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Does charge of a metal charged by induction determine by which part of the metal is ground to? I draw a diagram to make it simple to understand:enter image description here

Right diagram: When ground is touched with the positive side of the metal, electrons flow to the negative and the metal become negatively charged.

Left diagram: When ground is touched with the negative side of the metal, electrons flow to the ground and the metal become positively charged.

Is this true? Because opposite charges repel.

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When charging a metal object with induction, another charged object is brought near to that neutral metal object but it is not touched. This will make the electrons move within the metal object according to polarity of the charged object. Keep in mind that opposite charges attract one another whereas like charges repel. This movement of electrons creates an imbalance of charge on opposite sides of the neutral metal object. When the charge is separated within the metal object, and a ground is contacted to one of the sides of the metal object, a flow of electrons between the object and the ground will start.

Now back to your example: In the left diagram, electrons will flow from the ground to the metal object and make it negatively charged. Because the ground is connected where the positive charges accumulated on the metal object. In the right diagram, the electrons in the metal object will flow to the ground as the ground is connected where they accumulated and this will make the metal object positively charged.

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Are these comments based on theory or has this effect been demonstrated? – David White Aug 12 '15 at 3:48

The ground is connected to every point of the metal plate by the metal wire and the metal plate.

I mean, there is a metal wire that connects the ground to the plate, and then there is the metal plate that connects every point on the metal plate to each other, therefore every part of the metal plate is equally connected to the ground.

When you touch the plate with the grounded wire, electrons move to some direction through the wire. As every point on the metal plate has exactly the same voltage, it does not make any difference what part of the plate you touch with the wire.

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