March 11, 2005
Magnetic Deflection of Electrons
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Photo by
Physics II Class
This is a
picture of a special cathode ray tube (CRT) - very similar to a
television picture tube and works on the same principles. In this
special tube, electrons originate from the right hand side of the
picture and are travelling to the left in a vacuum. The
electrons,
however, strike the graph paper (called a "graticle"). Each
major
division is one centimeter. The graticle is coated with a
"phosphor"
that glows when electrons of high eneough energy strike it. This
phosphor glows blue when the electrons strike it. A color TV has
three
different types of phosphors - those that glow red, green, and blue in
order to make a color display. In the picture above, the beam is
bent. This bending of the electrons' path is caused by the
magnetic
force provided by an electro-magnet surrounding the tube. The
magnetic
field from the coils is directed so that the field is directed coming
out toward the camera. If a magnetic compass were placed in front
of
this tube, the compass points towards the observer. The forces
that
magnetic fields exert on charges are unusual in that they are not like
ordinary non-magnetic forces. Magnetic forces are perpendicular
to
both the magnetic field direction and the velocity of the charged
particles. The electrons (charged particles) move basically from
right
to left in the above photo. The magnetic field is perpendicular
to the
plane of the photo and perpendicular to the velocity of the
electrons.
The magnetic force is perpendicular to both the velocity and the
magnetic field, so the force is down. The electrons follow a
circular
path because the force is always at right angles to the velocity.
By
analyzing and measuring the radius of the electron's orbit, knowing the
velocity of the electrons, the physical properties of the electrons,
(the ratio of charge to mass) can be calculated. This experiment
was
pioneered by Nobel laureate J. J. Thompson in 1897 trying to learn
about the mysterious cathode rays in evacuated tubes. This
contributed
to the discovery of the electron., and of course, the demise of cathode
ray tubes for the purpose of TV. Read an interesting biography on J. J.
Thompson at the American Institute of Physics website.
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