A charge Q is uniformly distributed over a large plastic plate. The electric field at a point P close to the centre of the plate is 10 V m–1. If the plastic plate is replaced by a copper plate of the same geometrical dimensions and carrying the same charge Q, the electric field at the point P will become
A large plastic plate and copper plate is an example of infinite long plates.
As we know,
The electric field due to infinite long sheet is 
Where
σ is the surface charge density of the thin conducting sheet
ε0 is the permittivity of the free space
The electric field due to the plastic plate is 10 Vm-1
Thus, the electric field for the copper would be same as that of the plastic plate.
The electric field of the copper plate id 10 Vm-1
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