In electrostatics free charges in a good conductor reside only on the surface. So the free charge inside the conductor is zero. So the field in it is caused by charges on the surface. Since charges are of the same nature and distribution is UNIFORM, the electric fields cancel each other
It’s a conductor. There are free charges (free to move throughout the bulk of conductor). So, in presence of external field, these charges experience a force.
And here's the most crucial bit:
“These charges rearrange themselves until they no longer experience a force.”
As electric field is switched on, electrons move against electric field and accumulate at one side of metal. Naturally, other side gets positively charged. So, the result is an internal field due to accumulation of charges. The charges in middle stop moving a bit later because the internal field has exact same strength as external field. (The red dots represent positive charge accumulated on other side. Note that they are not protons)
Inside the conductor, now there are actually 2 equally balanced opposite Electric Fields. So net field inside conductor is zero.
A great physics professor had nice way of putting it across like this:
“The charges move in a conductor so as to kill the external field.”
You yourself need to think and realize why its true.
By the way, this phenomenon has major application in Electrostatic Shielding.
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can you tell me 10 questions