If the sizes of charged bodies are very small as compared to the distances between them, we treat them as point charges. All the charge content of the body is assumed to be concentrated at one point in space. Some other properties of the electric charge are
- Additive of charges If a system contains n charges q1, q2, q3, …, qn, then the total charge of the system is q1 + q2 + q3 + … + qn .
Charge has magnitude but no direction. Proper signs have to be used while adding the charges in a system.
- Charge is conserved It is not possible to create or destroy net charge carried by any isolated system.
Sometimes nature creates charged particles: a neutron turns into a proton and an electron. The proton and electron thus created have equal and opposite charges and the total charge is zero before and after the creation.
- Quantization of charge Charges are integral multiples of a basic unit of charge denoted by e. Thus charge q on a body is always given by q = ne. where n is any integer, positive or negative.
By convention, the charge on an electron is taken to be negative; therefore charge on an electron is written as –e and that on a proton as +e. The fact that electric charge is always an integral multiple of e is termed as quantization of charge.
In the International System (SI) of Units, a unit of charge is called a coulomb and is denoted by the symbol C.In this system, the value of the basic unit of charge is e = 1.602192 × 10–19 C.
Some of these questions which may be asked in your Board Examination 2012-2013
Q1: When a plastic comb is passed through dry hair, what type of charge is acquire by comb?
Q2: Does motion of a body affect its charge?
Q3: What is the origin of frictional forces?
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