Why is the electric field inside a conductor zero? How does the electric field become zero in a conductor?

Imagine you have a conductor.

Normally, if there is no external magnetic field, the conductor is free from any field.

Let’s look at the other. In the absence of an external field, the conductor was initially in the position shown below. (Blue dots represent electrons). However, not all electrons have been shown. It is believed that all charges are equally distributed.

Imagine that you turn on an Electric Field and bring a large, charged metallic sheet.

Now, you might say, “Aha! There’s also the field inside conductor. You’re missing a lot. It’s a conductor. It is free to move around the bulk of conductor without incurring any charges. These charges are experienced by an external force when there is one.

Here’s the crucial part:

“These charges rearrange until they don’t experience any force.”

When electric field is turned on, electrons travel against it and accumulate on one side of the metal. Naturally, the other side becomes positively charged. The result is an internal magnetic field that has been accumulated with charges. Because the internal field is stronger than the external field, charges in the middle stop moving later.

(The red dots represent positive charge accumulated on other side. Note that they are not protons)

There are now two equally balanced opposite Electric Fields inside the conductor. The net field within conductor is therefore zero.

This is how a great professor of Physics put it:
“The charges are moved in a conductor to destroy the external field.”

It is up to you to see the truth for yourself.

This phenomenon is also used extensively in Electrostatic Shielding.

Update: I have forgotten to mention that the electric fields are not zero when their ends are connected with a battery. Charges follow the same pattern as above, but there are no end points.