A block a mass m having a charge q is placed on a smooth horizontal table and is connected to a wall through an unstressed spring of spring constant κ as shown in figure. A horizontal electric field E parallel to the spring is switched on. Find the amplitude of the resulting SHM of the block.
Given:
Mass of the block: m
Charge of the block: q
Formula used:
When a charged body of charge q and mass m is brought under an horizontal electric field E, it will experience electric force Fe in the direction of the field.
Fe=qE
When the block is accelerated due to the electric force, the spring will cause a restoring spring force in the opposite direction.
Spring force is:
Fs = -kx
Where, k is the spring constant and x is the distance the spring is stretched or compressed.
Here x is the amplitude.
Thus,
Fe = Fs
∴ qE = -kx
We used modulus as the amplitude cannot be negative.
Hence, 
is the amplitude of the resulting SHM of the block.