When we touch a pith ball with an electrified plastic rod, some of the negative charges on the rod are transferred to the pith ball and it also gets charged. Thus the pith ball is charged by contact. It is then repelled by the plastic rod but is attracted by a glass rod which is oppositely charged. The process of charging of body by contact/rubbing of body is called charging by conduction.
In charging by induction, a charged body A imparts to another body B, some charge of opposite sign without any contact between A and B. The process of charging of body by without making any contact with other of body is called charging by induction. Body A shall not lose any charge as it is not in contact with B. Charging by induction is explained as below.
(i). Bring two metal spheres, A and B, supported on insulating stands, in contact as shown in Fig. a.
(ii) Bring a positively charged rod near one of the spheres; say A. The free electrons in the spheres are attracted towards the rod. This leaves an excess of positive charge on the rear surface of sphere B. Both kinds of charges are resided on the surfaces, as shown in Fig. b.
The left surface of sphere A, has an excess of negative charge and the right surface of sphere B, has an excess of positive charge. However, not all of the electrons in the spheres have accumulated on the left surface of A.
As the negative charge starts building up at the left surface of A, other electrons are repelled by these. In a short time, equilibrium is reached under the action of force of attraction of the rod and the force of repulsion due to the accumulated charges.
The process is called induction of charge and happens almost instantly. The accumulated charges remain on the surface, as shown, till the glass rod is held near the sphere. If the rod is removed, the charges are not acted by any outside force and they redistribute to their original neutral state.
(iii) Separate the spheres by a small distance while the glass rod is still held near sphere A, as shown in Fig. c. the two spheres are found to be oppositely charged and attract each other.
(iv)Remove the rod. The charges on spheres rearrange themselves as shown in Fig. d.
Now, separate the spheres quite apart. The charges on them get uniformly distributed over them, as shown in Fig. e.
In this process, the metal spheres will each be equal and oppositely charged.
Note: In the process of electric induction, the positively charged glass rod does not lose any charge. This is contrary to charging by conduction, i.e. charging by actual contact where the charged glass rod loses some charge.
Some of these questions which may be asked in your Board Examination 2012-2013
Q1: Does transfer of electron from one material to another depends upon the work function of the material?
Q2: Can two similar charged balls attract each other?
Q3: A glass rod rubbed with the silk is brought close to two uncharged metallic spheres in contact with each other, inducing charges on them. Describe whar happens when (i) the spheres are slightly separated and (ii) the glass rod is subsequently removed and finally (iii) the spheres are separated far apart?