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NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 22
Total: 25
A monoenergetic (18 keV) electron beam initially in the horizontal direction is subjected to a horizontal magnetic field of 0.4 G normal to the initial direction. Estimate the up or down deflection of the beam over a distance of 30 cm (me = 9.11 × 10–31 kg). [Note: Data in this exercise are so chosen that the answer will give you an idea of the effect of earth’s magnetic field on the motion of the electron beam from the electron gun to the screen in a TV set.]
Solution:  

Kinetic energy of electron
= 18 keV
mev2
2
= 18 × 103 × 1.6 × 10−19 joule
∴ velocity , v = 8 × 107 m s−1
Velocity in x direction remains constant,
hence time to cross 30 cm
t =
30×10−2
8×107
 =
3
8
×10−8 s
A magnetic force F = evB is acting in vertical direction, which provides acceleration in vertical direction
ay =
evB
me
 = 5.63 × 1014 m s−2
Vertical deflection , Sy = uyt+
1
2
ayt2
sy = 0 +
1
2
 × 5.63 × 1014 × [
3
8
×10−8]2 or Sy =
1
2
×
9
64
 × 5.63 × 10−2 = 4 mm
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