The electric field within a conductor is zero due to the movement of free electrons, which redistribute themselves to neutralize any external electric field.

To elaborate, conductors are materials that contain free electrons. These electrons are not bound to specific atoms, allowing them to move freely throughout the material. When an external electric field is applied to a conductor, these free electrons respond by moving in the direction dictated by the electric field. They will continue to move until they achieve a state of equilibrium, where the forces acting on them are balanced.

The free electrons rearrange themselves in such a manner that they generate an internal electric field that precisely opposes the external electric field. This occurs because electrons, which carry a negative charge, move in the opposite direction of the applied electric field. As they move, they create their own electric field in the opposite direction. Eventually, their arrangement results in an internal electric field that completely cancels out the external electric field. At this equilibrium point, the net electric field inside the conductor becomes zero.

This characteristic is a fundamental property of conductors and explains their usage in various applications, such as electrical wiring and shielding. For instance, the metal casing surrounding electrical cables acts as a shield, preventing the electric field generated by the cable from influencing nearby electronic devices. This occurs because the free electrons in the metal casing rearrange themselves to negate the electric field from the cable, ensuring that the net electric field inside the casing—and consequently outside the cable—remains zero.

It is also crucial to understand that this phenomenon applies only when the conductor is in electrostatic equilibrium. In this state, there are no net forces acting on the charges within the conductor, and the charges are stationary. If the conductor is not in electrostatic equilibrium, the electric field inside it will not be zero.