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The very familiar phenomenon of charging by induction includes bringing a charged object near an uncharged sphere to induce a separation of charges in it and then grounding the charge (same as the charge of the body brought close), which is at the end away from the one where the object is brought close. But my question:-

The fact, is the resultant electric field in any conductor is zero and it is an equipotential surface. Therefore before the grounding, the field due to the charged body brought near (say +), the induced negative charge(say q-), and that due to the positive induced charge(q+) at the other end gave a null field within the sphere. But when the q+ is grounded, wouldn't the resultant field of the two remaining charges turn out to be non-zero as the charge which was grounded had some contribution in making the field zero.
Next question, the "grounding" occurs as the earth is almost at zero potential and therefore when a charged body at some higher or lower potential is attached to earth, the charge flows in and out to ultimately bring the object at zero potential. But in the induction example above, the positive charge got grounded due to potential difference but the negative charge didn't. why? Shouldn't the negative charge too get grounded?

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up vote 1 down vote accepted

Answer to your second question:

If you take any point inside the conductor, there is a non-zero potential at that point due to the external field, and as you correctly said, the potential of the sphere should be zero as it is grounded. So a net charge (+ or - depending on the field) will accumulate on the conductor to bring the potential inside the conductor to zero. That's why those negative charges don't flow to the ground.

I'm not exactly sure about the answer to the first question.

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