A simple pendulum has a bob with mass 
and charge 
. The pendulum string has negligible mass. When a uniform and horizontal electric field 
is applied, the tension in the string changes. The final tension in the string, when pendulum attains an equilibrium position is 
. (g: acceleration due to gravity)
Text Solution
Verified by ExpertsA
The charged bob experiences two forces at equilibrium:
Gravitational force: 
(downward)
Electric force: 
(horizontal, in direction of field)
These forces are perpendicular to each other. The string tension must balance their resultant.
At equilibrium, the string makes angle 
with vertical such that:
Vertical: 
Horizontal: 
The net force from gravity and electric field is 
Therefore, the tension in the string is: 
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