Electrostatics (Study Notes)

ELECTRIC CHARGES AND FIELDS

Electrostatics:

Electrostatics is the study of electrical charges at rest (static electricity).

Electric Charge:

       Have you ever noticed what happens when a comb is ruffled through hair? It now becomes capable of attracting small pieces of paper. Similarly, a glass rod rubbed with silk cloth acquires the property of attracting small pieces of paper. The property of a substance acquired in this way is called electric charge.
         An atom is electrically neutral. The number of protons (positive charges) is equal to the number of electrons (negative charges) in atom. The electric charge is produced by adding or removing electrons to/from a body.
         If the electrons are removed from a body, it becomes positively charged. A glass rod becomes positively charged when it is rubbed with silk or wool. If the electrons are added to a body, it becomes negatively charged. A rubber rod becomes negatively charged when it is rubbed with fur.
         This nomenclature of electric charges as positive and negative charges was given by Benjamin Franklin. This choice was quite arbitrary. When a glass rod is rubbed with a silk cloth, some electrons are transferred from glass to silk. According to him, the charge accumulated on the glass rod is positive and that on the silk is negative. Also equal and opposite charges are produced on two bodies when they are rubbed together. In any such process, the net charge produced is zero.
         When bodies are rubbed together, the electrons are transferred from one body to the other. hence, one object becomes positively charged and the other becomes negatively charged.

Properties of Electric Charge:

Additivity:

The charges add like scalars. The total charge q of a system containing n charges q1, q2, q3….. qn is given by

          q = q1+ q2+ q3+ ….. +qn

Clearly, when a positive charge is added to a negative charge of the same magnitude, the net charge becomes zero. It means the proper signs should be used while adding the charges in a system.

Conservation of Electric Charge:

The total charge of an isolated system remains constant, i.e., charge is neither created nor destroyed. For such a system, the total positive charge is equal to total negative charge if it initially electrically neutral. If one region or body becomes positively charged, then an equal amount of negative charge will be developed in neighboring regions or bodies. The processes of pair production and pair annihilation follow the law of conservation of charges.

Quantization of Electric Charge:

The electric charge is quantized, i.e., it can have only discrete values which are an integral multiple of the charge of an electron (e = 1.6 10􀬵􀬽 C). Thus,
              q =  ne (n = 1, 2, 3, – – – -)
‘+’ indicates the deficiency of electrons and ‘’ their excess on a body.

The quantization of electric charge was experimentally demonstrated by Millikan in his famous Oil Drop experiment

At the macroscopic level, the change in the charge of a body in units of e (= 1.6 10􀬵􀬽 C) is not appreciable. At such level, the amount of the charge transferred is so large as
compared to e that it appears to be continuous and not discrete. Even a charge of 1 C contains 6.25  10􀬵􀬼 times the electronic charge. At this scale, it is better to take charge as continuous instead of dealing in units of e. Clearly, the quantization of charge can be ignored at the macroscopic level. Though, it is still important at the microscopic level.

Invariance of Electric Charge:

Charge at rest = charge in motion (or frames of
reference in relative motion)
But this is not true for mass. Therefore,

Mass at rest  mass in motion
Also, a charge cannot exist without mass (an electron has mass) though a mass can exist without charge. Therefore, in the process of charging, mass of a body changes.

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