Question

Three large identical conducting parallel plates carrying chrage +Q, -Q, and +2Q, respectively, are placed as shown in figure. If $E_A,E_B$ and $E_C$ refers to the magnitudes of the electric fields at points A, B, C, respectively, then :

• A. $E_A>E_B>E_C$
• B. $E_A=E_B>E_C$
• C. $E_A=0$ and $E_B>E_C$
• D. $E_A=0$ and $E_B=E_C$

Answer 1

The correct option is D $E_A=0$ and $E_B=E_C$

The electric field at any point outside of conducting plate is $E=\frac{\sigma }{2{ϵ}_0}=\frac{Q}{2A{ϵ}_0}$ where $Q=$ charge on plate and $A=$ area of it.
Here the field at A is $E_A=\frac{1}{2A{ϵ}_0}[Q-(-Q+2Q\left)\right]=0$
The field at B is $E_B=\frac{1}{2A{ϵ}_0}\left[\right(Q-Q)-2Q)]=-\frac{1}{A{ϵ}_0}$
The field at C is $E_C=\frac{1}{2A{ϵ}_0}[Q-Q+2Q)]=\frac{1}{A{ϵ}_0}$
In magnitude, $E_B=E_C$