A negatively-charged balloon is brought near a neutral, conducting sphere. When the balloon is held near, the opposite side of the sphere is touched (see figure below). What happens to the charge on the sphere? Explain. Be sure to include a description of particle movement in your explanation.
The process can be explained in two steps
1) When balloon is brought closer to the conductor but not touched by finger
When the negatively charged balloon is brought closer to the conductor, by the process of conduction separation of charges occur in the conductor. The positive charges move towards the right side near to balloon, and the negative charges are repelled to the farther side of the conductor as shown in the figure below.
2) When balloon is brought closer to the conductor and the conductor is touched by the finger
When the conductor is touched by finger, it is equivalent to earthing the conductor, as human body is a good conductor.
Now the negative charges repelled by the balloon on the far side of conductor moves via finger to the body leaving behind only positive charges which are held by the negative charge of balloon. Hence the conductor becomes positively charged.
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