Question

An uncharged metal sphere hangs from a nylon thread. When a positively charged glass rod is brought close to the metal sphere, the sphere is drawn toward the rod. But if the sphere touches the rod, it suddenly flies away from the rod. Explain why the sphere is first attracted and then repelled.

Short answer

The objects attract initially because negative charge is induced on the metal sphere. But when they touch, charges flow in both direction with a surplus of positive charge build up finally that causes repulsion.

Step-by-step solution

Explanation for the attraction of the sphere towards the positively charged rod.

When the sphere which is neutral initially is brought close to the positively charged rod, the free electrons are drawn to the surface which induces a negative charge all over the side that faces the rod. This effect is called polarizing the sphere, by separating the negative and positive charges while keeping the total charge of the sphere unchanged, which is neutral.

Explanation of the sudden repulsion of the sphere when it is touched to the rod transfer.

Polarization creates a force field that is attractive between the negative charge side of the sphere and the rod and also a force field that is repulsive between the side of the sphere which is positive. As the electric force according to Coulomb’s law varies with the square of the distance between charges, therefore the attraction force will dominate as shown in the figure below.

Due to the attraction, the sphere will finally touch the rod which allows the charges to flow and get transferred in both directions, i.e. from rod to sphere and sphere to rod. Some of the electrons will flow to the rod from the metal sphere but the rod has an abundance of positive charge which gets transferred to the sphere. So, now the rod is still positively charged but with less number of excess charges. Thus, both the objects being positively charged now will surely repel each other quiet abruptly.