Question

The distance between the plates of a charged parallel plate capacitor is $5cm$ and electric field inside the plates is $200Vc{m}^{-1}$. An uncharged metal bar of width $2cm$ is fully immersed into the capacitor. The length of the metal bar is same as that of capacitor. The voltage across capacitor after the immersion of the bar is:

• A. zero
• B. $400V$
• C. $600V$
• D. $100V$

Answer 1

The correct option is C $600V$

When the uncharged metal bar of width of $2cm$ is immersed into the capacitor, the distance between the plates of capacitor reduces and becomes: $(5-2)cm=3cm$
Since , the electric field: $E=\frac{V}{d}$, where $E=200V/cm,d=3cm$,
so potential: $V=E\times d=200\times 3=600V$