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Q.

An electron is made to enter symmetrically between two parallel and equally but oppositely charged metal plates, each of 10 cm length. The electron emerges out of the field region with a horizontal component of velocity 106 m/s. If the magnitude of the electric between the plates is 9.1 V/cm, then the vertical component of velocity of electron is 
(mass of electron =9.1×1031kg and charge of electron =1.6×1019C

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a

0

b

16×106m/s

c

16×104m/s

d

1×106m/s

answer is C.

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Detailed Solution

Question Image

To calculate the vertical component of the velocity of the electron when it exits the electric field, we use the following steps:

Step 1: Force acting on the electron

The force FF on the electron due to the electric field EE is given by:

F=qEF = qE

where:

  • q=1.6×1019Cq = 1.6 \times 10^{-19} \, \text{C} (charge of the electron),
  • E=9.1V/cm=9.1×102V/mE = 9.1 \, \text{V/cm} = 9.1 \times 10^2 \, \text{V/m}.

F=(1.6×1019)(9.1×102)=1.456×1016N.F = (1.6 \times 10^{-19}) \cdot (9.1 \times 10^2) = 1.456 \times 10^{-16} \, \text{N}.

Step 2: Acceleration of the electron

The vertical acceleration aa of the electron is given by Newton's second law:

a=Fma = \frac{F}{m}

where:

  • m=9.1×1031kgm = 9.1 \times 10^{-31} \, \text{kg} (mass of the electron).

a=1.456×10169.1×1031=1.6×1014m/s2.a = \frac{1.456 \times 10^{-16}}{9.1 \times 10^{-31}} = 1.6 \times 10^{14} \, \text{m/s}^2.

Step 3: Time spent in the electric field

The time tt the electron spends in the field is determined by the horizontal motion. Since the electron moves with a horizontal velocity vx=106m/sv_x = 10^6 \, \text{m/s} and the length of the plates is L=10cm=0.1mL = 10 \, \text{cm} = 0.1 \, \text{m}, the time is:

t=Lvxt = \frac{L}{v_x}

 t=0.1106=107s.t = \frac{0.1}{10^6} = 10^{-7} \, \text{s}. 

Step 4: Vertical velocity

The vertical velocity vyv_y acquired by the electron due to acceleration in the electric field is:

vy=atv_y = a \cdot t

Substitute a=1.6×1014m/s2a = 1.6 \times 10^{14} \, \text{m/s}^2 and t=107st = 10^{-7} \, \text{s}:

vy=(1.6×1014)(107)=16×106m/s.

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