particles is the
region of crossed fields. A region of crossed electric and magnetic fields, when equipped withsuitable
collimating slits, can act as avelocity selector.
A charged particle with chargeqenters the device on the left by passing through collimating
slits that ensure that its velocity is in thex-direction only. Inside the device a pair of parallel plates
creates a uniform electric fieldE~down, while a magnetic coil creates a uniform magnetic fieldB~
into the page as drawn.
From the right hand rule, the magnetic force on the charged particle isFB=qvB (414)up. The electric force, however, is
FE=qE (415)