Summary of chapter 22

Prepared by Dr. A. Mekki





           There are two types of electric charges in nature;

Positive charge (proton) and negative charge (electron).

        An object is positively charged if it has lost electrons.

        An object is negatively charged if it has gained electrons.


  1. Conductors are materials in which electric charge (electrons) can move quite freely. Metals such as copper and aluminum are conductors.

Insulators are materials in which electric charge (electrons) are not free to move. Materials such as glass, rubber, and plastic are insulators.

Semi-conductors have electrical properties between metals and insulators.


  1. Coulombs Law state that the electrostatic force between two charged particles separated by a distance r is given by:



       N m2/C2

eo is the permitivity of free space and k is the electrostatic constant, q1 and q2 are the charges of the two particles and r is the distance between the two charges.      



        If the two charges have same signs, there is repulsion between them.



                          +                             +





                          -                           -         


        If the two charges have opposite signs, there is attraction between them.













4.     The shell theorem

(i) A charge q is outside a uniformly charged shell carrying a charge Q.







The force on the charge q in both cases is




If the charge q in inside the uniformly charged shell then the force on the charge q is ZERO.



        The elementary charge is that of the electron e = 1.6 x 10-19 C.

Any charge on a body is an integer multiple of the electron charge,i.e.,

Q = n e, n = 1, 2,


        Electric charge is always conserved. It can be transferred from one body to another but cannot be lost.