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Question

A pendulum bob of mass 80 mg and carrying a charge of 4×108 C is at rest in a horizontal uniform electric field of magnitude 20000 V/m. The tension in the thread of the pendulum and the angle it makes with the vertical are (Acceleration due to gravity =10ms2):

A
82×104N,45o
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B
8×104N,60o
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C
62×104N,30o
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D
6×104N,37o
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Solution

The correct option is A 82×104N,45o
Since bob is at rest, net force on it is zero.
Balancing horizontal forces, qE=Tsinθ
Balancing vertical forces, mg=Tcosθ

Solving above two equations,
T=(mg)2+(qE)2
T=(80×106×10)2+(4×108×2×104)2
T=82×104 N

tanθ=qEmg
tanθ=4×108×2×10480×106×10
tanθ=1
θ=45o

623091_586362_ans.png

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