Engineering
Physics
Electric Potential Energy and Electric Potential
Question

A thin spherical insulating shell of radius R carries a uniformly distributed charge such that the potential at its surface is V0. A hole with a small area α4πR2 (α<<1) is made on the shell without affecting the rest of the shell. Which one of the following statements is correct ?

The potential at the center of the shell is reduced by 2αV0

The magnitude of electric field at a point, located on a line passing through the hole and shell's center on a distance 2R from the center of the spherical shell will be reduced by αV02R

The ratio of the potential at the center of the shell to that of the point at 12 R from center towards the hole will be 1α12α

The magnitude of electric field at the center of the shell is reduced by αV02R

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Solution

Let charge on the sphere initially be

  kQRV0

and charge removed = αQ

 

and VP=kQR2KαQR=kQR(12α)

VC=kQ(1α)R

   VCVP=1α12α 

(B) (EC)initial = zero                       

(EC)final=kαQR2      

⇒ Electric field increases

(C) (EP)initial=kQ4R2     

 (EP)final=kQ4R2kαQR2

 ΔEP=kQ4R2kQ4R2+kαQR2=V0αR     

(D)    (VC)initial=kQR     

 (VC)final=kQ(1α)R

 ΔVC=kQR(α)=αV0