47
Therefore the total displacement (y) in the spot position on the screen S due to the
application of electric field between the plates VV
/
is
y = CF + FO
/
= CF + BD
±
²
³
´
µ
+
=
L
2
a
v
m
a
E
e
y
2
0
 (7)
Thus
±
²
³
´
µ
+
=
L
2
a
a
E
y
v
m
e
2
0
 (8)
Hence, if the velocity of electron along the Xaxis (v
0
) is known, the value of (e/m)
can be calculated.
I.3
Let B be the magnetic field produced by the two bar magnets placed symmetrically
on either side of the cathode ray tube at a distance d from it. The magnetic field of
the bar magnets will be in the east west direction. The magnetic force in the electron
is given by
( )
B
v
e
F
×

=
. This step up a force Bev
0
on the moving electron along
the Ydirection. As a result, the electrons path becomes circular, with radius of
curvature r given by
0
2
0
v
e
B
r
v
m
=
 (9)
When the force on the electron beam due to crossed electric field and magnetic field
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 Fall '09
 LIND
 Electron, Magnetic Field, BM

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