Electrostatic Induction is the first topic of the electrostatics.
In the presence of a charged body, an insulated conductor
develops positive charge at one end and negative charge at the
other end. This process is called the electrostatic induction.
If we bring charged plastic rod near neutral copper
rod, both rods attract each other as shown in Fig. 13.4.
This attraction between the charged and uncharged rods
shows as if both rods have different charges. But this is not
true. Charged plastic rod produces displacement of positive
and negative charges on the neutral Copper rod which is the
cause of attraction between them. But total charge on
Copper rod is still zero. It implies that attraction is not the
sure test of charge on a body.
It also shows that electrostatic induction is another method
of charging a body and is described below.
Consider a metallic sphere placed on an insulated stand
The sphere is neutral as it carries equal number
of positive and negative charges. Now bring a negatively
charged rubber rod near the conducting sphere. As shown in
left part of the sphere that is close to the rod becomes positively charged while the right part that is away
from the rod becomes negatively charged. Negative charge in
the rod repels the negative charge of the sphere and shifts it
to the opposite region of the sphere that is away from the
rod. Thus there is excess of positive charge in the region of
sphere close to the rod while there is excess of negative
charge in the region of the sphere away from the rod. But as a
whole the sphere is still neutral, since no charge has been
added or subtracted.
Now if we remove the rod away from the sphere, the charge
again will spread uniformly on the whole surface of the
sphere.Now earth the sphere through a conducting wire in the
presence of the rod (Fig.13.6-a).
The negative charge will flow
to the earth and leaves the sphere with net positive charge.
Now if we first break the earth connection and then remove
the rod from the sphere it will get positive charge
Bring two metal spheres A and B and place them on
an insulating stand as shown in Fig. 13.7-a.
Now bring a positively
charged rod near sphere A as shown in Fig. 13.7-b.
Rod will attract negative charge towards it and repel positive charge away
from it. Negative charge will be developed on the left surface of
the sphere A which is close to the rod. While positive charge will
be developed on the right surface of the sphere B. Now separate
the spheres by a small distance while the rod is still near the
sphere A. The two spheres will be oppositely charged and attract
each other as shown in Fig.13.7-c.
Remove the rod. The charges on spheres rearrange themselves as shown in Fig. 13.7-d.
Now separate the spheres by a large distance. The charges are uniformly distributed over the surfaces of the spheres as shown in Fig.13.7-e.
In this process, an equal and opposite charge will be developed
on each metal sphere. This is charging by induction.