A charged bead is capable of sliding freely through a string held vertically in tension. An electric field is applied parallel to the string so that the bead stays at rest of the middle of the string. If the electric field is switched off momentarily and switched on

Concept Used: Concept of electric field 

Detailed Solution: 

Initally we say that, 

$\text{ Electric Force }=\text{ mg. }$

If electric field is switched off, there whould be net downwards gravitational force hence downward acceleration will there and it gain a velocity. 

If electric field is switched on, then force balance each other and acc become zero and it moves with constant velocity in downward direction.