magnetic field. When an electron beam is produced by the electron gun, the lowpressure
Helium appears as a circular trace and its magnitude can be approximated by using the
built in cm ruler which determines diameter readings. Cathode K is heated, releasing
heated electrons, the motion of the electrons is accelerated by the electric potential (V)
( all this can be seen by figure 2 in appendix A); applied between anode P and Cathode K.
Using the law of Conservation of Energy, the velocity of the electrons accelerated by V
can be determined with the equation:
(1)
which results in:
(2)
Where the mass of the electron (m) is measured in kg and the electrical charge of
the electron (e) is measured in C. A Lorentz Force
F = evB
(3)
is established due to the electrons perpendicular motion with velocity (v) to the
uniform magnetic field (B). This force gives the electrons their circular path in a plane
perpendicular to that of the magnetic field’s.
Establishing a magnetic field with magnetic flux density equal to B [b/m
2
], we
can obtain the equation
(4)
Where r is the radius and v is the velocity of the circular motion respectively.
From the previous equation, we determine
,
(5)
which allows for the calculation of the charge to mass ratio, or e/m, being
(6)
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 Spring '12
 Dr.Phaneuf
 Physics, Magnetic Field, power supply

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