motion of the electrons is accelerated by the electric potential (V); applied between anode P and
Cathode K [1]. Using the law of Conservation of Energy, we can determine the velocity of the
electrons accelerated by V with the equation
which results in
; 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) 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
where r is the radius and v is the velocity
of the circular motion respectively. From the previous equation, we determine
, which allows for
the calculation of the charge to mass ratio, or e/m, being
.
The magnitude of the magnetic field created due to the Helmholtz coils (H), can be
calculated using the coil’s radius (R) and passing current (I) using . Here we determine
, where
N denotes the turns of the wire in each coil. The e/m apparatus used for this laboratory
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experiment is built of 130 turns per wire with a radius of 0.15m, establishing our final calculation
of the magnetic field being
.
Procedure
: (Broken into parts for convenience)
A] Preparation
1.
Connect the 6.3V power supply to the apparatus’ power terminal, then connect the 500V
DC “B power supply” to the “B power” terminal of the apparatus (P = + [red terminal]
and K =  [black terminal]).
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 Spring '12
 Dr.Phaneuf
 Physics, Charge, Magnetic Field, power supply

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