First slide

Electric field

Question

There is a uniform electric field of strength 10³ V/m along y-axis. A body of mass 1g and charge 10^-6C is projected into the field from origin along the positive x-axis with a velocity 10 m/s. Its speed in m/s after 10 s is (Neglect gravitation)

Moderate

A

10
B

5 $\sqrt{2}$
C

10 $\sqrt{2}$
D

20

Solution

Body moves along the parabolic path.
For vertical motion: By using $v = u + a t$
$\Rightarrow v_{y} = 0 + \frac{Q E}{m} . t = \frac{10^{- 6} \times 10^{3}}{10^{- 3}} \times 10 = 10 m / \sec$
For horizontal motion. Its horizontal velocity remains the same i.e. after 10 sec, horizontal velocity of body $v_{x} = 10 m / \sec$
$Velocity after 10 sec v = \sqrt{v_{x}^{2} + v_{y}^{2}} = 10 \sqrt{2} m / \sec$

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