A rod made from insulating material carries a net charge,while a copper sphere i

Question

A rod made from insulating material carries a net charge,while a copper sphere is neutral. The rod and the sphere do nottouch. Is it possible for the rod and the sphere to

(a) attract one another?
(b) repel one another? Select all that apply.

If thecopper sphere is grounded before the charged rod is brought close,the rod brought close, and then the ground connection removed; therod and the copper sphere will attract each other. The effect ofthe grounding is to allow charge opposite to the charge on the rodto move onto the copper sphere.

In orderfor the charged rod and uncharged copper sphere to repel there mustbe charge on the copper sphere, on the side of the copper sphereclose to the approaching rod, that has the same sign as the chargeon the rod. Without deliberately adding this like charge to thecopper sphere it is not possible to get the rod and the sphere torepel. It ispossible for the rod and sphere to attract. They will attractnaturally if the two are brought close to each other withouttouching. As the rod nears the uncharged copper sphere mobileelectrons on the copper will either move to or away from the sideof the sphere the chraged rod is close to depending on whether therod is positively or negatively charged, respectively. Theresulting unbalanced and unequal charge condition leads toattraction between the rod and the copper sphere. Thecharged rod and the uncharged copper sphere will always repel eachother when the two are brought close to each other withouttouching. Thecharged rod and the uncharged sphere can repel each other if thecharge on the rod, when the rod attracted the neutral coppersphere, is removed and replaced with the opposite charge. Then therod and copper sphere will repel. A rod made from insulating material carries a net charge,while a copper sphere is neutral. The rod and the sphere do nottouch. Is it possible for the rod and the sphere to If thecopper sphere is grounded before the charged rod is brought close,the rod brought close, and then the ground connection removed; therod and the copper sphere will attract each other. The effect ofthe grounding is to allow charge opposite to the charge on the rodto move onto the copper sphere.

In orderfor the charged rod and uncharged copper sphere to repel there mustbe charge on the copper sphere, on the side of the copper sphereclose to the approaching rod, that has the same sign as the chargeon the rod. Without deliberately adding this like charge to thecopper sphere it is not possible to get the rod and the sphere torepel. It ispossible for the rod and sphere to attract. They will attractnaturally if the two are brought close to each other withouttouching. As the rod nears the uncharged copper sphere mobileelectrons on the copper will either move to or away from the sideof the sphere the chraged rod is close to depending on whether therod is positively or negatively charged, respectively. Theresulting unbalanced and unequal charge condition leads toattraction between the rod and the copper sphere. Thecharged rod and the uncharged copper sphere will always repel eachother when the two are brought close to each other withouttouching. Thecharged rod and the uncharged sphere can repel each other if thecharge on the rod, when the rod attracted the neutral coppersphere, is removed and replaced with the opposite charge. Then therod and copper sphere will repel.

Explanation / Answer

(a) Yes. Suppose the rod has a positive charge. Bring it close tothe copper sphere. The positive charge on the rod will (dependingon how you want to look at it) either repel positive charges in thesphere or attract negative charges. In either case, the sphere willend up being a dipole, with negative charge close to the rod andpositive charge further away. The rod will then attract thesphere. You can see this for yourself. Charge a comb by running it throughyour hair and put it near a stream of water from a faucet. You'llsee the water bend towards the comb. The water is neutral, but thecomb causes the stream to become a dipole and then attracts thewater. (b) Nope. The induced charge will always be attractive. Now: if youwere to touch the rod to the sphere, the charge on rod and spherewould equalize. The rod and sphere would then repel. However, youspecified that the rod and sphere could not touch, which leavesthis out.

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