# A point charge q is placed in a cavity in a metal block. If a charge Q is brought outside the metal, will the charge q feel an electric force?

Metals are conductors of electric charge, the charge placed in metal will always collect on the surface of the metal. However, it has an electrostatic shielding effect. That is, any charge in a cavity inside the metal block will be completely isolated from the outside world. This is because, if we take a Gaussian surface surrounding the cavity, the charge enclosed is zero. By Gauss’ law,

,

Where

= electric flux, E = electric field, ds = area element, q = charge enclosed, 𝜖0= electric permittivity of vacuum.

Here, q = 0 since the charge inside a conductor is 0. This justifies why the cavity in an external electric field will be completely shielded or isolated.

There will be no lines of force that can penetrate into the cavity. Therefore, even if a charge Q is brought outside the metal, the charge q inside the cavity will experience no force. Since charge distributed on the surface hence total charge will be on the surface is q+Q.

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